Re: [RFC PATCH v3 11/12] firmware: arm_scmi: Add virtio transport

[Cristian Marussi]

Hi Peter,

On Tue, May 11, 2021 at 02:20:39AM +0200, Peter Hilber wrote:
> From: Igor Skalkin <igor.skalkin@xxxxxxxxxxxxxxx>
> 
> This transport enables accessing an SCMI platform as a virtio device.
> 
> Implement an SCMI virtio driver according to the virtio SCMI device spec
> [1]. Virtio device id 32 has been reserved for the SCMI device [2].
> 
> The virtio transport has one Tx channel (virtio cmdq, A2P channel) and
> at most one Rx channel (virtio eventq, P2A channel).
> 
> The following feature bit defined in [1] is not implemented:
> VIRTIO_SCMI_F_SHARED_MEMORY.
> 
> After the preparatory patches, this implements the virtio transport as
> paraphrased:
> 
> Only support a single arm-scmi device (which is consistent with the SCMI
> spec). scmi-virtio init is called from arm-scmi module init. During the
> arm-scmi probing, link to the first probed scmi-virtio device. Defer
> arm-scmi probing if no scmi-virtio device is bound yet.
> 
> For simplicity, restrict the number of messages which can be pending
> simultaneously according to the virtqueue capacity. (The virtqueue sizes
> are negotiated with the virtio device.)
> 
> As soon as Rx channel message buffers are allocated or have been read
> out by the arm-scmi driver, feed them to the virtio device.
> 
> Since some virtio devices may not have the short response time exhibited
> by SCMI platforms using other transports, set a generous response
> timeout.
> 
> Limitations:
> 
> - Polling is not supported.
> 
> - The timeout for delayed responses has not been adjusted.
> 

I want to give you some feedback/update while I'm reworking this series.

My main concerns currently arise from the logic of the interaction with
the core transport layer and the race resolution logic that you added in
the final patch, but I understand that, as said, this stems from the
current assumptions built into the core about transports.

So I tried to radically simplify this interactions modifying the core
accordingly and I now have something working (even though still a bit
dirty): so my plan would be in the next days to post an updated RFC V4 with
some core changes upfront, plus your V3 with patches 7,8,12 dropped and a
cumulative patch on virtio transport patch 11 (to better highlight the
differences) and get some feedback at first. (and tessing if you can :D)

I'm fine with the message abstractions introduced in patch 09.

I still have not addressed the probing sequence issues (as discussed),
nor included some work I'm doing on atomicity and polling of the transport
(and I've just seen Rob email about DT), so such RFC V4 will be anyway
somehow transitional, in order to share and get your feedback and see if
I missed something along the way.

Anyway, in a nutshell, I think that the main issues mentioned above derive
from the fact that the virtio transport is forced to cope with the assumption,
currently present in the core model, that a transport needs to handle msg
payloads and hdrs in two different steps, while virtio clearly is ready
to go as soon as the resp/notif/dresp msg are received from the vqueues
and this leads to the need of passing back and forth transport specific
ancillary data (like msg_handle) and complicates the handling and syncing
of the states of the inflight messages between the core and the virtio
transport.

So...I'm making fetch_response/notification optional to the core :D, such
that if a transport like virtio does not offer such ops, the new assumption
in the core is that, when scmi_rx_callback(cinfo, msg_hdr) is called,
a transport like virtio would have already completely filled up the xfer
and there's no need to call fetch_* since all is ready to go; in order to
do that, I added a couple of helpers in the core for the transport to be
able to grab, based only in msg_hdr, the proper pending xfer (for resp/
dresp since they are tied uniquely to the seq/token in the hdr matching the
original sent cmd) or have a freshly allocated xfer assigned (for notif):
this way the transport RX callback (like vio_complete) can now effectively
fillup completely the xfer and release the vqueue message before even
calling scmi_rx_callback. (and also I added a bit of additional sync logic
in and around xfer to avoid races against stale and possibly reused xfers.)

This seemed to me a bit less complicated and with fewer core/transport sync
issue sthan the actual solution so I gave it a try.

