Re: [RFC PATCH 04/11] firmware: arm_scmi: Add core notifications support

[Jonathan Cameron]

On Mon, 20 Jan 2020 12:23:26 +0000
Cristian Marussi <cristian.marussi@xxxxxxx> wrote:

> Add basic support for SCMI Notifications, using Kernel notification chains
> mechanism.
> 
> Each SCMI Protocol has a dedicated events' queue and deferred worker which
> is in charge of delivering the notifications to the interested users
> invoking the users' registered callbacks.
> 
> Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>

I'm not keen on the huge asymmetry on the setup and tear down paths.
For instance the lack of a scmi_unregister_protocol events.

The idr usage is not standard.  They aren't the right structure
if you known the value you want.

Not a particularly thorough review otherwise. Out of time for today and
may not get back to this for a while.

Thanks,

Jonathan

> ---
>  drivers/firmware/arm_scmi/Makefile |   2 +-
>  drivers/firmware/arm_scmi/common.h |   4 +
>  drivers/firmware/arm_scmi/driver.c |   2 +
>  drivers/firmware/arm_scmi/notify.c | 904 +++++++++++++++++++++++++++++
>  drivers/firmware/arm_scmi/notify.h |  79 +++
>  5 files changed, 990 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/firmware/arm_scmi/notify.c
>  create mode 100644 drivers/firmware/arm_scmi/notify.h
> 
> diff --git a/drivers/firmware/arm_scmi/Makefile b/drivers/firmware/arm_scmi/Makefile
> index 5f298f00a82e..26587ea4661f 100644
> --- a/drivers/firmware/arm_scmi/Makefile
> +++ b/drivers/firmware/arm_scmi/Makefile
> @@ -1,6 +1,6 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  obj-y	= scmi-bus.o scmi-driver.o scmi-protocols.o
>  scmi-bus-y = bus.o
> -scmi-driver-y = driver.o
> +scmi-driver-y = driver.o notify.o
>  scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o
>  obj-$(CONFIG_ARM_SCMI_POWER_DOMAIN) += scmi_pm_domain.o
> diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
> index df35358ff324..2621c05e9149 100644
> --- a/drivers/firmware/arm_scmi/common.h
> +++ b/drivers/firmware/arm_scmi/common.h
> @@ -6,6 +6,8 @@
>   *
>   * Copyright (C) 2018 ARM Ltd.
>   */
> +#ifndef _SCMI_COMMON_H
> +#define _SCMI_COMMON_H
>  
>  #include <linux/bitfield.h>
>  #include <linux/completion.h>
> @@ -113,3 +115,5 @@ void scmi_setup_protocol_implemented(const struct scmi_handle *handle,
>  				     u8 *prot_imp);
>  
>  int scmi_base_protocol_init(struct scmi_handle *h);
> +
> +#endif /* _SCMI_COMMON_H */
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index 28ed1f0cb417..a43fad29de11 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -350,12 +350,14 @@ __scmi_xfer_put(struct scmi_xfers_info *minfo, struct scmi_xfer *xfer)
>  
>  static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
>  {
> +	ktime_t ts;
>  	struct scmi_xfer *xfer;
>  	struct device *dev = cinfo->dev;
>  	struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
>  	struct scmi_xfers_info *minfo = &info->rx_minfo;
>  	struct scmi_shared_mem __iomem *mem = cinfo->payload;
>  
> +	ts = ktime_get_boottime();
>  	xfer = scmi_xfer_get(cinfo->handle, minfo);
>  	if (IS_ERR(xfer)) {
>  		dev_err(dev, "failed to get free message slot (%ld)\n",
> diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
> new file mode 100644
> index 000000000000..da342f43021e
> --- /dev/null
> +++ b/drivers/firmware/arm_scmi/notify.c
> @@ -0,0 +1,904 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * System Control and Management Interface (SCMI) Notification support
> + *
> + * Copyright (C) 2019 ARM Ltd.
> + *
> + * SCMI Protocol specification allows the platform to signal events to
> + * interested agents via notification messages: this in an implementation
> + * of the dispatch and delivery of such notifications to the interested users
> + * inside the Linux kernel.
> + *
> + * Each SCMI Protocol implementation, during its initialization, registers with
> + * this core notification framework its set of supported events via
> + * @scmi_register_protocol_events(), while Kernel users interested in some
> + * specific event can register their associated callbacks providing the usual
> + * notifier_block descriptor, since the notification system internally supports
> + * events delivery using customized notification chains.
> + *
> + * Given the number of possible events defined by SCMI and the extensibility
> + * of the SCMI Protocol itself, such underlying notification chains are created
> + * and destroyed dynamically on demand depending on the number of users
> + * effectively registered for an event, so that no structures or chains are
> + * allocated until at least one user has registered a notifier_block for such
> + * event. Similarly, events' generation itself is enabled at the platform level
> + * only after at least one user has registered, and it is shutdown after the
> + * last user for that event has gone.
> + *
> + * An event is identified univocally by the tuple (proto_id, evt_id, src_id)
> + * and is served by its own dedicated notification chain: given that such chain
> + * is dynamically created, the registration API simply let the users associate
> + * their callbacks with the above tuple.
> + *
> + * Here proto_id and evt_id are simply the protocol_id and message_id numbers as
> + * reported in the SCMI Protocol specification, while src_id represents an
> + * optional, protocol dependent, source identifier (like domain_id, perf_id
> + * or sensor_id and so forth), so that a user can register its callbacks for a
> + * particular event coming only from a defined source (like CPU vs GPU).
> + * When the source is not specified the user callback will be registered for
> + * all existing sources for that event (if any).
> + *
> + * Upon reception of a notification message from the platform the SCMI RX ISR
> + * passes the received message payload and some ancillary information (including
> + * an arrival timestamp in nanoseconds) to the core via @scmi_notify(), which,
> + * in turn, after having looked up the event in the registered events mapping,
> + * pushes the event-data itself on a protocol dedicated kfifo queue for deferred
> + * processing.
> + *
> + * Such dedicated protocols' queues are allocated once for all at initialization
> + * time, together with a dedicated work_item running the common delivery logic
> + * of @scmi_events_dispatcher(), so that each protocol has it own dedicated
> + * worker which, once kicked by the ISR, takes care to empty is own dedicated
> + * queue deliverying the queued items into the proper notification chain.
> + * Note that since the underlying cmwq workers run one distinct work_item per
> + * protocol and there are a number of max-active workers equal to the number of
> + * protocols, notifications processing can proceed concurrently only between
> + * events belonging to different protocols, while delivery of events within the
> + * same protocol is still strictly sequentially ordered by time of arrival; this
> + * separation effectively avoids the possibility that one particularly verbose
> + * protocol, flooding the queues with events, can cause other protocols' events
> + * to be lost or their processing starved.
> + *
> + * Event's information is converted to a custom per-event report struct before
> + * being fed as void *data param to the user callback embedded in the registered
> + * notifier_block so that it looks like as follow from the user perspective:
> + *
> + * int user_cb(struct notifier_block *nb, unsigned long event_id, void *report)
> + *
No point in having blank lines at the end of a comment block.
> + *
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/bug.h>
> +#include <linux/bitfield.h>
> +#include <linux/err.h>
> +#include <linux/idr.h>
> +#include <linux/kernel.h>
> +#include <linux/kfifo.h>
> +#include <linux/notifier.h>
> +#include <linux/refcount.h>
> +#include <linux/slab.h>
> +#include <linux/scmi_protocol.h>
> +#include <linux/types.h>
> +#include <linux/workqueue.h>
> +
> +#include "notify.h"
> +
> +#define SCMI_NOTIF_MAX_ACTIVE_QUEUES	7
> +
> +/*
> + * Builds an unsigned 32bit key from the given input tuple to be used as a key
> + * in IDR mappings; note that if __src_p is passed in as NULL, the returned key
> + * is built using SCMI_ALL_SRC_IDS as src_id.
> + */
> +#define MAKE_EVT_KEY(__proto, __evt, __src_p)				\
> +	({								\
> +		u32 __evt_key, __src_id;				\
> +									\
> +		__src_id = ((__src_p)) ?				\
> +			    (*((u32 *)(__src_p)) & SCMI_ALL_SRC_IDS) :	\
> +			    SCMI_ALL_SRC_IDS;				\
> +		__evt_key = ((__proto) << 24) |				\
> +				((__evt) << 16) | __src_id;		\
> +		__evt_key;						\
> +	})
> +
> +#define PROTO_ID_MASK			GENMASK(31, 24)
> +#define EVT_ID_MASK			GENMASK(23, 16)
> +#define SRC_ID_MASK			GENMASK(15, 0)
> +#define KEY_XTRACT_PROTO_ID(key)	FIELD_GET(PROTO_ID_MASK, (key))
> +#define KEY_XTRACT_EVT_ID(key)		FIELD_GET(EVT_ID_MASK, (key))
> +#define KEY_XTRACT_SRC_ID(key)		FIELD_GET(SRC_ID_MASK, (key))
> +
> +/**
> + * events_queue  - Describes a queue and its associated worker

