[PATCH v6 00/17] Introduce SCMI transport based on VirtIO

@@ -166,7 +166,8 @@ struct scmi_msg_sensor_reading_get {
 
 struct scmi_resp_sensor_reading_complete {
 	__le32 id;
-	__le64 readings;
+	__le32 readings_low;
+	__le32 readings_high;
 };
 
 struct scmi_sensor_reading_resp {

@@ -717,7 +718,8 @@ static int scmi_sensor_reading_get(const struct scmi_protocol_handle *ph,
 
 			resp = t->rx.buf;
 			if (le32_to_cpu(resp->id) == sensor_id)
-				*value = get_unaligned_le64(&resp->readings);
+				*value =
+					get_unaligned_le64(&resp->readings_low);
 			else
 				ret = -EPROTO;
 		}

@@ -1025,8 +1025,9 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
 	const struct scmi_desc *desc = sinfo->desc;
 
 	/* Pre-allocated messages, no more than what hdr.seq can support */
-	if (WARN_ON(desc->max_msg >= MSG_TOKEN_MAX)) {
-		dev_err(dev, "Maximum message of %d exceeds supported %ld\n",
+	if (WARN_ON(!desc->max_msg || desc->max_msg > MSG_TOKEN_MAX)) {
+		dev_err(dev,
+			"Invalid max_msg %d. Maximum messages supported %lu.\n",
 			desc->max_msg, MSG_TOKEN_MAX);
 		return -EINVAL;
 	}

@@ -70,6 +70,7 @@ struct scmi_msg_resp_prot_version {
  *
  * @id: The identifier of the message being sent
  * @protocol_id: The identifier of the protocol used to send @id message
+ * @type: The SCMI type for this message
  * @seq: The token to identify the message. When a message returns, the
  *	platform returns the whole message header unmodified including the
  *	token

@@ -80,6 +81,7 @@ struct scmi_msg_resp_prot_version {
 struct scmi_msg_hdr {
 	u8 id;
 	u8 protocol_id;
+	u8 type;
 	u16 seq;
 	u32 status;
 	bool poll_completion;

@@ -89,13 +91,14 @@ struct scmi_msg_hdr {
  * pack_scmi_header() - packs and returns 32-bit header
  *
  * @hdr: pointer to header containing all the information on message id,
- *	protocol id and sequence id.
+ *	protocol id, sequence id and type.
  *
  * Return: 32-bit packed message header to be sent to the platform.
  */
 static inline u32 pack_scmi_header(struct scmi_msg_hdr *hdr)
 {
 	return FIELD_PREP(MSG_ID_MASK, hdr->id) |
+		FIELD_PREP(MSG_TYPE_MASK, hdr->type) |
 		FIELD_PREP(MSG_TOKEN_ID_MASK, hdr->seq) |
 		FIELD_PREP(MSG_PROTOCOL_ID_MASK, hdr->protocol_id);
 }

@@ -110,6 +113,7 @@ static inline void unpack_scmi_header(u32 msg_hdr, struct scmi_msg_hdr *hdr)
 {
 	hdr->id = MSG_XTRACT_ID(msg_hdr);
 	hdr->protocol_id = MSG_XTRACT_PROT_ID(msg_hdr);
+	hdr->type = MSG_XTRACT_TYPE(msg_hdr);
 }
 
 /**

@@ -565,6 +565,7 @@ static int xfer_get_init(const struct scmi_protocol_handle *ph,
 
 	xfer->tx.len = tx_size;
 	xfer->rx.len = rx_size ? : info->desc->max_msg_size;
+	xfer->hdr.type = MSG_TYPE_COMMAND;
 	xfer->hdr.id = msg_id;
 	xfer->hdr.poll_completion = false;
 

@@ -171,19 +171,6 @@ static inline int scmi_to_linux_errno(int errno)
 	return -EIO;
 }
 
-/**
- * scmi_dump_header_dbg() - Helper to dump a message header.
- *
- * @dev: Device pointer corresponding to the SCMI entity
- * @hdr: pointer to header.
- */
-static inline void scmi_dump_header_dbg(struct device *dev,
-					struct scmi_msg_hdr *hdr)
-{
-	dev_dbg(dev, "Message ID: %x Sequence ID: %x Protocol: %x\n",
-		hdr->id, hdr->seq, hdr->protocol_id);
-}
-
 void scmi_notification_instance_data_set(const struct scmi_handle *handle,
 					 void *priv)
 {