As of now it works for me in my minimal emulated setup for cmd/resp/dresp/
notif using sensors proto via scmi_iio/hwmon as a testbed (like it was
working indeed with your V3 original series, so this is conforting :D),
but I want to make more test on a real virtio backend and cleanup a bit
the series before posting an intermediate V4 RFC as said.

Thanks,
Cristian

> [1] https://github.com/oasis-tcs/virtio-spec/blob/master/virtio-scmi.tex
> [2] https://www.oasis-open.org/committees/ballot.php?id=3496
> 
> Signed-off-by: Igor Skalkin <igor.skalkin@xxxxxxxxxxxxxxx>
> [ Peter: Adapted patch for submission to upstream. ]
> Co-developed-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
> Signed-off-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
> ---
>  MAINTAINERS                        |   1 +
>  drivers/firmware/Kconfig           |  12 +
>  drivers/firmware/arm_scmi/Makefile |   1 +
>  drivers/firmware/arm_scmi/common.h |   3 +
>  drivers/firmware/arm_scmi/driver.c |   3 +
>  drivers/firmware/arm_scmi/virtio.c | 523 +++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_ids.h    |   1 +
>  include/uapi/linux/virtio_scmi.h   |  25 ++
>  8 files changed, 569 insertions(+)
>  create mode 100644 drivers/firmware/arm_scmi/virtio.c
>  create mode 100644 include/uapi/linux/virtio_scmi.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index bd7aff0c120f..449c336872f3 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -17705,6 +17705,7 @@ F:	drivers/regulator/scmi-regulator.c
>  F:	drivers/reset/reset-scmi.c
>  F:	include/linux/sc[mp]i_protocol.h
>  F:	include/trace/events/scmi.h
> +F:	include/uapi/linux/virtio_scmi.h
>  
>  SYSTEM RESET/SHUTDOWN DRIVERS
>  M:	Sebastian Reichel <sre@xxxxxxxxxx>
> diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
> index e8377b12e4d0..7e9eafdd9b63 100644
> --- a/drivers/firmware/Kconfig
> +++ b/drivers/firmware/Kconfig
> @@ -39,6 +39,18 @@ config ARM_SCMI_HAVE_MSG
>  	  This declares whether a message passing based transport for SCMI is
>  	  available.
>  
> +# This config option includes the dependencies of ARM_SCMI_PROTOCOL so that
> +# this config doesn't show up when SCMI wouldn't be available.
> +config VIRTIO_SCMI
> +	bool "Virtio transport for SCMI"
> +	select ARM_SCMI_HAVE_MSG
> +	depends on VIRTIO && (ARM || ARM64 || COMPILE_TEST)
> +	help
> +	  This enables the virtio based transport for SCMI.
> +
> +	  If you want to use the ARM SCMI protocol between the virtio guest and
> +	  a host providing a virtio SCMI device, answer Y.
> +
>  config ARM_SCMI_POWER_DOMAIN
>  	tristate "SCMI power domain driver"
>  	depends on ARM_SCMI_PROTOCOL || (COMPILE_TEST && OF)
> diff --git a/drivers/firmware/arm_scmi/Makefile b/drivers/firmware/arm_scmi/Makefile
> index f6b4acb8abdb..db1787606fb2 100644
> --- a/drivers/firmware/arm_scmi/Makefile
> +++ b/drivers/firmware/arm_scmi/Makefile
> @@ -5,6 +5,7 @@ scmi-transport-$(CONFIG_ARM_SCMI_HAVE_SHMEM) = shmem.o
>  scmi-transport-$(CONFIG_MAILBOX) += mailbox.o
>  scmi-transport-$(CONFIG_HAVE_ARM_SMCCC_DISCOVERY) += smc.o
>  scmi-transport-$(CONFIG_ARM_SCMI_HAVE_MSG) += msg.o
> +scmi-transport-$(CONFIG_VIRTIO_SCMI) += virtio.o
>  scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o system.o voltage.o
>  scmi-module-objs := $(scmi-bus-y) $(scmi-driver-y) $(scmi-protocols-y) \
>  		    $(scmi-transport-y)
> diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