Not valid kernel-doc.    I'll assume all other instances will get fixed
and not bother talking about them.

> + *
> + * Each protocol has its own dedicated events_queue descriptor.
> + *
> + * @sz: Size in bytes of the related kfifo
> + * @kfifo: A dedicated Kernel kfifo
> + * @notify_work: A custom work item bound to this queue
> + * @wq: A reference to the related workqueue
> + */
> +struct events_queue {
> +	size_t			sz;
> +	struct kfifo		kfifo;
> +	struct work_struct	notify_work;
> +	struct workqueue_struct	*wq;
> +};
> +
> +/**
> + * scmi_registered_protocol_events_desc  - Protocol Specific information
> + *
> + * All protocols that registers at least an event have their protocol-specific
> + * information stored here, together with a pointer to the allocated
> + * events_queue.
> + *
> + * @id: Protocol ID
> + * @ops: Protocol specific and event-related operations
> + * @equeue: A reference to the associated per-protocol events_queue
> + */
> +struct scmi_registered_protocol_events_desc {
> +	u8					id;
> +	const struct scmi_protocol_event_ops	*ops;
> +	struct events_queue			*equeue;
> +};
> +
> +/**
> + * scmi_registered_event  - Event Specific Information
> + *
> + * All registered events are represented by one of these structures.
> + *
> + * @proto: A reference to the associated protocol descriptor
> + * @evt: A reference to the associated event descriptor (as provided at
> + *       registration time)
> + * @scratch_isr: A pre-allocated buffer to be used as a scratch area by ISR
> + * @scratch_isr: A pre-allocated buffer to be used as a scratch area by the
> + *		 deferred worker
scratch_bh.

