Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 941 insertions, 0 deletions

diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
new file mode 100644
index 000000000..1db975c08
--- /dev/null
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -0,0 +1,941 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Virtio Transport driver for Arm System Control and Management Interface
+ * (SCMI).
+ *
+ * Copyright (C) 2020-2022 OpenSynergy.
+ * Copyright (C) 2021-2022 ARM Ltd.
+ */
+
+/**
+ * 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/completion.h>
+#include <linux/errno.h>
+#include <linux/refcount.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_MAX_RX_TIMEOUT_MS	60000
+#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
+ *
+ * @vqueue: Associated virtqueue
+ * @cinfo: SCMI Tx or Rx channel
+ * @free_lock: Protects access to the @free_list.
+ * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
+ * @deferred_tx_work: Worker for TX deferred replies processing
+ * @deferred_tx_wq: Workqueue for TX deferred replies
+ * @pending_lock: Protects access to the @pending_cmds_list.
+ * @pending_cmds_list: List of pre-fetched commands queueud for later processing
+ * @is_rx: Whether channel is an Rx channel
+ * @max_msg: Maximum number of pending messages for this channel.
+ * @lock: Protects access to all members except users, free_list and
+ *	  pending_cmds_list.
+ * @shutdown_done: A reference to a completion used when freeing this channel.
+ * @users: A reference count to currently active users of this channel.
+ */
+struct scmi_vio_channel {
+	struct virtqueue *vqueue;
+	struct scmi_chan_info *cinfo;
+	/* lock to protect access to the free list. */
+	spinlock_t free_lock;
+	struct list_head free_list;
+	/* lock to protect access to the pending list. */
+	spinlock_t pending_lock;
+	struct list_head pending_cmds_list;
+	struct work_struct deferred_tx_work;
+	struct workqueue_struct *deferred_tx_wq;
+	bool is_rx;
+	unsigned int max_msg;
+	/*
+	 * Lock to protect access to all members except users, free_list and
+	 * pending_cmds_list
+	 */
+	spinlock_t lock;
+	struct completion *shutdown_done;
+	refcount_t users;
+};
+
+enum poll_states {
+	VIO_MSG_NOT_POLLED,
+	VIO_MSG_POLL_TIMEOUT,
+	VIO_MSG_POLLING,
+	VIO_MSG_POLL_DONE,
+};
+
+/**
+ * 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
+ * @poll_idx: Last used index registered for polling purposes if this message
+ *	      transaction reply was configured for polling.
+ * @poll_status: Polling state for this message.
+ * @poll_lock: A lock to protect @poll_status
+ * @users: A reference count to track this message users and avoid premature
+ *	   freeing (and reuse) when polling and IRQ execution paths interleave.
+ */
+struct scmi_vio_msg {
+	struct scmi_msg_payld *request;
+	struct scmi_msg_payld *input;
+	struct list_head list;
+	unsigned int rx_len;
+	unsigned int poll_idx;
+	enum poll_states poll_status;
+	/* Lock to protect access to poll_status */
+	spinlock_t poll_lock;
+	refcount_t users;
+};
+
+/* Only one SCMI VirtIO device can possibly exist */
+static struct virtio_device *scmi_vdev;
+
+static void scmi_vio_channel_ready(struct scmi_vio_channel *vioch,
+				   struct scmi_chan_info *cinfo)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	cinfo->transport_info = vioch;
+	/* Indirectly setting channel not available any more */
+	vioch->cinfo = cinfo;
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	refcount_set(&vioch->users, 1);
+}
+
+static inline bool scmi_vio_channel_acquire(struct scmi_vio_channel *vioch)
+{
+	return refcount_inc_not_zero(&vioch->users);
+}
+
+static inline void scmi_vio_channel_release(struct scmi_vio_channel *vioch)
+{
+	if (refcount_dec_and_test(&vioch->users)) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&vioch->lock, flags);
+		if (vioch->shutdown_done) {
+			vioch->cinfo = NULL;
+			complete(vioch->shutdown_done);
+		}
+		spin_unlock_irqrestore(&vioch->lock, flags);
+	}
+}
+
+static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
+{
+	unsigned long flags;
+	DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
+
+	/*
+	 * Prepare to wait for the last release if not already released
+	 * or in progress.