> index 4cb6571c7aaf..bada06cfd33d 100644
> --- a/drivers/firmware/arm_scmi/common.h
> +++ b/drivers/firmware/arm_scmi/common.h
> @@ -349,6 +349,9 @@ extern const struct scmi_desc scmi_mailbox_desc;
>  #ifdef CONFIG_HAVE_ARM_SMCCC
>  extern const struct scmi_desc scmi_smc_desc;
>  #endif
> +#ifdef CONFIG_VIRTIO_SCMI
> +extern const struct scmi_desc scmi_virtio_desc;
> +#endif
>  
>  int scmi_set_transport_info(struct device *dev, void *transport_info);
>  void *scmi_get_transport_info(struct device *dev);
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index e04e7c8e6928..a31187385470 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -1637,6 +1637,9 @@ static const struct of_device_id scmi_of_match[] = {
>  #endif
>  #ifdef CONFIG_HAVE_ARM_SMCCC_DISCOVERY
>  	{ .compatible = "arm,scmi-smc", .data = &scmi_smc_desc},
> +#endif
> +#ifdef CONFIG_VIRTIO_SCMI
> +	{ .compatible = "arm,scmi-virtio", .data = &scmi_virtio_desc},
>  #endif
>  	{ /* Sentinel */ },
>  };
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> new file mode 100644
> index 000000000000..20972adf6dc7
> --- /dev/null
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -0,0 +1,523 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Virtio Transport driver for Arm System Control and Management Interface
> + * (SCMI).
> + *
> + * Copyright (C) 2020 OpenSynergy.
> + */
> +
> +/**
> + * DOC: Theory of Operation
> + *
> + * The scmi-virtio transport implements a driver for the virtio SCMI device.
> + *
> + * There is one Tx channel (virtio cmdq, A2P channel) and at most one Rx
> + * channel (virtio eventq, P2A channel). Each channel is implemented through a
> + * virtqueue. Access to each virtqueue is protected by spinlocks.
> + */
> +
> +#include <linux/errno.h>
> +#include <linux/of.h>
> +#include <linux/of_platform.h>
> +#include <linux/platform_device.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/virtio.h>
> +#include <linux/virtio_config.h>
> +#include <uapi/linux/virtio_ids.h>
> +#include <uapi/linux/virtio_scmi.h>
> +
> +#include "common.h"
> +
> +#define VIRTIO_SCMI_MAX_MSG_SIZE 128 /* Value may be increased. */
> +#define VIRTIO_SCMI_MAX_PDU_SIZE \
> +	(VIRTIO_SCMI_MAX_MSG_SIZE + SCMI_MSG_MAX_PROT_OVERHEAD)
> +#define DESCRIPTORS_PER_TX_MSG 2
> +
> +/**
> + * struct scmi_vio_channel - Transport channel information
> + *
> + * @lock: Protects access to all members except ready.
> + * @ready_lock: Protects access to ready. If required, it must be taken before
> + *              lock.
> + * @vqueue: Associated virtqueue
> + * @cinfo: SCMI Tx or Rx channel
> + * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
> + * @is_rx: Whether channel is an Rx channel
> + * @ready: Whether transport user is ready to hear about channel
> + */
> +struct scmi_vio_channel {
> +	spinlock_t lock;
> +	spinlock_t ready_lock;
> +	struct virtqueue *vqueue;
> +	struct scmi_chan_info *cinfo;
> +	struct list_head free_list;
> +	u8 is_rx;
> +	u8 ready;
> +};
> +
> +/**
> + * struct scmi_vio_msg - Transport PDU information
> + *
> + * @request: SDU used for commands
> + * @input: SDU used for (delayed) responses and notifications
> + * @list: List which scmi_vio_msg may be part of
> + * @rx_len: Input SDU size in bytes, once input has been received
> + */
> +struct scmi_vio_msg {
> +	struct scmi_msg_payld *request;
> +	struct scmi_msg_payld *input;
> +	struct list_head list;