> + * @report: A pre-allocated buffer used by the deferred workr to fill a

worker

> + *	    customized event report
> + */
> +struct scmi_registered_event {
> +	struct scmi_registered_protocol_events_desc	*proto;
> +	const struct scmi_event				*evt;
> +	void						*scratch_isr;
> +	void						*scratch_bh;
> +	void						*report;
> +};
> +
> +/**
> + * scmi_event_handler  - Event handler information
> + *
> + * This structure collects all the information needed to process a received
> + * event, calling all the registered notifier callbacks.
> + * Note this an event handler is associated to a tuple of values like:
> + *	(proto_id, evt_id, src_id)
> + * through the means of a key and an IDR mapping.
> + *
> + * @evt_key: The unsigned 32bit key associated to this descriptor in the
> + *	     related IDR mapping
> + * @r_evt: A reference to the underlying registered event
> + * @chain: The notification chain dedicated to this specific event tuple
> + */
> +struct scmi_event_handler {
> +	u32				evt_key;
> +	bool				enabled;
> +	refcount_t			users;
> +	struct scmi_registered_event	*r_evt;
> +	struct blocking_notifier_head	chain;
> +};
> +
> +/**
> + * scmi_event_header  - A utility header

That's not a good description.

> + *
> + * This header is prepended to each received event message payload before
> + * being queued on the related events_queue, and describes the attached event
> + * message.
> + *
> + * @timestamp: The timestamp, innanoseconds (boottime), which was associated
> + *	       to this event as soon as it entered the SCMI RX ISR
> + * @proto_id: Protocol ID
> + * @evt_id: Event ID (corresponds to the Event MsgID for this Protocol)
> + * @payld_sz: Effective size of the attached message payload which follows
> + * @payld: A reference to the included Event payload
> + */
> +struct scmi_event_header {
> +	u64	timestamp;
> +	u8	proto_id;
> +	u8	evt_id;
> +	size_t	payld_sz;
> +	u8	payld[];
> +} __packed;
> +
> +/*
> + * A few IDR maps to track:
> + *
> + * - @scmi_registered_events: All event's descriptors registered by the
> + *			      protocols, together with their ancillary data
> + * - @scmi_registered_events_handlers: All event's handlers descriptors, created
> + *				       to collect all the users' notifier_block
> + *				       callbacks and related notification chains
> + * - @scmi_registered_protocols: All protocol-level specific information related
> + *				 to events' handling
> + */
> +static struct idr scmi_registered_events;
> +static struct idr scmi_registered_events_handlers;
> +static DEFINE_MUTEX(scmi_registered_events_handler_mtx);
> +static struct idr scmi_registered_protocols;
> +
> +/* Common Kernel cmwq workqueue used by notifications core */
> +static struct workqueue_struct *scmi_notify_wq;
> +
> +static bool scmi_notifications_initialized;
> +
> +static struct scmi_event_handler *scmi_get_event_handler(u32 evt_key);
> +static void scmi_put_event_handler(struct scmi_event_handler *hndl);
> +
> +/**
> + * scmi_discard_bad_evt_payloadi() - Discard data from a kfifo

function name is wrong.