+	 */
+	spin_lock_irqsave(&vioch->lock, flags);
+	if (!vioch->cinfo || vioch->shutdown_done) {
+		spin_unlock_irqrestore(&vioch->lock, flags);
+		return;
+	}
+
+	vioch->shutdown_done = &vioch_shutdown_done;
+	if (!vioch->is_rx && vioch->deferred_tx_wq)
+		/* Cannot be kicked anymore after this...*/
+		vioch->deferred_tx_wq = NULL;
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	scmi_vio_channel_release(vioch);
+
+	/* Let any possibly concurrent RX path release the channel */
+	wait_for_completion(vioch->shutdown_done);
+}
+
+/* Assumes to be called with vio channel acquired already */
+static struct scmi_vio_msg *
+scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
+{
+	unsigned long flags;
+	struct scmi_vio_msg *msg;
+
+	spin_lock_irqsave(&vioch->free_lock, flags);
+	if (list_empty(&vioch->free_list)) {
+		spin_unlock_irqrestore(&vioch->free_lock, flags);
+		return NULL;
+	}
+
+	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
+	list_del_init(&msg->list);
+	spin_unlock_irqrestore(&vioch->free_lock, flags);
+
+	/* Still no users, no need to acquire poll_lock */
+	msg->poll_status = VIO_MSG_NOT_POLLED;
+	refcount_set(&msg->users, 1);
+
+	return msg;
+}
+
+static inline bool scmi_vio_msg_acquire(struct scmi_vio_msg *msg)
+{
+	return refcount_inc_not_zero(&msg->users);
+}
+
+/* Assumes to be called with vio channel acquired already */
+static inline bool scmi_vio_msg_release(struct scmi_vio_channel *vioch,
+					struct scmi_vio_msg *msg)
+{
+	bool ret;
+
+	ret = refcount_dec_and_test(&msg->users);
+	if (ret) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&vioch->free_lock, flags);
+		list_add_tail(&msg->list, &vioch->free_list);
+		spin_unlock_irqrestore(&vioch->free_lock, flags);
+	}
+
+	return ret;
+}
+
+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;
+	struct device *dev = &vioch->vqueue->vdev->dev;
+
+	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(dev, "failed to add to RX virtqueue (%d)\n", rc);
+	else
+		virtqueue_kick(vioch->vqueue);
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	return rc;
+}
+
+/*
+ * Assume to be called with channel already acquired or not ready at all;
+ * vioch->lock MUST NOT have been already acquired.
+ */
+static void scmi_finalize_message(struct scmi_vio_channel *vioch,
+				  struct scmi_vio_msg *msg)
+{
+	if (vioch->is_rx)
+		scmi_vio_feed_vq_rx(vioch, msg);
+	else
+		scmi_vio_msg_release(vioch, msg);
+}
+
+static void scmi_vio_complete_cb(struct virtqueue *vqueue)
+{
+	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 (;;) {
+		if (!scmi_vio_channel_acquire(vioch))
+			return;
+
+		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)) {
+				spin_unlock_irqrestore(&vioch->lock, flags);
+				scmi_vio_channel_release(vioch);
+				return;
+			}
+			cb_enabled = true;
+		}
+		spin_unlock_irqrestore(&vioch->lock, flags);
+
+		if (msg) {
+			msg->rx_len = length;
+			scmi_rx_callback(vioch->cinfo,
+					 msg_read_header(msg->input), msg);
+
+			scmi_finalize_message(vioch, msg);
+		}
+
+		/*
+		 * Release vio channel between loop iterations to allow
+		 * virtio_chan_free() to eventually fully release it when
+		 * shutting down; in such a case, any outstanding message will
+		 * be ignored since this loop will bail out at the next
+		 * iteration.