> +	unsigned int rx_len;
> +};
> +
> +static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
> +{
> +	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
> +}
> +
> +static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
> +			       struct scmi_vio_msg *msg)
> +{
> +	struct scatterlist sg_in;
> +	int rc;
> +	unsigned long flags;
> +
> +	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
> +
> +	spin_lock_irqsave(&vioch->lock, flags);
> +
> +	rc = virtqueue_add_inbuf(vioch->vqueue, &sg_in, 1, msg, GFP_ATOMIC);
> +	if (rc)
> +		dev_err_once(vioch->cinfo->dev,
> +			     "%s() failed to add to virtqueue (%d)\n", __func__,
> +			     rc);
> +	else
> +		virtqueue_kick(vioch->vqueue);
> +
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	return rc;
> +}
> +
> +static void scmi_vio_complete_cb(struct virtqueue *vqueue)
> +{
> +	unsigned long ready_flags;
> +	unsigned long flags;
> +	unsigned int length;
> +	struct scmi_vio_channel *vioch;
> +	struct scmi_vio_msg *msg;
> +	bool cb_enabled = true;
> +
> +	if (WARN_ON_ONCE(!vqueue->vdev->priv))
> +		return;
> +	vioch = &((struct scmi_vio_channel *)vqueue->vdev->priv)[vqueue->index];
> +
> +	for (;;) {
> +		spin_lock_irqsave(&vioch->ready_lock, ready_flags);
> +
> +		if (!vioch->ready) {
> +			if (!cb_enabled)
> +				(void)virtqueue_enable_cb(vqueue);
> +			goto unlock_ready_out;
> +		}
> +
> +		spin_lock_irqsave(&vioch->lock, flags);
> +		if (cb_enabled) {
> +			virtqueue_disable_cb(vqueue);
> +			cb_enabled = false;
> +		}
> +		msg = virtqueue_get_buf(vqueue, &length);
> +		if (!msg) {
> +			if (virtqueue_enable_cb(vqueue))
> +				goto unlock_out;
> +			else
> +				cb_enabled = true;
> +		}
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +		if (msg) {
> +			msg->rx_len = length;
> +
> +			/*
> +			 * Hold the ready_lock during the callback to avoid
> +			 * races when the arm-scmi driver is unbinding while
> +			 * the virtio device is not quiesced yet.
> +			 */
> +			scmi_rx_callback(vioch->cinfo,
> +					 msg_read_header(msg->input), msg);
> +		}
> +		spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
> +	}
> +
> +unlock_out:
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +unlock_ready_out:
> +	spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
> +}
> +
> +static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
> +
> +static vq_callback_t *scmi_vio_complete_callbacks[] = {
> +	scmi_vio_complete_cb,
> +	scmi_vio_complete_cb
> +};
> +
> +static unsigned int virtio_get_max_msg(bool tx,
> +				       struct scmi_chan_info *base_cinfo)
> +{
> +	struct scmi_vio_channel *vioch = base_cinfo->transport_info;
> +	unsigned int ret;
> +
> +	ret = virtqueue_get_vring_size(vioch->vqueue);
> +
> +	/* Tx messages need multiple descriptors. */
> +	if (tx)
> +		ret /= DESCRIPTORS_PER_TX_MSG;
> +
> +	if (ret > MSG_TOKEN_MAX) {
> +		dev_info_once(
> +			base_cinfo->dev,
> +			"Only %ld messages can be pending simultaneously, while the %s virtqueue could hold %d\n",
> +			MSG_TOKEN_MAX, tx ? "tx" : "rx", ret);
> +		ret = MSG_TOKEN_MAX;
> +	}
> +
> +	return ret;
> +}
> +
> +static int scmi_vio_match_any_dev(struct device *dev, const void *data)
> +{
> +	return 1;
> +}
> +
> +static struct virtio_driver virtio_scmi_driver; /* Forward declaration */
> +
> +static int virtio_link_supplier(struct device *dev)
> +{
> +	struct device *vdev = driver_find_device(