> + *
> + * @kq: The kfifo to act on
> + * @count: Number of bytes to flush
> + */
> +static inline void scmi_discard_bad_evt_payload(struct kfifo *kq,
> +						const unsigned int count)
> +{
> +	int i = 0;
> +
> +	pr_warn("SCMI Notification WQ :: skipping bad EVT Payload - %d bytes\n",
> +		count);
> +	/* Discard stale pending queued payload. */
> +	for (i = 0; i < count; i++)
> +		kfifo_skip(kq);
> +}
> +
> +/**
> + * scmi_lookup_and_call_event_chain  - Lookup the proper chain and call it
> + *
> + * @evt_key: The event key to use to lookup the related notification chain
> + * @report: The customized event-specific report to pass down to the callbacks
> + *	    as their *data parameter.
> + */
> +static inline void scmi_lookup_and_call_event_chain(u32 evt_key, void *report)
> +{
> +	int ret;
> +	struct scmi_event_handler *hndl;
> +
> +	hndl = scmi_get_event_handler(evt_key);
> +	if (IS_ERR_OR_NULL(hndl))
> +		return;
> +
> +	ret = blocking_notifier_call_chain(&hndl->chain,
> +					   KEY_XTRACT_EVT_ID(evt_key),
> +					   report);
> +	/* Notifiers are NOT supposed to cut the chain */
> +	WARN_ON_ONCE(ret & NOTIFY_STOP_MASK);
> +
> +	scmi_put_event_handler(hndl);
> +}
> +
> +/**
> + * scmi_events_dispatcher  - Common worker logic for all work items.
> + *
> + * In turn:
> + *  1. dequeue one pending RX notification (queued in SCMI RX ISR context)
> + *  2. generate a custom event report from the received event message
> + *  3. lookup for any registered ALL_SRC_IDs handler
> + *     - > call the related notification chain passing in the report
> + *  4. lookup for any registered specific SRC_ID handler
> + *     - > call the related notification chain passing in the report
> + *
> + * Note that:
> + * - a dedicated per-protocol kfifo queue is used: in this way an anomalous
> + *   flood of events cannot saturate other protocols' queues.
> + *
> + * - each per-protocol queue is associated to a distinct work_item, which
> + *   means, in turn, that:
> + *   + all protocol can process their dedicated queues concurrently
> + *     (since scmi_notify_wq:max_active > 1)
> + *   + anyway at most one worker instance is allowed to run on the same queue
> + *     concurrently: this ensures that we can have only one concurrent
> + *     reader/writer on the associated kfifo (needed for lock-less access)
> + *
> + * @work: The work item to use, which is associated to the proper events_queue
> + */
> +static void scmi_events_dispatcher(struct work_struct *work)
> +{
> +	struct events_queue *equeue;
> +	struct scmi_event_header eh;
> +
> +	equeue = container_of(work, struct events_queue, notify_work);
> +	while (kfifo_out(&equeue->kfifo, &eh, sizeof(eh))) {
> +		u32 src_id, evt_key;
> +		unsigned int outs;
> +		struct scmi_registered_event *r_evt;
> +		void *report = NULL;
> +
> +		evt_key = MAKE_EVT_KEY(eh.proto_id, eh.evt_id, NULL);
> +		r_evt = idr_find(&scmi_registered_events, evt_key);
> +		if (!r_evt) {
> +			scmi_discard_bad_evt_payload(&equeue->kfifo,
> +						     eh.payld_sz);
> +			continue;
> +		}
> +
> +		outs = kfifo_out(&equeue->kfifo, r_evt->scratch_bh,
> +				 eh.payld_sz);
> +		if (outs != eh.payld_sz) {
> +			pr_warn("SCMI Notification WQ :: SKIP corrupted EVT Payload.\n");
> +			continue;
> +		}
> +
> +		/* Reset and fill custom report */
> +		memset(r_evt->report, 0x00, r_evt->evt->max_report_sz);
> +		report = r_evt->proto->ops->fill_custom_report(eh.evt_id,
> +							eh.timestamp,
> +							r_evt->scratch_bh,
> +							eh.payld_sz,
> +							r_evt->report,
> +							&src_id);
> +		if (!report)
> +			continue;
> +
> +		/* At first search for a generic ALL src_ids handler... */
> +		scmi_lookup_and_call_event_chain(evt_key, report);
> +		if (src_id != SCMI_ALL_SRC_IDS) {
> +			u32 *__sub_p = &src_id;
> +
> +			evt_key = MAKE_EVT_KEY(eh.proto_id, eh.evt_id, __sub_p);
> +			scmi_lookup_and_call_event_chain(evt_key, report);
> +		}
> +	}
> +}
> +
> +/**
> + * scmi_notify  - Queues a notification for further deferred processing
> + *
> + * This is called in interrupt context to queue a received event for
> + * deferred processing.
> + *
> + * @proto_id: Protocol ID
> + * @evt_id: Event ID (msgID)
> + * @buf: Event Message Payload (without the header)
> + * @len: Event Message Payload size
> + * @ts: RX Timestamp in nanoseconds (boottime)
> + *
> + * Return: 0 on Success
> + */
> +int scmi_notify(u8 proto_id, u8 evt_id, const void *buf, size_t len, u64 ts)
> +{
> +	struct scmi_registered_event *r_evt;
> +	struct scmi_event_header *eh;
> +
> +	if (unlikely(!scmi_notifications_initialized))
> +		return 0;
> +
> +	r_evt = idr_find(&scmi_registered_events,
> +			 MAKE_EVT_KEY(proto_id, evt_id, NULL));
> +	if (unlikely(!r_evt || !r_evt->proto->equeue))
> +		return -EINVAL;
> +
> +	if (unlikely(len > r_evt->evt->max_payld_sz)) {
> +		pr_err("SCMI Notification discard badly sized message\n");
> +		return -EINVAL;
> +	}
> +	if (unlikely(kfifo_avail(&r_evt->proto->equeue->kfifo) <
> +		     sizeof(*eh) + len)) {
> +		pr_warn("SCMI Notification queue full: dropping proto_id:%d  evt_id:%d  ts:%lld\n",
> +			proto_id, evt_id, ts);
> +		return -ENOMEM;
> +	}
> +
> +	/*
> +	 * Note that we can use the per protocol kfifo in a lock-less manner
> +	 * since we have only one concurrent reader and writer but, in order
> +	 * to avoid any trouble on the reader side, here we perform one single
> +	 * write, so we have to collate event header and payload in a scratch
> +	 * area at first.
> +	 */
> +	eh = r_evt->scratch_isr;
> +	eh->timestamp = ts;
> +	eh->proto_id = proto_id;
> +	eh->evt_id = evt_id;
> +	eh->payld_sz = len;
> +	memcpy(eh->payld, buf, eh->payld_sz);
> +	kfifo_in(&r_evt->proto->equeue->kfifo, eh, sizeof(*eh) + eh->payld_sz);
> +	queue_work(r_evt->proto->equeue->wq,
> +		   &r_evt->proto->equeue->notify_work);
> +
> +	return 0;
> +}
> +
> +/**
> + * scmi_allocate_events_queue  - Allocate an events_queue descriptor
> + *
> + * @sz: Size of the kfifo to initialize
> + *
> + * Return: A valid pointer to the allocated events_queue on Success
> + */
> +static struct events_queue *scmi_allocate_events_queue(size_t sz)
> +{
> +	int ret;
> +	struct events_queue *equeue;
> +
> +	equeue = kzalloc(sizeof(*equeue), GFP_KERNEL);
> +	if (!equeue)
> +		return ERR_PTR(-ENOMEM);
> +	ret = kfifo_alloc(&equeue->kfifo, sz, GFP_KERNEL);
> +	if (ret) {
> +		kfree(equeue);
> +		return ERR_PTR(ret);
> +	}
> +	equeue->sz = sz;
> +	INIT_WORK(&equeue->notify_work, scmi_events_dispatcher);
> +	equeue->wq = scmi_notify_wq;
> +
> +	return equeue;
> +}
> +
> +/**
> + * scmi_deallocate_events_queue  - Deallocate am events_queue descriptor
> + *
> + * @equeue: The events_queue to free
> + */
> +static void scmi_deallocate_events_queue(struct events_queue *equeue)
> +{
> +	kfifo_free(&equeue->kfifo);
> +	kfree(equeue);
> +}
> +
> +/**
> + * scmi_allocate_registered_protocol_desc  - Allocate a registered protocol
> + * events' descriptor
> + *
> + * Used to keep protocol specific information related to events handling for any
> + * protocl which has registered at least one event.