+		 */
+		scmi_vio_channel_release(vioch);
+	}
+}
+
+static void scmi_vio_deferred_tx_worker(struct work_struct *work)
+{
+	unsigned long flags;
+	struct scmi_vio_channel *vioch;
+	struct scmi_vio_msg *msg, *tmp;
+
+	vioch = container_of(work, struct scmi_vio_channel, deferred_tx_work);
+
+	if (!scmi_vio_channel_acquire(vioch))
+		return;
+
+	/*
+	 * Process pre-fetched messages: these could be non-polled messages or
+	 * late timed-out replies to polled messages dequeued by chance while
+	 * polling for some other messages: this worker is in charge to process
+	 * the valid non-expired messages and anyway finally free all of them.
+	 */
+	spin_lock_irqsave(&vioch->pending_lock, flags);
+
+	/* Scan the list of possibly pre-fetched messages during polling. */
+	list_for_each_entry_safe(msg, tmp, &vioch->pending_cmds_list, list) {
+		list_del(&msg->list);
+
+		/*
+		 * Channel is acquired here (cannot vanish) and this message
+		 * is no more processed elsewhere so no poll_lock needed.
+		 */
+		if (msg->poll_status == VIO_MSG_NOT_POLLED)
+			scmi_rx_callback(vioch->cinfo,
+					 msg_read_header(msg->input), msg);
+
+		/* Free the processed message once done */
+		scmi_vio_msg_release(vioch, msg);
+	}
+
+	spin_unlock_irqrestore(&vioch->pending_lock, flags);
+
+	/* Process possibly still pending messages */
+	scmi_vio_complete_cb(vioch->vqueue);
+
+	scmi_vio_channel_release(vioch);
+}
+
+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(struct scmi_chan_info *base_cinfo)
+{
+	struct scmi_vio_channel *vioch = base_cinfo->transport_info;
+
+	return vioch->max_msg;
+}
+
+static int virtio_link_supplier(struct device *dev)
+{
+	if (!scmi_vdev) {
+		dev_notice(dev,
+			   "Deferring probe after not finding a bound scmi-virtio device\n");
+		return -EPROBE_DEFER;
+	}
+
+	if (!device_link_add(dev, &scmi_vdev->dev,
+			     DL_FLAG_AUTOREMOVE_CONSUMER)) {
+		dev_err(dev, "Adding link to supplier virtio device failed\n");
+		return -ECANCELED;
+	}
+
+	return 0;
+}
+
+static bool virtio_chan_available(struct device *dev, int idx)
+{
+	struct scmi_vio_channel *channels, *vioch = NULL;
+
+	if (WARN_ON_ONCE(!scmi_vdev))
+		return false;
+
+	channels = (struct scmi_vio_channel *)scmi_vdev->priv;
+
+	switch (idx) {
+	case VIRTIO_SCMI_VQ_TX:
+		vioch = &channels[VIRTIO_SCMI_VQ_TX];
+		break;
+	case VIRTIO_SCMI_VQ_RX:
+		if (scmi_vio_have_vq_rx(scmi_vdev))
+			vioch = &channels[VIRTIO_SCMI_VQ_RX];
+		break;
+	default:
+		return false;
+	}
+
+	return vioch && !vioch->cinfo;
+}
+
+static void scmi_destroy_tx_workqueue(void *deferred_tx_wq)
+{
+	destroy_workqueue(deferred_tx_wq);
+}
+
+static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
+			     bool tx)
+{
+	struct scmi_vio_channel *vioch;
+	int index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
+	int i;
+
+	if (!scmi_vdev)
+		return -EPROBE_DEFER;
+
+	vioch = &((struct scmi_vio_channel *)scmi_vdev->priv)[index];
+
+	/* Setup a deferred worker for polling. */
+	if (tx && !vioch->deferred_tx_wq) {
+		int ret;
+
+		vioch->deferred_tx_wq =
+			alloc_workqueue(dev_name(&scmi_vdev->dev),
+					WQ_UNBOUND | WQ_FREEZABLE | WQ_SYSFS,
+					0);
+		if (!vioch->deferred_tx_wq)