@@ -287,7 +274,6 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
 	}
 
 	unpack_scmi_header(msg_hdr, &xfer->hdr);
-	scmi_dump_header_dbg(dev, &xfer->hdr);
 	info->desc->ops->fetch_notification(cinfo, info->desc->max_msg_size,
 					    xfer);
 	scmi_notify(cinfo->handle, xfer->hdr.protocol_id,

@@ -338,8 +324,6 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 	if (msg_type == MSG_TYPE_DELAYED_RESP)
 		xfer->rx.len = info->desc->max_msg_size;
 
-	scmi_dump_header_dbg(dev, &xfer->hdr);
-
 	info->desc->ops->fetch_response(cinfo, xfer);
 
 	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,

@@ -321,6 +321,12 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
 /**
  * struct scmi_desc - Description of SoC integration
  *
+ * @init: An optional function that a transport can provide to initialize some
+ *	  transport-specific setup during SCMI core initialization, so ahead of
+ *	  SCMI core probing.
+ * @exit: An optional function that a transport can provide to de-initialize
+ *	  some transport-specific setup during SCMI core de-initialization, so
+ *	  after SCMI core removal.
  * @ops: Pointer to the transport specific ops structure
  * @max_rx_timeout_ms: Timeout for communication with SoC (in Milliseconds)
  * @max_msg: Maximum number of messages that can be pending

@@ -328,6 +334,8 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
  * @max_msg_size: Maximum size of data per message that can be handled.
  */
 struct scmi_desc {
+	int (*init)(void);
+	void (*exit)(void);
 	const struct scmi_transport_ops *ops;
 	int max_rx_timeout_ms;
 	int max_msg;

@@ -1578,10 +1578,64 @@ static struct platform_driver scmi_driver = {
 	.remove = scmi_remove,
 };
 
+/**
+ * __scmi_transports_setup  - Common helper to call transport-specific
+ * .init/.exit code if provided.
+ *
+ * @init: A flag to distinguish between init and exit.
+ *
+ * Note that, if provided, we invoke .init/.exit functions for all the
+ * transports currently compiled in.
+ *
+ * Return: 0 on Success.
+ */
+static inline int __scmi_transports_setup(bool init)
+{
+	int ret = 0;
+	const struct of_device_id *trans;
+
+	for (trans = scmi_of_match; trans->data; trans++) {
+		const struct scmi_desc *tdesc = trans->data;
+
+		if ((init && !tdesc->init) || (!init && !tdesc->exit))
+			continue;
+
+		if (init)
+			ret = tdesc->init();
+		else
+			tdesc->exit();
+
+		if (ret) {
+			pr_err("SCMI transport %s FAILED initialization!\n",
+			       trans->compatible);
+			break;
+		}
+	}
+
+	return ret;
+}
+
+static int __init scmi_transports_init(void)
+{
+	return __scmi_transports_setup(true);
+}
+
+static void __exit scmi_transports_exit(void)
+{
+	__scmi_transports_setup(false);
+}
+
 static int __init scmi_driver_init(void)
 {
+	int ret;
+
 	scmi_bus_init();
 
+	/* Initialize any compiled-in transport which provided an init/exit */
+	ret = scmi_transports_init();
+	if (ret)
+		return ret;
+
 	scmi_base_register();
 
 	scmi_clock_register();

@@ -1610,6 +1664,8 @@ static void __exit scmi_driver_exit(void)
 
 	scmi_bus_exit();
 
+	scmi_transports_exit();
+
 	platform_driver_unregister(&scmi_driver);
 }
 module_exit(scmi_driver_exit);

@@ -14,7 +14,10 @@
 #include <linux/device.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
+#include <linux/hashtable.h>
+#include <linux/list.h>
 #include <linux/module.h>
+#include <linux/refcount.h>
 #include <linux/scmi_protocol.h>
 #include <linux/types.h>
 