protocol

> + *
> + * @proto_id: Protocol ID
> + * @queue_sz: Size of the associated queue to allocate
> + * @ops: Pointer to a struct holding references to protocol specific helpers
> + *	 needed during events handling
> + */
> +static struct scmi_registered_protocol_events_desc *
> +scmi_allocate_registered_protocol_desc(u8 proto_id, size_t queue_sz,
> +				const struct scmi_protocol_event_ops *ops)
> +{
> +	int ret;
> +	struct scmi_registered_protocol_events_desc *pdesc;
> +
> +	pdesc = idr_find(&scmi_registered_protocols, proto_id);

Perhaps add a comment here on why this might get called multiple times
but is still safe from point of view of release path.

> +	if (pdesc)
> +		return pdesc;


> +
> +	pdesc = kzalloc(sizeof(*pdesc), GFP_KERNEL);
> +	if (!pdesc)
> +		return ERR_PTR(-ENOMEM);
> +	pdesc->id = proto_id;
> +	pdesc->ops = ops;
> +
> +	pdesc->equeue = scmi_allocate_events_queue(queue_sz);
> +	if (IS_ERR(pdesc->equeue)) {
> +		kfree(pdesc);
> +		return ERR_CAST(pdesc->equeue);
> +	}
> +
> +	ret = idr_alloc(&scmi_registered_protocols, pdesc,
> +			pdesc->id, pdesc->id + 1, GFP_KERNEL);
> +	if (ret < 0) {
> +		pr_err("SCMI Failed to allocate PDESC IDR - key:%d - err:%d\n",
> +		       pdesc->id, ret);
> +		scmi_deallocate_events_queue(pdesc->equeue);
> +		kfree(pdesc);
> +		return ERR_PTR(ret);
> +	}
> +
> +	return pdesc;
> +}
> +
> +/**
> + * scmi_register_protocol_events  - Register Protocol Events with the core
> + *
> + * Used by SCMI Protocols initialization code to register with the notification
> + * core the list of supported events and their description: this takes care to
> + * pre-allocate all needed scratch buffers and link the proper event queue to
> + * this event.
> + *
> + * @proto_id: Protocol ID
> + * @queue_sz: Size in bytes of the associated queue to be allocated
> + * @ops: Protocol specific event-related operations
> + * @evt: Event descriptor array
> + * @num_events: Number of events in @evt array
> + *
> + * Return: 0 on Success
> + */
> +int scmi_register_protocol_events(u8 proto_id, size_t queue_sz,
> +				  const struct scmi_protocol_event_ops *ops,
> +				  const struct scmi_event *evt, int num_events)
> +{
> +	int i;
> +	struct scmi_registered_protocol_events_desc *pdesc;
> +
> +	if (!scmi_notifications_initialized)
> +		return -EAGAIN;
> +
> +	if (!ops || !evt)
> +		return -EINVAL;
> +
> +	pdesc = scmi_allocate_registered_protocol_desc(proto_id, queue_sz, ops);
> +	if (IS_ERR(pdesc))
> +		return -ENOMEM;
> +
> +	for (i = 0; i < num_events; i++, evt++) {
> +		int ret;
> +		u32 key;
> +		struct scmi_registered_event *r_evt;
> +
> +		r_evt = kzalloc(sizeof(*r_evt), GFP_KERNEL);
> +		if (!r_evt)
> +			continue;
> +		r_evt->proto = pdesc;
> +		r_evt->evt = evt;
> +
> +		r_evt->scratch_isr = kzalloc(sizeof(struct scmi_event_header) +
> +					     evt->max_payld_sz, GFP_KERNEL);
> +		if (!r_evt->scratch_isr)
> +			goto continue_isr_fail;
> +
> +		r_evt->scratch_bh = kzalloc(evt->max_payld_sz, GFP_KERNEL);
> +		if (!r_evt->scratch_bh)
> +			goto continue_bh_fail;
> +
> +		r_evt->report = kzalloc(evt->max_report_sz, GFP_KERNEL);
> +		if (!r_evt->report)
> +			goto continue_report_fail;
> +
> +		key = MAKE_EVT_KEY(r_evt->proto->id, evt->evt_id, NULL);
> +		ret = idr_alloc(&scmi_registered_events, r_evt,
> +				key, key + 1, GFP_KERNEL);
> +		if (ret < 0) {
> +			pr_err("SCMI Failed to allocate EVENT IDR - key:%X - err:%d\n",
> +			       key, ret);
> +			goto continue_idr_fail;
> +		}
> +
> +		pr_info("SCMI Notification registered event - key:%X\n", key);
> +		continue;
> +
> +	/* yes, this is not nice ... */
> +continue_idr_fail:
> +		kfree(r_evt->report);
> +continue_report_fail:
> +		kfree(r_evt->scratch_bh);
> +continue_bh_fail:
> +		kfree(r_evt->scratch_isr);
> +continue_isr_fail:
> +		kfree(r_evt);
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * scmi_register_event_handler  - Allocate an Event handler
> + *
> + * Allocate an event handler and related notification chain associated with
> + * the event identified by the provided event key. Fails if the associated
> + * event is unknown to the core (i.e. it had not been successfully registered
> + * upfront by some protocol)
> + *
> + * @evt_key: 32bit key uniquely bind to the event identified by the tuple
> + *	     (proto_id, evt_id, src_id)
> + *
> + * Return: the freshly allocated structure on Success
> + */
> +static struct scmi_event_handler *scmi_register_event_handler(u32 evt_key)
> +{
> +	int id;
> +	u8 proto_id, evt_id;
> +	struct scmi_registered_event *r_evt;
> +	struct scmi_event_handler *hndl;
> +
> +	proto_id = KEY_XTRACT_PROTO_ID(evt_key);
> +	evt_id = KEY_XTRACT_EVT_ID(evt_key);
> +	r_evt = idr_find(&scmi_registered_events,
> +			 MAKE_EVT_KEY(proto_id, evt_id, NULL));
> +	if (!r_evt)
> +		return ERR_PTR(-EINVAL);
> +
> +	hndl = kzalloc(sizeof(*hndl), GFP_KERNEL);
> +	if (!hndl)
> +		return ERR_PTR(-ENOMEM);
> +	hndl->r_evt = r_evt;
> +	hndl->evt_key = evt_key;
> +	BLOCKING_INIT_NOTIFIER_HEAD(&hndl->chain);
> +	refcount_set(&hndl->users, 1);
> +
> +	/* Register freshly allocated event handler */
> +	id = idr_alloc(&scmi_registered_events_handlers, hndl,
> +		       evt_key, evt_key + 1, GFP_KERNEL);