+			return -ENOMEM;
+
+		ret = devm_add_action_or_reset(dev, scmi_destroy_tx_workqueue,
+					       vioch->deferred_tx_wq);
+		if (ret)
+			return ret;
+
+		INIT_WORK(&vioch->deferred_tx_work,
+			  scmi_vio_deferred_tx_worker);
+	}
+
+	for (i = 0; i < vioch->max_msg; i++) {
+		struct scmi_vio_msg *msg;
+
+		msg = devm_kzalloc(dev, sizeof(*msg), GFP_KERNEL);
+		if (!msg)
+			return -ENOMEM;
+
+		if (tx) {
+			msg->request = devm_kzalloc(dev,
+						    VIRTIO_SCMI_MAX_PDU_SIZE,
+						    GFP_KERNEL);
+			if (!msg->request)
+				return -ENOMEM;
+			spin_lock_init(&msg->poll_lock);
+			refcount_set(&msg->users, 1);
+		}
+
+		msg->input = devm_kzalloc(dev, VIRTIO_SCMI_MAX_PDU_SIZE,
+					  GFP_KERNEL);
+		if (!msg->input)
+			return -ENOMEM;
+
+		scmi_finalize_message(vioch, msg);
+	}
+
+	scmi_vio_channel_ready(vioch, cinfo);
+
+	return 0;
+}
+
+static int virtio_chan_free(int id, void *p, void *data)
+{
+	struct scmi_chan_info *cinfo = p;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+	/*
+	 * Break device to inhibit further traffic flowing while shutting down
+	 * the channels: doing it later holding vioch->lock creates unsafe
+	 * locking dependency chains as reported by LOCKDEP.
+	 */
+	virtio_break_device(vioch->vqueue->vdev);
+	scmi_vio_channel_cleanup_sync(vioch);
+
+	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;
+
+	if (!scmi_vio_channel_acquire(vioch))
+		return -EINVAL;
+
+	msg = scmi_virtio_get_free_msg(vioch);
+	if (!msg) {
+		scmi_vio_channel_release(vioch);
+		return -EBUSY;
+	}
+
+	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));
+
+	spin_lock_irqsave(&vioch->lock, flags);
+
+	/*
+	 * If polling was requested for this transaction:
+	 *  - retrieve last used index (will be used as polling reference)
+	 *  - bind the polled message to the xfer via .priv
+	 *  - grab an additional msg refcount for the poll-path
+	 */
+	if (xfer->hdr.poll_completion) {
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		/* Still no users, no need to acquire poll_lock */
+		msg->poll_status = VIO_MSG_POLLING;
+		scmi_vio_msg_acquire(msg);
+		/* Ensure initialized msg is visibly bound to xfer */
+		smp_store_mb(xfer->priv, msg);
+	}
+
+	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
+	if (rc)
+		dev_err(vioch->cinfo->dev,
+			"failed to add to TX virtqueue (%d)\n", rc);
+	else
+		virtqueue_kick(vioch->vqueue);
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	if (rc) {
+		/* Ensure order between xfer->priv clear and vq feeding */
+		smp_store_mb(xfer->priv, NULL);
+		if (xfer->hdr.poll_completion)
+			scmi_vio_msg_release(vioch, msg);
+		scmi_vio_msg_release(vioch, msg);
+	}
+
+	scmi_vio_channel_release(vioch);
+
+	return rc;
+}
+
+static void virtio_fetch_response(struct scmi_chan_info *cinfo,
+				  struct scmi_xfer *xfer)
+{
+	struct scmi_vio_msg *msg = xfer->priv;
+
+	if (msg)
+		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)
+{
+	struct scmi_vio_msg *msg = xfer->priv;
+
+	if (msg)
+		msg_fetch_notification(msg->input, msg->rx_len, max_len, xfer);
+}
+
+/**
+ * virtio_mark_txdone  - Mark transmission done
+ *
+ * Free only completed polling transfer messages.