> +		&virtio_scmi_driver.driver, NULL, NULL, scmi_vio_match_any_dev);
> +
> +	if (!vdev) {
> +		dev_notice_once(
> +			dev,
> +			"Deferring probe after not finding a bound scmi-virtio device\n");
> +		return -EPROBE_DEFER;
> +	}
> +
> +	/* Add device link for remove order and sysfs link. */
> +	if (!device_link_add(dev, vdev, DL_FLAG_AUTOREMOVE_CONSUMER)) {
> +		put_device(vdev);
> +		dev_err(dev, "Adding link to supplier virtio device failed\n");
> +		return -ECANCELED;
> +	}
> +
> +	put_device(vdev);
> +	return scmi_set_transport_info(dev, dev_to_virtio(vdev));
> +}
> +
> +static bool virtio_chan_available(struct device *dev, int idx)
> +{
> +	struct virtio_device *vdev;
> +
> +	/* scmi-virtio doesn't support per-protocol channels */
> +	if (is_scmi_protocol_device(dev))
> +		return false;
> +
> +	vdev = scmi_get_transport_info(dev);
> +	if (!vdev)
> +		return false;
> +
> +	switch (idx) {
> +	case VIRTIO_SCMI_VQ_TX:
> +		return true;
> +	case VIRTIO_SCMI_VQ_RX:
> +		return scmi_vio_have_vq_rx(vdev);
> +	default:
> +		return false;
> +	}
> +}
> +
> +static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
> +			     bool tx)
> +{
> +	unsigned long flags;
> +	struct virtio_device *vdev;
> +	struct scmi_vio_channel *vioch;
> +	int index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
> +	int max_msg;
> +	int i;
> +
> +	if (!virtio_chan_available(dev, index))
> +		return -ENODEV;
> +
> +	vdev = scmi_get_transport_info(dev);
> +	vioch = &((struct scmi_vio_channel *)vdev->priv)[index];
> +
> +	spin_lock_irqsave(&vioch->lock, flags);
> +	cinfo->transport_info = vioch;
> +	vioch->cinfo = cinfo;
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	max_msg = virtio_get_max_msg(tx, cinfo);
> +
> +	for (i = 0; i < max_msg; i++) {
> +		struct scmi_vio_msg *msg;
> +
> +		msg = devm_kzalloc(cinfo->dev, sizeof(*msg), GFP_KERNEL);
> +		if (!msg)
> +			return -ENOMEM;
> +
> +		if (tx) {
> +			msg->request = devm_kzalloc(cinfo->dev,
> +						    VIRTIO_SCMI_MAX_PDU_SIZE,
> +						    GFP_KERNEL);
> +			if (!msg->request)
> +				return -ENOMEM;
> +		}
> +
> +		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
> +					  GFP_KERNEL);
> +		if (!msg->input)
> +			return -ENOMEM;
> +
> +		if (tx) {
> +			spin_lock_irqsave(&vioch->lock, flags);
> +			list_add_tail(&msg->list, &vioch->free_list);
> +			spin_unlock_irqrestore(&vioch->lock, flags);
> +		} else {
> +			scmi_vio_feed_vq_rx(vioch, msg);
> +		}
> +	}
> +
> +	spin_lock_irqsave(&vioch->ready_lock, flags);
> +	vioch->ready = true;
> +	spin_unlock_irqrestore(&vioch->ready_lock, flags);
> +
> +	return 0;
> +}
> +
> +static int virtio_chan_free(int id, void *p, void *data)
> +{
> +	unsigned long flags;
> +	struct scmi_chan_info *cinfo = p;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +
> +	spin_lock_irqsave(&vioch->ready_lock, flags);
> +	vioch->ready = false;
> +	spin_unlock_irqrestore(&vioch->ready_lock, flags);
> +
> +	scmi_free_channel(cinfo, data, id);
> +	return 0;
> +}
> +
> +static int virtio_send_message(struct scmi_chan_info *cinfo,
> +			       struct scmi_xfer *xfer)
> +{
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +	struct scatterlist sg_out;
> +	struct scatterlist sg_in;
> +	struct scatterlist *sgs[DESCRIPTORS_PER_TX_MSG] = { &sg_out, &sg_in };
> +	unsigned long flags;
> +	int rc;
> +	struct scmi_vio_msg *msg;
> +
> +	/*
> +	 * TODO: For now, we don't support polling. But it should not be