Umm. That's not using an idr for what it is meant for. You aren't allocating an id, you
are using it as a poor wrapper around an xarray.

> +	if (id < 0) {
> +		pr_err("SCMI Failed to allocate HNDL IDR - key:%X  err:%d\n",
> +		       evt_key, id);
> +		kfree(hndl);
> +		return ERR_PTR(id);
> +	}
> +
> +	return hndl;
> +}
> +
> +/**
> + * scmi_unregister_event_handler  - Free the provided Event handler
> + *
> + * @hndl: The event handler structure to free
> + */
> +static void scmi_unregister_event_handler(struct scmi_event_handler *hndl)
> +{
> +	idr_remove(&scmi_registered_events_handlers, hndl->evt_key);
> +	kfree(hndl);
> +}
> +
> +/**
> + * __scmi_event_handler_get_ops  - Get or create an event handler
> + *
> + * After having got exclusive access to the registered events map, searches for
> + * the desired handler matching the key: when found adjust refcount, when not
> + * create and register a new handler is asked to do so.
> + * Events generation is NOT enabled within this routine since at creation time
> + * we usually want to have all setup and registered before events starts
> + * flowing.
> + *
> + * @evt_key: The event key to use
> + * @create: A boolean flag to specify if a handler must be created
> + *	    when not found
> + */
> +static inline struct scmi_event_handler *
> +__scmi_event_handler_get_ops(u32 evt_key, bool create)
> +{
> +	struct scmi_event_handler *hndl = NULL;
> +
> +	mutex_lock(&scmi_registered_events_handler_mtx);
> +	hndl = idr_find(&scmi_registered_events_handlers, evt_key);
> +	if (hndl)
> +		refcount_inc(&hndl->users);
> +	else if (create)
> +		hndl = scmi_register_event_handler(evt_key);
> +	mutex_unlock(&scmi_registered_events_handler_mtx);
> +
> +	return hndl;
> +}
> +
> +static struct scmi_event_handler *scmi_get_event_handler(u32 evt_key)
> +{
> +	return __scmi_event_handler_get_ops(evt_key, false);
> +}
> +
> +static struct scmi_event_handler *scmi_get_or_create_event_handler(u32 evt_key)
> +{
> +	return __scmi_event_handler_get_ops(evt_key, true);
> +}
> +
> +/**
> + * __scmi_enable_events_ops  - Enable/disable events generation
> + *
> + * @hndl: The handler specifying the events enable/disable
> + * @action: The action to perform: true->Enable false->Disable
> + *
> + * Return: True when the required @action has been successfully executed
> + */
> +static inline bool __scmi_enable_events_ops(struct scmi_event_handler *hndl,
> +					    bool action)
> +{
> +	bool ret = true;
> +
> +	if (hndl->enabled != action) {
> +		u8 evt_id;
> +		u32 src_id;
> +
> +		evt_id = KEY_XTRACT_EVT_ID(hndl->evt_key);
> +		src_id = KEY_XTRACT_SRC_ID(hndl->evt_key);
> +		ret = hndl->r_evt->proto->ops->set_notify_enabled(evt_id,
> +			      src_id != SCMI_ALL_SRC_IDS ? &src_id : NULL,
> +			      action);
> +		if (ret)
> +			hndl->enabled = action;
> +	}
> +
> +	return ret;
> +}
> +
> +static bool scmi_enable_events(struct scmi_event_handler *hndl)
> +{
> +	return __scmi_enable_events_ops(hndl, true);
> +}
> +
> +static bool scmi_disable_events(struct scmi_event_handler *hndl)
> +{
> +	return __scmi_enable_events_ops(hndl, false);
> +}
> +
> +/**
> + * scmi_put_event_handler  - Put an event handler
> + *
> + * After having got exclusive access to the registered events map, update
> + * the refcount and if @hndl is no more in use by anyone:
> + *  - disable the events generation
> + *  - unregister and free the handler itself
> + *
> + * @hndl: The handler to act upon
> + */
> +static void scmi_put_event_handler(struct scmi_event_handler *hndl)
> +{
> +	mutex_lock(&scmi_registered_events_handler_mtx);
> +	if (refcount_dec_and_test(&hndl->users)) {
> +		scmi_disable_events(hndl);
> +		scmi_unregister_event_handler(hndl);
> +	}
> +	mutex_unlock(&scmi_registered_events_handler_mtx);
> +}
> +
> +/**
> + * scmi_register_event_notifier  - Register a notifier_block for an event
> + *
> + * Generic helper to register a notifier_block attached to a protocol event.
> + *
> + * A notifier_block @nb will be registered for each distinct event identified
> + * by the tuple (proto_id, evt_id, src_id) on a dedicated notification chain
> + * so that:
> + *
> + *	(proto_X, evt_Y, src_Z) --> chain_X_Y_Z
> + *
> + * @src_id meaning is protocol specific and identifies the origin of the event
> + * (like domain_id, sensor_id and os forth); @src_id can be NULL to signify that