+ *
+ * Note that in the SCMI VirtIO transport we never explicitly release still
+ * outstanding but timed-out messages by forcibly re-adding them to the
+ * free-list inside the TX code path; we instead let IRQ/RX callbacks, or the
+ * TX deferred worker, eventually clean up such messages once, finally, a late
+ * reply is received and discarded (if ever).
+ *
+ * This approach was deemed preferable since those pending timed-out buffers are
+ * still effectively owned by the SCMI platform VirtIO device even after timeout
+ * expiration: forcibly freeing and reusing them before they had been returned
+ * explicitly by the SCMI platform could lead to subtle bugs due to message
+ * corruption.
+ * An SCMI platform VirtIO device which never returns message buffers is
+ * anyway broken and it will quickly lead to exhaustion of available messages.
+ *
+ * For this same reason, here, we take care to free only the polled messages
+ * that had been somehow replied (only if not by chance already processed on the
+ * IRQ path - the initial scmi_vio_msg_release() takes care of this) and also
+ * any timed-out polled message if that indeed appears to have been at least
+ * dequeued from the virtqueues (VIO_MSG_POLL_DONE): this is needed since such
+ * messages won't be freed elsewhere. Any other polled message is marked as
+ * VIO_MSG_POLL_TIMEOUT.
+ *
+ * Possible late replies to timed-out polled messages will be eventually freed
+ * by RX callbacks if delivered on the IRQ path or by the deferred TX worker if
+ * dequeued on some other polling path.
+ *
+ * @cinfo: SCMI channel info
+ * @ret: Transmission return code
+ * @xfer: Transfer descriptor
+ */
+static void virtio_mark_txdone(struct scmi_chan_info *cinfo, int ret,
+			       struct scmi_xfer *xfer)
+{
+	unsigned long flags;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct scmi_vio_msg *msg = xfer->priv;
+
+	if (!msg || !scmi_vio_channel_acquire(vioch))
+		return;
+
+	/* Ensure msg is unbound from xfer anyway at this point */
+	smp_store_mb(xfer->priv, NULL);
+
+	/* Must be a polled xfer and not already freed on the IRQ path */
+	if (!xfer->hdr.poll_completion || scmi_vio_msg_release(vioch, msg)) {
+		scmi_vio_channel_release(vioch);
+		return;
+	}
+
+	spin_lock_irqsave(&msg->poll_lock, flags);
+	/* Do not free timedout polled messages only if still inflight */
+	if (ret != -ETIMEDOUT || msg->poll_status == VIO_MSG_POLL_DONE)
+		scmi_vio_msg_release(vioch, msg);
+	else if (msg->poll_status == VIO_MSG_POLLING)
+		msg->poll_status = VIO_MSG_POLL_TIMEOUT;
+	spin_unlock_irqrestore(&msg->poll_lock, flags);
+
+	scmi_vio_channel_release(vioch);
+}
+
+/**
+ * virtio_poll_done  - Provide polling support for VirtIO transport
+ *
+ * @cinfo: SCMI channel info
+ * @xfer: Reference to the transfer being poll for.
+ *
+ * VirtIO core provides a polling mechanism based only on last used indexes:
+ * this means that it is possible to poll the virtqueues waiting for something
+ * new to arrive from the host side, but the only way to check if the freshly
+ * arrived buffer was indeed what we were waiting for is to compare the newly
+ * arrived message descriptor with the one we are polling on.
+ *
+ * As a consequence it can happen to dequeue something different from the buffer
+ * we were poll-waiting for: if that is the case such early fetched buffers are
+ * then added to a the @pending_cmds_list list for later processing by a
+ * dedicated deferred worker.