@@ -138,6 +141,10 @@ struct scmi_msg {
  *	buffer for the rx path as we use for the tx path.
  * @done: command message transmit completion event
  * @async_done: pointer to delayed response message received event completion
+ * @users: A refcount to track the active users for this xfer
+ * @pending: True for xfers added to @pending_xfers hashtable
+ * @node: An hlist_node reference used to store this xfer, alternatively, on
+ *	  the free list @free_xfers or in the @pending_xfers hashtable
  */
 struct scmi_xfer {
 	int transfer_id;

@@ -146,8 +153,26 @@ struct scmi_xfer {
 	struct scmi_msg rx;
 	struct completion done;
 	struct completion *async_done;
+	refcount_t users;
+	bool pending;
+	struct hlist_node node;
 };
 
+/*
+ * An helper macro to lookup an xfer from the @pending_xfers hashtable
+ * using the message sequence number token as a key.
+ */
+#define XFER_FIND(__ht, __k)					\
+({								\
+	typeof(__k) k_ = __k;					\
+	struct scmi_xfer *xfer_ = NULL;				\
+								\
+	hash_for_each_possible((__ht), xfer_, node, k_)		\
+		if (xfer_->hdr.seq == k_)			\
+			break;					\
+	xfer_;							\
+})
+
 struct scmi_xfer_ops;
 
 /**

@@ -21,6 +21,7 @@
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/ktime.h>
+#include <linux/hashtable.h>
 #include <linux/list.h>
 #include <linux/module.h>
 #include <linux/of_address.h>

@@ -65,19 +66,29 @@ struct scmi_requested_dev {
 	struct list_head node;
 };
 
+#define SCMI_PENDING_XFERS_HT_ORDER_SZ	9
+
 /**
  * struct scmi_xfers_info - Structure to manage transfer information
  *
- * @xfer_block: Preallocated Message array
  * @xfer_alloc_table: Bitmap table for allocated messages.
  *	Index of this bitmap table is also used for message
  *	sequence identifier.
  * @xfer_lock: Protection for message allocation
+ * @last_token: A counter to use as base to generate for monotonically
+ *		increasing tokens.
+ * @free_xfers: A free list for available to use xfers. It is initialized with
+ *		a number of xfers equal to the maximum allowed in-flight
+ *		messages.
+ * @pending_xfers: An hashtable, indexed by msg_hdr.seq, used to keep all the
+ *		   currently in-flight messages.
  */
 struct scmi_xfers_info {
-	struct scmi_xfer *xfer_block;
 	unsigned long *xfer_alloc_table;
 	spinlock_t xfer_lock;
+	atomic_t last_token;
+	struct hlist_head free_xfers;
+	DECLARE_HASHTABLE(pending_xfers, SCMI_PENDING_XFERS_HT_ORDER_SZ);
 };
 
 /**

@@ -190,45 +201,177 @@ void *scmi_notification_instance_data_get(const struct scmi_handle *handle)
 	return info->notify_priv;
 }
 