so

> + * the caller is interested in receiving notifications from ALL the available
> + * sources for that protocol OR simply that the protocol does not support
> + * distinct sources: in these cases @nb will be attached to a generic
> + * notification chain defined for ALL src_id of that proto_id/evt_id pair like:
> + *
> + *	(proto_X, evt_Y, NULL) --> chain_X_Y_ALL
> + *
> + * Any received event will be then dispatched to both such chains if at least
> + * one user had registered an @nb on them.

both should be all I think.

> + *
> + * Note also that notification chains are created dynamically: a new chain is
> + * created each time a new distinct tuple is detected and then the provided @nb
> + * is bound to such chain; at creation time the related SCMI notifications are
> + * also enabled: this way, only when at least one user has registered its
> + * interest for a specific event tuple, the underlying notification chain is
> + * created and the related notifications are enabled in the platform.

That's one of the most complex bits of punctuation I've seenin a comment for
a long time.   Please break that sentence up for readability.

> + *
> + * @proto_id: Protocol ID
> + * @evt_id: Event ID
> + * @src_id: Source ID
> + * @nb: A standard notifier block to register for the specified event
> + *
> + * Return: Return 0 on Success
> + */
> +int scmi_register_event_notifier(u8 proto_id, u8 evt_id, u32 *src_id,
> +				 struct notifier_block *nb)
> +{
> +	u32 evt_key;
> +	struct scmi_event_handler *hndl;
> +
> +	evt_key = MAKE_EVT_KEY(proto_id, evt_id, src_id);
> +	hndl = scmi_get_or_create_event_handler(evt_key);
> +	if (IS_ERR_OR_NULL(hndl))
> +		return PTR_ERR(hndl);
> +
> +	blocking_notifier_chain_register(&hndl->chain, nb);
> +
> +	if (!scmi_enable_events(hndl)) {
> +		pr_err("SCMI Failed to ENABLE events for key:%X !\n", evt_key);
> +		blocking_notifier_chain_unregister(&hndl->chain, nb);
> +		scmi_put_event_handler(hndl);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * scmi_unregister_event_notifier  - Unregister a notifier_block for an event
> + *
> + * Takes care to unregister the provided @nb from the notification chain
> + * associated to the specified event and, if there are no more users for the
> + * event handler, frees also the associated event handler structures.
> + *
> + * @proto_id: Protocol ID
> + * @evt_id: Event ID
> + * @src_id: Source ID
> + * @nb: The notifier_block to unregister
> + *
> + * Return: 0 on Success
> + */
> +int scmi_unregister_event_notifier(u8 proto_id, u8 evt_id, u32 *src_id,
> +				   struct notifier_block *nb)
> +{
> +	u32 evt_key;
> +	struct scmi_event_handler *hndl;
> +
> +	evt_key = MAKE_EVT_KEY(proto_id, evt_id, src_id);
> +	hndl = scmi_get_event_handler(evt_key);
> +	if (IS_ERR_OR_NULL(hndl))
> +		return -EINVAL;
> +
> +	blocking_notifier_chain_unregister(&hndl->chain, nb);
> +
> +	scmi_put_event_handler(hndl);
> +	/*
> +	 * If this was the last user callback for this handler, this last put
> +	 * will force the handler to be freed.
> +	 * Note that if a call_chain walk is ongoing it will be the call_chain
> +	 * put request which will finally free the handler; note also that any
> +	 * operation on the inner notifier_block chain is protected on its own.
> +	 */
> +	scmi_put_event_handler(hndl);
> +
> +	return 0;
> +}
> +
> +/**
> + * scmi_notification_init  - Initializes Notification Core Support
> + *
> + * Return: 0 on Success
> + */
> +int __init scmi_notification_init(void)
> +{
> +	scmi_notify_wq = alloc_workqueue("scmi_notify",
> +					 WQ_UNBOUND | WQ_FREEZABLE,
> +					 SCMI_NOTIF_MAX_ACTIVE_QUEUES);
> +	if (!scmi_notify_wq)
> +		return -ENOMEM;
> +	idr_init(&scmi_registered_events);
> +	idr_init(&scmi_registered_events_handlers);
> +	idr_init(&scmi_registered_protocols);
> +
> +	scmi_notifications_initialized = true;
> +
> +	pr_info("SCMI Notifications enabled.\n");
> +
> +	return 0;
> +}
> +
> +/**
> + * scmi_notification_exit  - Shutdown and clean Notification core
> + */
> +void __exit scmi_notification_exit(void)
> +{
> +	int k;
> +	struct scmi_event_handler *hndl;
> +	struct scmi_registered_protocol_events_desc *pdesc;
> +	struct scmi_registered_event *r_evt;
> +
> +	scmi_notifications_initialized = false;
> +
> +	/* Disable notifications' generation */
> +	idr_for_each_entry(&scmi_registered_events_handlers, hndl, k)
> +		scmi_disable_events(hndl);
> +
> +	/* Let pending work complete */
> +	destroy_workqueue(scmi_notify_wq);
> +
> +	/* Remove ALL events handlers */
> +	idr_for_each_entry(&scmi_registered_events_handlers, hndl, k)
> +		scmi_put_event_handler(hndl);
> +	idr_destroy(&scmi_registered_events_handlers);
> +
> +	/* Finally remove all registered events */