+ *
+ * So, basically, once something new is spotted we proceed to de-queue all the
+ * freshly received used buffers until we found the one we were polling on, or,
+ * we have 'seemingly' emptied the virtqueue; if some buffers are still pending
+ * in the vqueue at the end of the polling loop (possible due to inherent races
+ * in virtqueues handling mechanisms), we similarly kick the deferred worker
+ * and let it process those, to avoid indefinitely looping in the .poll_done
+ * busy-waiting helper.
+ *
+ * Finally, we delegate to the deferred worker also the final free of any timed
+ * out reply to a polled message that we should dequeue.
+ *
+ * Note that, since we do NOT have per-message suppress notification mechanism,
+ * the message we are polling for could be alternatively delivered via usual
+ * IRQs callbacks on another core which happened to have IRQs enabled while we
+ * are actively polling for it here: in such a case it will be handled as such
+ * by scmi_rx_callback() and the polling loop in the SCMI Core TX path will be
+ * transparently terminated anyway.
+ *
+ * Return: True once polling has successfully completed.
+ */
+static bool virtio_poll_done(struct scmi_chan_info *cinfo,
+			     struct scmi_xfer *xfer)
+{
+	bool pending, found = false;
+	unsigned int length, any_prefetched = 0;
+	unsigned long flags;
+	struct scmi_vio_msg *next_msg, *msg = xfer->priv;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+	if (!msg)
+		return true;
+
+	/*
+	 * Processed already by other polling loop on another CPU ?
+	 *
+	 * Note that this message is acquired on the poll path so cannot vanish
+	 * while inside this loop iteration even if concurrently processed on
+	 * the IRQ path.
+	 *
+	 * Avoid to acquire poll_lock since polled_status can be changed
+	 * in a relevant manner only later in this same thread of execution:
+	 * any other possible changes made concurrently by other polling loops
+	 * or by a reply delivered on the IRQ path have no meaningful impact on
+	 * this loop iteration: in other words it is harmless to allow this
+	 * possible race but let has avoid spinlocking with irqs off in this
+	 * initial part of the polling loop.
+	 */
+	if (msg->poll_status == VIO_MSG_POLL_DONE)
+		return true;
+
+	if (!scmi_vio_channel_acquire(vioch))
+		return true;
+
+	/* Has cmdq index moved at all ? */
+	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	if (!pending) {
+		scmi_vio_channel_release(vioch);
+		return false;
+	}
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	virtqueue_disable_cb(vioch->vqueue);
+
+	/*
+	 * Process all new messages till the polled-for message is found OR
+	 * the vqueue is empty.
+	 */
+	while ((next_msg = virtqueue_get_buf(vioch->vqueue, &length))) {
+		bool next_msg_done = false;
+
+		/*
+		 * Mark any dequeued buffer message as VIO_MSG_POLL_DONE so
+		 * that can be properly freed even on timeout in mark_txdone.
+		 */
+		spin_lock(&next_msg->poll_lock);
+		if (next_msg->poll_status == VIO_MSG_POLLING) {
+			next_msg->poll_status = VIO_MSG_POLL_DONE;
+			next_msg_done = true;
+		}
+		spin_unlock(&next_msg->poll_lock);
+
+		next_msg->rx_len = length;
+		/* Is the message we were polling for ? */
+		if (next_msg == msg) {
+			found = true;
+			break;
+		} else if (next_msg_done) {
+			/* Skip the rest if this was another polled msg */
+			continue;
+		}
+
+		/*
+		 * Enqueue for later processing any non-polled message and any
+		 * timed-out polled one that we happen to have dequeued.
+		 */
+		spin_lock(&next_msg->poll_lock);
+		if (next_msg->poll_status == VIO_MSG_NOT_POLLED ||
+		    next_msg->poll_status == VIO_MSG_POLL_TIMEOUT) {
+			spin_unlock(&next_msg->poll_lock);
+
+			any_prefetched++;
+			spin_lock(&vioch->pending_lock);
+			list_add_tail(&next_msg->list,
+				      &vioch->pending_cmds_list);
+			spin_unlock(&vioch->pending_lock);
+		} else {
+			spin_unlock(&next_msg->poll_lock);
+		}
+	}
+
+	/*
+	 * When the polling loop has successfully terminated if something
+	 * else was queued in the meantime, it will be served by a deferred
+	 * worker OR by the normal IRQ/callback OR by other poll loops.