+/**
+ * scmi_xfer_token_set  - Reserve and set new token for the xfer at hand
+ *
+ * @minfo: Pointer to Tx/Rx Message management info based on channel type
+ * @xfer: The xfer to act upon
+ *
+ * Pick the next unused monotonically increasing token and set it into
+ * xfer->hdr.seq: picking a monotonically increasing value avoids immediate
+ * reuse of freshly completed or timed-out xfers, thus mitigating the risk
+ * of incorrect association of a late and expired xfer with a live in-flight
+ * transaction, both happening to re-use the same token identifier.
+ *
+ * Since platform is NOT required to answer our request in-order we should
+ * account for a few rare but possible scenarios:
+ *
+ *  - exactly 'next_token' may be NOT available so pick xfer_id >= next_token
+ *    using find_next_zero_bit() starting from candidate next_token bit
+ *
+ *  - all tokens ahead upto (MSG_TOKEN_ID_MASK - 1) are used in-flight but we
+ *    are plenty of free tokens at start, so try a second pass using
+ *    find_next_zero_bit() and starting from 0.
+ *
+ *  X = used in-flight
+ *
+ * Normal
+ * ------
+ *
+ *		|- xfer_id picked
+ *   -----------+----------------------------------------------------------
+ *   | | |X|X|X| | | | | | ... ... ... ... ... ... ... ... ... ... ...|X|X|
+ *   ----------------------------------------------------------------------
+ *		^
+ *		|- next_token
+ *
+ * Out-of-order pending at start
+ * -----------------------------
+ *
+ *	  |- xfer_id picked, last_token fixed
+ *   -----+----------------------------------------------------------------
+ *   |X|X| | | | |X|X| ... ... ... ... ... ... ... ... ... ... ... ...|X| |
+ *   ----------------------------------------------------------------------
+ *    ^
+ *    |- next_token
+ *
+ *
+ * Out-of-order pending at end
+ * ---------------------------
+ *
+ *	  |- xfer_id picked, last_token fixed
+ *   -----+----------------------------------------------------------------
+ *   |X|X| | | | |X|X| ... ... ... ... ... ... ... ... ... ... |X|X|X||X|X|
+ *   ----------------------------------------------------------------------
+ *								^
+ *								|- next_token
+ *
+ * Context: Assumes to be called with @xfer_lock already acquired.
+ *
+ * Return: 0 on Success or error
+ */
+static int scmi_xfer_token_set(struct scmi_xfers_info *minfo,
+			       struct scmi_xfer *xfer)
+{
+	unsigned long xfer_id, next_token;
+
+	/* Pick a candidate monotonic token in range [0, MSG_TOKEN_MAX - 1] */
+	next_token = (atomic_inc_return(&minfo->last_token) &
+		      (MSG_TOKEN_MAX - 1));
+
+	/* Pick the next available xfer_id >= next_token */
+	xfer_id = find_next_zero_bit(minfo->xfer_alloc_table,
+				     MSG_TOKEN_MAX, next_token);
+	if (xfer_id == MSG_TOKEN_MAX) {
+		/*
+		 * After heavily out-of-order responses, there are no free
+		 * tokens ahead, but only at start of xfer_alloc_table so
+		 * try again from the beginning.
+		 */
+		xfer_id = find_next_zero_bit(minfo->xfer_alloc_table,
+					     MSG_TOKEN_MAX, 0);
+		/*
+		 * Something is wrong if we got here since there can be a
+		 * maximum number of (MSG_TOKEN_MAX - 1) in-flight messages
+		 * but we have not found any free token [0, MSG_TOKEN_MAX - 1].
+		 */
+		if (WARN_ON_ONCE(xfer_id == MSG_TOKEN_MAX))
+			return -ENOMEM;
+	}
+
+	/* Update +/- last_token accordingly if we skipped some hole */
+	if (xfer_id != next_token)
+		atomic_add((int)(xfer_id - next_token), &minfo->last_token);
+
+	/* Set in-flight */
+	set_bit(xfer_id, minfo->xfer_alloc_table);
+	xfer->hdr.seq = (u16)xfer_id;
+
+	return 0;
+}
+
+/**
+ * scmi_xfer_token_clear  - Release the token
+ *
+ * @minfo: Pointer to Tx/Rx Message management info based on channel type
+ * @xfer: The xfer to act upon
+ */
+static inline void scmi_xfer_token_clear(struct scmi_xfers_info *minfo,
+					 struct scmi_xfer *xfer)
+{
+	clear_bit(xfer->hdr.seq, minfo->xfer_alloc_table);
+}
+
 /**
  * scmi_xfer_get() - Allocate one message
  *
  * @handle: Pointer to SCMI entity handle
  * @minfo: Pointer to Tx/Rx Message management info based on channel type
+ * @set_pending: If true a monotonic token is picked and the xfer is added to
+ *		 the pending hash table.
  *
  * Helper function which is used by various message functions that are
  * exposed to clients of this driver for allocating a message traffic event.
  *
- * This function can sleep depending on pending requests already in the system
- * for the SCMI entity. Further, this also holds a spinlock to maintain
- * integrity of internal data structures.
+ * Picks an xfer from the free list @free_xfers (if any available) and, if
+ * required, sets a monotonically increasing token and stores the inflight xfer
+ * into the @pending_xfers hashtable for later retrieval.
+ *
+ * The successfully initialized xfer is refcounted.
+ *
+ * Context: Holds @xfer_lock while manipulating @xfer_alloc_table and
+ *	    @free_xfers.
  *
  * Return: 0 if all went fine, else corresponding error.
  */
 static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
-				       struct scmi_xfers_info *minfo)
+				       struct scmi_xfers_info *minfo,
+				       bool set_pending)
 {
-	u16 xfer_id;
+	int ret;
+	unsigned long flags;
 	struct scmi_xfer *xfer;
-	unsigned long flags, bit_pos;
-	struct scmi_info *info = handle_to_scmi_info(handle);
 