Finally seems a bit premature here ;)

> +	idr_for_each_entry(&scmi_registered_events, r_evt, k) {
> +		idr_remove(&scmi_registered_events, k);
> +		kfree(r_evt->report);
> +		kfree(r_evt->scratch_isr);
> +		kfree(r_evt->scratch_bh);
> +		kfree(r_evt);
> +	}
> +	idr_destroy(&scmi_registered_events);
> +
> +	/* free any remaining protocol data */
> +	idr_for_each_entry(&scmi_registered_protocols, pdesc, k) {
> +		idr_remove(&scmi_registered_protocols, k);
> +		scmi_deallocate_events_queue(pdesc->equeue);
> +		kfree(pdesc);
> +	}
> +	idr_destroy(&scmi_registered_protocols);
> +}
> diff --git a/drivers/firmware/arm_scmi/notify.h b/drivers/firmware/arm_scmi/notify.h
> new file mode 100644
> index 000000000000..6fa07b244b14
> --- /dev/null
> +++ b/drivers/firmware/arm_scmi/notify.h
> @@ -0,0 +1,79 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * System Control and Management Interface (SCMI) Message Protocol
> + * notification header file containing some definitions, structures
> + * and function prototypes related to SCMI Notification handling.
> + *
> + * Copyright (C) 2019 ARM Ltd.
> + */
> +#ifndef _SCMI_NOTIFY_H
> +#define _SCMI_NOTIFY_H
> +
> +#include <linux/bug.h>
> +#include <linux/notifier.h>
> +#include <linux/types.h>
> +
> +#define	SCMI_ALL_SRC_IDS		0x0000ffffL
> +
> +#define MAP_EVT_TO_ENABLE_CMD(id, map)			\
> +({							\
> +	int ret = -1;					\
> +							\
> +	if (likely((id) < ARRAY_SIZE((map))))		\
> +		ret = (map)[(id)];			\
> +	else						\
> +		WARN(1, "UN-KNOWN evt_id:%d\n", (id));	\
> +	ret;						\
> +})
> +
> +/**
> + * scmi_event  - Describes an event to be supported

I don't think this is valid kernel-doc.  Make sure to point the scripts at the
file and check and fix any warnings.

> + *
> + * Each SCMI protocol, during its initialization phase, can describe the events
> + * it wishes to support in a few struct scmi_event and pass them to the core
> + * using scmi_register_protocol_events().
> + *
> + * @evt_id: Event ID
> + * @max_payld_sz: Max possible size for the payload of a notif msg of this kind
> + * @max_report_sz: Max possible size for the report of a notif msg of this kind
> + */
> +struct scmi_event {
> +	u8	evt_id;
> +	size_t	max_payld_sz;
> +	size_t	max_report_sz;
> +
> +};
> +
> +/**
> + * scmi_protocol_event_ops  - Helpers called by notification core.
> + *
> + * These are called only in process context.
> + *
> + * @set_notify_enabled: Enable/disable the required evt_id/src_id notifications
> + *			using the proper custom protocol commands.
> + *			Return true if at least one the required src_id
> + *			has been successfully enabled/disabled
> + * @fill_custom_report: fills a custom event report from the provided

Nitpick.  Inconsistent capitalization after the :

> + *			event message payld identifying the event
> + *			specific src_id.
> + *			Return NULL on failure otherwise @report now fully
> + *			populated
> + */
> +struct scmi_protocol_event_ops {
> +	bool (*set_notify_enabled)(u8 evt_id, const u32 *src_id, bool enabled);
> +	void *(*fill_custom_report)(u8 evt_id, u64 timestamp, const void *payld,
> +				    size_t payld_sz, void *report, u32 *src_id);
> +};
> +
> +int scmi_notification_init(void);
> +void scmi_notification_exit(void);
> +int scmi_register_protocol_events(u8 proto_id, size_t queue_sz,
> +				  const struct scmi_protocol_event_ops *ops,
> +				  const struct scmi_event *evt, int num_events);
> +int scmi_register_event_notifier(u8 proto_id, u8 evt_id, u32 *sub_id,
> +				 struct notifier_block *nb);
> +int scmi_unregister_event_notifier(u8 proto_id, u8 evt_id, u32 *sub_id,
> +				   struct notifier_block *nb);
> +int scmi_notify(u8 proto_id, u8 evt_id, const void *buf, size_t len, u64 ts);
> +
> +#endif /* _SCMI_NOTIFY_H */