+	 *
+	 * If we are still looking for the polled reply, the polling index has
+	 * to be updated to the current vqueue last used index.
+	 */
+	if (found) {
+		pending = !virtqueue_enable_cb(vioch->vqueue);
+	} else {
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	}
+
+	if (vioch->deferred_tx_wq && (any_prefetched || pending))
+		queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	scmi_vio_channel_release(vioch);
+
+	return found;
+}
+
+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,
+	.mark_txdone = virtio_mark_txdone,
+	.poll_done = virtio_poll_done,
+};
+
+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];
+
+	/* Only one SCMI VirtiO device allowed */
+	if (scmi_vdev) {
+		dev_err(dev,
+			"One SCMI Virtio device was already initialized: only one allowed.\n");
+		return -EBUSY;
+	}
+
+	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;
+	}
+
+	for (i = 0; i < vq_cnt; i++) {
+		unsigned int sz;
+
+		spin_lock_init(&channels[i].lock);
+		spin_lock_init(&channels[i].free_lock);
+		INIT_LIST_HEAD(&channels[i].free_list);
+		spin_lock_init(&channels[i].pending_lock);
+		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
+		channels[i].vqueue = vqs[i];
+
+		sz = virtqueue_get_vring_size(channels[i].vqueue);
+		/* Tx messages need multiple descriptors. */
+		if (!channels[i].is_rx)
+			sz /= DESCRIPTORS_PER_TX_MSG;
+
+		if (sz > MSG_TOKEN_MAX) {
+			dev_info(dev,
+				 "%s virtqueue could hold %d messages. Only %ld allowed to be pending.\n",
+				 channels[i].is_rx ? "rx" : "tx",
+				 sz, MSG_TOKEN_MAX);
+			sz = MSG_TOKEN_MAX;
+		}
+		channels[i].max_msg = sz;
+	}
+
+	vdev->priv = channels;
+	/* Ensure initialized scmi_vdev is visible */
+	smp_store_mb(scmi_vdev, vdev);
+
+	return 0;
+}
+
+static void scmi_vio_remove(struct virtio_device *vdev)
+{
+	/*
+	 * Once we get here, virtio_chan_free() will have already been called by
+	 * the SCMI core for any existing channel and, as a consequence, all the
+	 * virtio channels will have been already marked NOT ready, causing any
+	 * outstanding message on any vqueue to be ignored by complete_cb: now
+	 * we can just stop processing buffers and destroy the vqueues.
+	 */
+	virtio_reset_device(vdev);
+	vdev->config->del_vqs(vdev);
+	/* Ensure scmi_vdev is visible as NULL */
+	smp_store_mb(scmi_vdev, NULL);
+}
+
+static int scmi_vio_validate(struct virtio_device *vdev)
+{
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
+	if (!virtio_has_feature(vdev, VIRTIO_F_VERSION_1)) {
+		dev_err(&vdev->dev,
+			"device does not comply with spec version 1.x\n");
+		return -EINVAL;
+	}
+#endif
+	return 0;
+}
+
+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,
+	.validate = scmi_vio_validate,
+};
+
+static int __init virtio_scmi_init(void)
+{
+	return register_virtio_driver(&virtio_scmi_driver);
+}
+
+static void virtio_scmi_exit(void)
+{
+	unregister_virtio_driver(&virtio_scmi_driver);
+}
+
+const struct scmi_desc scmi_virtio_desc = {
+	.transport_init = virtio_scmi_init,
+	.transport_exit = virtio_scmi_exit,
+	.ops = &scmi_virtio_ops,
+	/* for non-realtime virtio devices */
+	.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
+	.max_msg = 0, /* overridden by virtio_get_max_msg() */
+	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
+	.atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE),
+};