-	/* Keep the locked section as small as possible */
 	spin_lock_irqsave(&minfo->xfer_lock, flags);
-	bit_pos = find_first_zero_bit(minfo->xfer_alloc_table,
-				      info->desc->max_msg);
-	if (bit_pos == info->desc->max_msg) {
+	if (hlist_empty(&minfo->free_xfers)) {
 		spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 		return ERR_PTR(-ENOMEM);
 	}
-	set_bit(bit_pos, minfo->xfer_alloc_table);
-	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 
-	xfer_id = bit_pos;
+	/* grab an xfer from the free_list */
+	xfer = hlist_entry(minfo->free_xfers.first, struct scmi_xfer, node);
+	hlist_del_init(&xfer->node);
+
+	if (set_pending) {
+		/* Pick and set monotonic token */
+		ret = scmi_xfer_token_set(minfo, xfer);
+		if (!ret) {
+			hash_add(minfo->pending_xfers, &xfer->node,
+				 xfer->hdr.seq);
+			xfer->pending = true;
+		} else {
+			dev_err(handle->dev,
+				"Failed to get monotonic token %d\n", ret);
+			hlist_add_head(&xfer->node, &minfo->free_xfers);
+			xfer = ERR_PTR(ret);
+		}
+	}
 
-	xfer = &minfo->xfer_block[xfer_id];
-	xfer->hdr.seq = xfer_id;
-	xfer->transfer_id = atomic_inc_return(&transfer_last_id);
+	if (!IS_ERR(xfer)) {
+		refcount_set(&xfer->users, 1);
+		xfer->transfer_id = atomic_inc_return(&transfer_last_id);
+	}
+	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 
 	return xfer;
 }

@@ -239,6 +382,9 @@ static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
  * @minfo: Pointer to Tx/Rx Message management info based on channel type
  * @xfer: message that was reserved by scmi_xfer_get
  *
+ * After refcount check, possibly release an xfer, clearing the token slot,
+ * removing xfer from @pending_xfers and putting it back into free_xfers.
+ *
  * This holds a spinlock to maintain integrity of internal data structures.
  */
 static void

@@ -246,16 +392,44 @@ __scmi_xfer_put(struct scmi_xfers_info *minfo, struct scmi_xfer *xfer)
 {
 	unsigned long flags;
 
-	/*
-	 * Keep the locked section as small as possible
-	 * NOTE: we might escape with smp_mb and no lock here..
-	 * but just be conservative and symmetric.
-	 */
 	spin_lock_irqsave(&minfo->xfer_lock, flags);
-	clear_bit(xfer->hdr.seq, minfo->xfer_alloc_table);
+	if (refcount_dec_and_test(&xfer->users)) {
+		if (xfer->pending) {
+			scmi_xfer_token_clear(minfo, xfer);
+			hash_del(&xfer->node);
+			xfer->pending = false;
+		}
+		hlist_add_head(&xfer->node, &minfo->free_xfers);
+	}
 	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
 }
 
+/**
+ * scmi_xfer_lookup_unlocked  -  Helper to lookup an xfer_id
+ *
+ * @minfo: Pointer to Tx/Rx Message management info based on channel type
+ * @xfer_id: Token ID to lookup in @pending_xfers
+ *
+ * Refcounting is untouched.
+ *
+ * Context: Assumes to be called with @xfer_lock already acquired.
+ *
+ * Return: A valid xfer on Success or error otherwise
+ */
+static struct scmi_xfer *
+scmi_xfer_lookup_unlocked(struct scmi_xfers_info *minfo, u16 xfer_id)
+{
+	struct scmi_xfer *xfer = NULL;
+
+	if (xfer_id >= MSG_TOKEN_MAX)
+		return ERR_PTR(-EINVAL);
+
+	if (test_bit(xfer_id, minfo->xfer_alloc_table))
+		xfer = XFER_FIND(minfo->pending_xfers, xfer_id);
+
+	return xfer ?: ERR_PTR(-EINVAL);
+}
+
 static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
 {
 	struct scmi_xfer *xfer;