> +	 * difficult to add support.
> +	 */
> +	if (xfer->hdr.poll_completion)
> +		return -EINVAL;
> +
> +	spin_lock_irqsave(&vioch->lock, flags);
> +
> +	if (list_empty(&vioch->free_list)) {
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +		return -EBUSY;
> +	}
> +
> +	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> +	list_del(&msg->list);
> +
> +	msg_tx_prepare(msg->request, xfer);
> +
> +	sg_init_one(&sg_out, msg->request, msg_command_size(xfer));
> +	sg_init_one(&sg_in, msg->input, msg_response_size(xfer));
> +
> +	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
> +	if (rc) {
> +		list_add(&msg->list, &vioch->free_list);
> +		dev_err_once(vioch->cinfo->dev,
> +			     "%s() failed to add to virtqueue (%d)\n", __func__,
> +			     rc);
> +	} else {
> +		virtqueue_kick(vioch->vqueue);
> +	}
> +
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	return rc;
> +}
> +
> +static void virtio_fetch_response(struct scmi_chan_info *cinfo,
> +				  struct scmi_xfer *xfer, void *msg_handle)
> +{
> +	struct scmi_vio_msg *msg = msg_handle;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +
> +	if (!msg) {
> +		dev_dbg_once(&vioch->vqueue->vdev->dev,
> +			     "Ignoring %s() call with NULL msg_handle\n",
> +			     __func__);
> +		return;
> +	}
> +
> +	msg_fetch_response(msg->input, msg->rx_len, xfer);
> +}
> +
> +static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
> +				      size_t max_len, struct scmi_xfer *xfer,
> +				      void *msg_handle)
> +{
> +	struct scmi_vio_msg *msg = msg_handle;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +
> +	if (!msg) {
> +		dev_dbg_once(&vioch->vqueue->vdev->dev,
> +			     "Ignoring %s() call with NULL msg_handle\n",
> +			     __func__);
> +		return;
> +	}
> +
> +	msg_fetch_notification(msg->input, msg->rx_len, max_len, xfer);
> +}
> +
> +static void dummy_clear_channel(struct scmi_chan_info *cinfo)
> +{
> +}
> +
> +static bool dummy_poll_done(struct scmi_chan_info *cinfo,
> +			    struct scmi_xfer *xfer)
> +{
> +	return false;
> +}
> +
> +static void virtio_drop_message(struct scmi_chan_info *cinfo, void *msg_handle)
> +{
> +	unsigned long flags;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +	struct scmi_vio_msg *msg = msg_handle;
> +
> +	if (!msg) {
> +		dev_dbg_once(&vioch->vqueue->vdev->dev,
> +			     "Ignoring %s() call with NULL msg_handle\n",
> +			     __func__);
> +		return;
> +	}
> +
> +	if (vioch->is_rx) {
> +		scmi_vio_feed_vq_rx(vioch, msg);
> +	} else {
> +		spin_lock_irqsave(&vioch->lock, flags);
> +		list_add(&msg->list, &vioch->free_list);
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +	}
> +}
> +
> +static const struct scmi_transport_ops scmi_virtio_ops = {
> +	.link_supplier = virtio_link_supplier,
> +	.chan_available = virtio_chan_available,
> +	.chan_setup = virtio_chan_setup,
> +	.chan_free = virtio_chan_free,
> +	.get_max_msg = virtio_get_max_msg,
> +	.send_message = virtio_send_message,
> +	.fetch_response = virtio_fetch_response,
> +	.fetch_notification = virtio_fetch_notification,
> +	.clear_channel = dummy_clear_channel,
> +	.poll_done = dummy_poll_done,
> +	.drop_message = virtio_drop_message,
> +};
> +
> +static int scmi_vio_probe(struct virtio_device *vdev)
> +{
> +	struct device *dev = &vdev->dev;
> +	struct scmi_vio_channel *channels;
> +	bool have_vq_rx;
> +	int vq_cnt;
> +	int i;
> +	int ret;
> +	struct virtqueue *vqs[VIRTIO_SCMI_VQ_MAX_CNT];
