Re: [PATCH v4 14/16] firmware: arm_scmi: Add virtio transport

[Cristian Marussi]

Hi,

On Mon, Jun 14, 2021 at 03:23:15PM +0100, Jonathan Cameron wrote:
> On Fri, 11 Jun 2021 17:59:35 +0100
> Cristian Marussi <cristian.marussi@xxxxxxx> 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.
> > 
> > [1] https://github.com/oasis-tcs/virtio-spec/blob/master/virtio-scmi.tex
> > [2] https://www.oasis-open.org/committees/ballot.php?id=3496
> > 
> > [ Peter: Adapted patch for submission to upstream. ]
> > 
> > Co-developed-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
> > Signed-off-by: Igor Skalkin <igor.skalkin@xxxxxxxxxxxxxxx>
> > Signed-off-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
> 
> A few drive by comments inline.
> 
> J
> > ---
> >  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 f408cf2d2781..e1b27ed11060 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -17878,6 +17878,7 @@ 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 e74d25065d07..2c37d5af66ad 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 fa4075336580..c143a449d278 100644
> > --- a/drivers/firmware/arm_scmi/common.h
> > +++ b/drivers/firmware/arm_scmi/common.h
> > @@ -390,6 +390,9 @@ extern const struct scmi_desc scmi_mailbox_desc;
> >  #ifdef CONFIG_HAVE_ARM_SMCCC_DISCOVERY
> >  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 bb2de15554a9..ccc7dd49261e 100644
> > --- a/drivers/firmware/arm_scmi/driver.c
> > +++ b/drivers/firmware/arm_scmi/driver.c
> > @@ -1954,6 +1954,9 @@ 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>
> Why either of these?
> 
> perhaps
> mod_devicetable.h is appropriate.
> 

These are clearly leftovers from a previous version where there were
some DT configs, now dropped since not acceptable nor needed.

> > +#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
> 
> Drop the else as it doesn't add readability as far as I can see.
> 

Sure.

> > +				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);
> 
> Blank line here would be nice + consistent.
> 

Right, I'll do.

> > +	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);
> 
> Rather noisy given I assume there are plenty of other ways to se this
> succeeded (i.e. probe succeeded)
> 

Yes, indeed, the only additional info that was meant to be carried here
I suppose is that when you find 2 virtqueues (max possible) you know
(deduct really) that you have the P2A channel, so also notifications and
delayed responses are supported.

I'll review in general the info msgs and drop to debug when not neeeded.
(or explicit the above I-have-notif_dresp information.

> > +
> > +	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);
> > +}
> 
> module_virtio_driver() ?
> 

No that's not possible at the moment, and indeed it is the whole
reason I introduced early in the series the transport specific
.init()/.exit() optional helpers.

The issue is that currently SCMI transports are in fact part of the SCMI
stack compilation unit; if you enable some transports those get compiled
in the main SCMI module, and if you then (hopefully) define a DT entry
describing an SCMI instance using a specific transport (setting compatible
"arm,scmi-virtio" in this case) such SCMI stack will use that transport and
its ops: problem is the SCMI main module can be compiled as loadable
module on its own, including all transports configured and all base SCMI
protos, so if you use here the standard module_virtio_driver() it won't
link in case you built the SCMI core as module and complains about

> multiple definition of `init_module'; 
> multiple definition of `cleanup_module';

The above .init()/.exit() instead are registered with the core as
optional transport helpers and invoked when the SCMI core is
initialized, so also well before the SCMI core is probed.

Making it a full fledged module instead (and so then using normal
module_virtio_driver() macros) that registers with the SCMI core somehow
after its probed by its own susbsystem (like virtIO), seemed to
complicate a lot the interactions between the core and the transports
(even though I'd like it much more as an approach) so I have not gone
through this road still....but I cannot exclude I'll come back to this
kind of approach when I'll try to simplify a bit the probing sequence of
this SCMI VirtIO driver...it's just that for now seemed easier to add
these optional transport helpers to let transport provide their early setup
stuff (if needed), like registering with another subsystem.
(upcoming SCMI OPTEE transport posted in a different series by Etienne
has the same needs)

Thanks,
Cristian