@@ -265,7 +439,7 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
 	ktime_t ts;
 
 	ts = ktime_get_boottime();
-	xfer = scmi_xfer_get(cinfo->handle, minfo);
+	xfer = scmi_xfer_get(cinfo->handle, minfo, false);
 	if (IS_ERR(xfer)) {
 		dev_err(dev, "failed to get free message slot (%ld)\n",
 			PTR_ERR(xfer));

@@ -291,19 +465,22 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
 static void scmi_handle_response(struct scmi_chan_info *cinfo,
 				 u16 xfer_id, u8 msg_type)
 {
+	unsigned long flags;
 	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->tx_minfo;
 
 	/* Are we even expecting this? */
-	if (!test_bit(xfer_id, minfo->xfer_alloc_table)) {
+	spin_lock_irqsave(&minfo->xfer_lock, flags);
+	xfer = scmi_xfer_lookup_unlocked(minfo, xfer_id);
+	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
+	if (IS_ERR(xfer)) {
 		dev_err(dev, "message for %d is not expected!\n", xfer_id);
 		info->desc->ops->clear_channel(cinfo);
 		return;
 	}
 
-	xfer = &minfo->xfer_block[xfer_id];
 	/*
 	 * Even if a response was indeed expected on this slot at this point,
 	 * a buggy platform could wrongly reply feeding us an unexpected

@@ -540,7 +717,7 @@ static int xfer_get_init(const struct scmi_protocol_handle *ph,
 	    tx_size > info->desc->max_msg_size)
 		return -ERANGE;
 
-	xfer = scmi_xfer_get(pi->handle, minfo);
+	xfer = scmi_xfer_get(pi->handle, minfo, true);
 	if (IS_ERR(xfer)) {
 		ret = PTR_ERR(xfer);
 		dev_err(dev, "failed to get free message slot(%d)\n", ret);

@@ -1017,18 +1194,25 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
 		return -EINVAL;
 	}
 
-	info->xfer_block = devm_kcalloc(dev, desc->max_msg,
-					sizeof(*info->xfer_block), GFP_KERNEL);
-	if (!info->xfer_block)
-		return -ENOMEM;
+	hash_init(info->pending_xfers);
 
-	info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(desc->max_msg),
+	/* Allocate a bitmask sized to hold MSG_TOKEN_MAX tokens */
+	info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(MSG_TOKEN_MAX),
 					      sizeof(long), GFP_KERNEL);
 	if (!info->xfer_alloc_table)
 		return -ENOMEM;
 
-	/* Pre-initialize the buffer pointer to pre-allocated buffers */
-	for (i = 0, xfer = info->xfer_block; i < desc->max_msg; i++, xfer++) {
+	/*
+	 * Preallocate a number of xfers equal to max inflight messages,
+	 * pre-initialize the buffer pointer to pre-allocated buffers and
+	 * attach all of them to the free list
+	 */
+	INIT_HLIST_HEAD(&info->free_xfers);
+	for (i = 0; i < desc->max_msg; i++) {
+		xfer = devm_kzalloc(dev, sizeof(*xfer), GFP_KERNEL);
+		if (!xfer)
+			return -ENOMEM;
+
 		xfer->rx.buf = devm_kcalloc(dev, sizeof(u8), desc->max_msg_size,
 					    GFP_KERNEL);
 		if (!xfer->rx.buf)

@@ -1036,8 +1220,12 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
 
 		xfer->tx.buf = xfer->rx.buf;
 		init_completion(&xfer->done);
+
+		/* Add initialized xfer to the free list */
+		hlist_add_head(&xfer->node, &info->free_xfers);
 	}
 
+	atomic_set(&info->last_token, -1);
 	spin_lock_init(&info->xfer_lock);
 
 	return 0;

@@ -19,6 +19,7 @@
 #include <linux/module.h>
 #include <linux/refcount.h>
 #include <linux/scmi_protocol.h>
+#include <linux/spinlock.h>
 #include <linux/types.h>
 