> +
> +	have_vq_rx = scmi_vio_have_vq_rx(vdev);
> +	vq_cnt = have_vq_rx ? VIRTIO_SCMI_VQ_MAX_CNT : 1;
> +
> +	channels = devm_kcalloc(dev, vq_cnt, sizeof(*channels), GFP_KERNEL);
> +	if (!channels)
> +		return -ENOMEM;
> +
> +	if (have_vq_rx)
> +		channels[VIRTIO_SCMI_VQ_RX].is_rx = true;
> +
> +	ret = virtio_find_vqs(vdev, vq_cnt, vqs, scmi_vio_complete_callbacks,
> +			      scmi_vio_vqueue_names, NULL);
> +	if (ret) {
> +		dev_err(dev, "Failed to get %d virtqueue(s)\n", vq_cnt);
> +		return ret;
> +	}
> +	dev_info(dev, "Found %d virtqueue(s)\n", vq_cnt);
> +
> +	for (i = 0; i < vq_cnt; i++) {
> +		spin_lock_init(&channels[i].lock);
> +		spin_lock_init(&channels[i].ready_lock);
> +		INIT_LIST_HEAD(&channels[i].free_list);
> +		channels[i].vqueue = vqs[i];
> +	}
> +
> +	vdev->priv = channels;
> +
> +	return 0;
> +}
> +
> +static void scmi_vio_remove(struct virtio_device *vdev)
> +{
> +	vdev->config->reset(vdev);
> +	vdev->config->del_vqs(vdev);
> +}
> +
> +static unsigned int features[] = {
> +	VIRTIO_SCMI_F_P2A_CHANNELS,
> +};
> +
> +static const struct virtio_device_id id_table[] = {
> +	{ VIRTIO_ID_SCMI, VIRTIO_DEV_ANY_ID },
> +	{ 0 }
> +};
> +
> +static struct virtio_driver virtio_scmi_driver = {
> +	.driver.name = "scmi-virtio",
> +	.driver.owner = THIS_MODULE,
> +	.feature_table = features,
> +	.feature_table_size = ARRAY_SIZE(features),
> +	.id_table = id_table,
> +	.probe = scmi_vio_probe,
> +	.remove = scmi_vio_remove,
> +};
> +
> +static int __init virtio_scmi_init(void)
> +{
> +	return register_virtio_driver(&virtio_scmi_driver);
> +}
> +
> +static void __exit virtio_scmi_exit(void)
> +{
> +	unregister_virtio_driver(&virtio_scmi_driver);
> +}
> +
> +const struct scmi_desc scmi_virtio_desc = {
> +	.init = virtio_scmi_init,
> +	.exit = virtio_scmi_exit,
> +	.ops = &scmi_virtio_ops,
> +	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
> +	.max_msg = 0, /* overridden by virtio_get_max_msg() */
> +	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
> +};
> diff --git a/include/uapi/linux/virtio_ids.h b/include/uapi/linux/virtio_ids.h
> index f0c35ce8628c..c146fe30e589 100644
> --- a/include/uapi/linux/virtio_ids.h
> +++ b/include/uapi/linux/virtio_ids.h
> @@ -56,5 +56,6 @@
>  #define VIRTIO_ID_PMEM			27 /* virtio pmem */
>  #define VIRTIO_ID_BT			28 /* virtio bluetooth */
>  #define VIRTIO_ID_MAC80211_HWSIM	29 /* virtio mac80211-hwsim */
> +#define VIRTIO_ID_SCMI			32 /* virtio SCMI */
>  
>  #endif /* _LINUX_VIRTIO_IDS_H */
> diff --git a/include/uapi/linux/virtio_scmi.h b/include/uapi/linux/virtio_scmi.h
> new file mode 100644
> index 000000000000..732b01504c35
> --- /dev/null
> +++ b/include/uapi/linux/virtio_scmi.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) */
> +/*
> + * Copyright (C) 2020 OpenSynergy GmbH
> + */
> +
> +#ifndef _UAPI_LINUX_VIRTIO_SCMI_H
> +#define _UAPI_LINUX_VIRTIO_SCMI_H
> +
> +#include <linux/virtio_types.h>
> +
> +/* Feature bits */
> +
> +/* Device implements some SCMI notifications, or delayed responses. */
> +#define VIRTIO_SCMI_F_P2A_CHANNELS 0
> +
> +/* Device implements any SCMI statistics shared memory region */
> +#define VIRTIO_SCMI_F_SHARED_MEMORY 1
> +
> +/* Virtqueues */
> +
> +#define VIRTIO_SCMI_VQ_TX 0 /* cmdq */
> +#define VIRTIO_SCMI_VQ_RX 1 /* eventq */
> +#define VIRTIO_SCMI_VQ_MAX_CNT 2
> +
> +#endif /* _UAPI_LINUX_VIRTIO_SCMI_H */
> -- 
> 2.25.1
> 
>