 #include <asm/unaligned.h>

@@ -145,6 +146,13 @@ struct scmi_msg {
  * @pending: True for xfers added to @pending_xfers hashtable
  * @node: An hlist_node reference used to store this xfer, alternatively, on
  *	  the free list @free_xfers or in the @pending_xfers hashtable
+ * @busy: An atomic flag to ensure exclusive write access to this xfer
+ * @state: The current state of this transfer, with states transitions deemed
+ *	   valid being:
+ *	    - SCMI_XFER_SENT_OK -> SCMI_XFER_RESP_OK [ -> SCMI_XFER_DRESP_OK ]
+ *	    - SCMI_XFER_SENT_OK -> SCMI_XFER_DRESP_OK
+ *	      (Missing synchronous response is assumed OK and ignored)
+ * @lock: A spinlock to protect state and busy fields.
  */
 struct scmi_xfer {
 	int transfer_id;

@@ -156,6 +164,15 @@ struct scmi_xfer {
 	refcount_t users;
 	bool pending;
 	struct hlist_node node;
+#define SCMI_XFER_FREE		0
+#define SCMI_XFER_BUSY		1
+	atomic_t busy;
+#define SCMI_XFER_SENT_OK	0
+#define SCMI_XFER_RESP_OK	1
+#define SCMI_XFER_DRESP_OK	2
+	int state;
+	/* A lock to protect state and busy fields */
+	spinlock_t lock;
 };
 
 /*

@@ -392,5 +409,4 @@ bool shmem_poll_done(struct scmi_shared_mem __iomem *shmem,
 void scmi_notification_instance_data_set(const struct scmi_handle *handle,
 					 void *priv);
 void *scmi_notification_instance_data_get(const struct scmi_handle *handle);
-
 #endif /* _SCMI_COMMON_H */

@@ -369,6 +369,7 @@ static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
 
 	if (!IS_ERR(xfer)) {
 		refcount_set(&xfer->users, 1);
+		atomic_set(&xfer->busy, SCMI_XFER_FREE);
 		xfer->transfer_id = atomic_inc_return(&transfer_last_id);
 	}
 	spin_unlock_irqrestore(&minfo->xfer_lock, flags);

@@ -430,6 +431,168 @@ scmi_xfer_lookup_unlocked(struct scmi_xfers_info *minfo, u16 xfer_id)
 	return xfer ?: ERR_PTR(-EINVAL);
 }
 
+/**
+ * scmi_msg_response_validate  - Validate message type against state of related
+ * xfer
+ *
+ * @cinfo: A reference to the channel descriptor.
+ * @msg_type: Message type to check
+ * @xfer: A reference to the xfer to validate against @msg_type
+ *
+ * This function checks if @msg_type is congruent with the current state of
+ * a pending @xfer; if an asynchronous delayed response is received before the
+ * related synchronous response (Out-of-Order Delayed Response) the missing
+ * synchronous response is assumed to be OK and completed, carrying on with the
+ * Delayed Response: this is done to address the case in which the underlying
+ * SCMI transport can deliver such out-of-order responses.
+ *
+ * Context: Assumes to be called with xfer->lock already acquired.
+ *
+ * Return: 0 on Success, error otherwise
+ */
+static inline int scmi_msg_response_validate(struct scmi_chan_info *cinfo,
+					     u8 msg_type,
+					     struct scmi_xfer *xfer)
+{
+	/*
+	 * Even if a response was indeed expected on this slot at this point,
+	 * a buggy platform could wrongly reply feeding us an unexpected
+	 * delayed response we're not prepared to handle: bail-out safely
+	 * blaming firmware.
+	 */
+	if (msg_type == MSG_TYPE_DELAYED_RESP && !xfer->async_done) {
+		dev_err(cinfo->dev,
+			"Delayed Response for %d not expected! Buggy F/W ?\n",
+			xfer->hdr.seq);
+		return -EINVAL;
+	}
+
+	switch (xfer->state) {
+	case SCMI_XFER_SENT_OK:
+		if (msg_type == MSG_TYPE_DELAYED_RESP) {
+			/*
+			 * Delayed Response expected but delivered earlier.
+			 * Assume message RESPONSE was OK and skip state.
+			 */
+			xfer->hdr.status = SCMI_SUCCESS;
+			xfer->state = SCMI_XFER_RESP_OK;
+			complete(&xfer->done);
+			dev_warn(cinfo->dev,
+				 "Received valid OoO Delayed Response for %d\n",
+				 xfer->hdr.seq);
+		}
+		break;
+	case SCMI_XFER_RESP_OK:
+		if (msg_type != MSG_TYPE_DELAYED_RESP)
+			return -EINVAL;
+		break;
+	case SCMI_XFER_DRESP_OK:
+		/* No further message expected once in SCMI_XFER_DRESP_OK */
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static bool scmi_xfer_is_free(struct scmi_xfer *xfer)
+{
+	int ret;
+
+	ret = atomic_cmpxchg(&xfer->busy, SCMI_XFER_FREE, SCMI_XFER_BUSY);
+
+	return ret == SCMI_XFER_FREE;
+}
+
+/**
+ * scmi_xfer_command_acquire  -  Helper to lookup and acquire a command xfer
+ *
+ * @cinfo: A reference to the channel descriptor.
+ * @msg_hdr: A message header to use as lookup key
+ *
+ * When a valid xfer is found for the sequence number embedded in the provided
+ * msg_hdr, reference counting is properly updated and exclusive access to this
+ * xfer is granted till released with @scmi_xfer_command_release.
+ *
+ * Return: A valid @xfer on Success or error otherwise.
+ */
+static inline struct scmi_xfer *
+scmi_xfer_command_acquire(struct scmi_chan_info *cinfo, u32 msg_hdr)
+{
+	int ret;
+	unsigned long flags;
+	struct scmi_xfer *xfer;
+	struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
+	struct scmi_xfers_info *minfo = &info->tx_minfo;
+	u8 msg_type = MSG_XTRACT_TYPE(msg_hdr);
+	u16 xfer_id = MSG_XTRACT_TOKEN(msg_hdr);
+
+	/* Are we even expecting this? */
+	spin_lock_irqsave(&minfo->xfer_lock, flags);
+	xfer = scmi_xfer_lookup_unlocked(minfo, xfer_id);
+	if (IS_ERR(xfer)) {
+		dev_err(cinfo->dev,
+			"Message for %d type %d is not expected!\n",
+			xfer_id, msg_type);
+		spin_unlock_irqrestore(&minfo->xfer_lock, flags);
+		return xfer;
+	}
+	refcount_inc(&xfer->users);
+	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
+
+	spin_lock_irqsave(&xfer->lock, flags);
+	ret = scmi_msg_response_validate(cinfo, msg_type, xfer);
+	/*
+	 * If a pending xfer was found which was also in a congruent state with
+	 * the received message, acquire exclusive access to it setting the busy
+	 * flag.
+	 * Spins only on the rare limit condition of concurrent reception of
+	 * RESP and DRESP for the same xfer.
+	 */
+	if (!ret) {
+		spin_until_cond(scmi_xfer_is_free(xfer));
+		xfer->hdr.type = msg_type;
+	}
+	spin_unlock_irqrestore(&xfer->lock, flags);
+
+	if (ret) {
+		dev_err(cinfo->dev,
+			"Invalid message type:%d for %d - HDR:0x%X  state:%d\n",
+			msg_type, xfer_id, msg_hdr, xfer->state);
+		/* On error the refcount incremented above has to be dropped */
+		__scmi_xfer_put(minfo, xfer);
+		xfer = ERR_PTR(-EINVAL);
+	}
+
+	return xfer;
+}
+
+static inline void scmi_xfer_command_release(struct scmi_info *info,
+					     struct scmi_xfer *xfer)
+{
+	atomic_set(&xfer->busy, SCMI_XFER_FREE);
+	__scmi_xfer_put(&info->tx_minfo, xfer);
+}
+
+/**
+ * scmi_xfer_state_update  - Update xfer state
+ *
+ * @xfer: A reference to the xfer to update
+ *
+ * Context: Assumes to be called on an xfer exclusively acquired using the
+ *	    busy flag.
+ */
+static inline void scmi_xfer_state_update(struct scmi_xfer *xfer)
+{
+	switch (xfer->hdr.type) {
+	case MSG_TYPE_COMMAND:
+		xfer->state = SCMI_XFER_RESP_OK;
+		break;
+	case MSG_TYPE_DELAYED_RESP:
+		xfer->state = SCMI_XFER_DRESP_OK;
+		break;
+	}
+}
+
 static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
 {
 	struct scmi_xfer *xfer;