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Date:   Mon, 5 Mar 2018 13:52:53 +0000
From:   Jonathan Cameron <Jonathan.Cameron@huawei.com>
To:     Sudeep Holla <sudeep.holla@arm.com>
CC:     ALKML <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        DTML <devicetree@vger.kernel.org>,
        "Alexey Klimov" <klimov.linux@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v6 03/20] firmware: arm_scmi: add basic driver
 infrastructure for SCMI
Message-ID: <20180305145253.00004247@huawei.com>
In-Reply-To: <1519403030-21189-4-git-send-email-sudeep.holla@arm.com>
References: <1519403030-21189-1-git-send-email-sudeep.holla@arm.com>
        <1519403030-21189-4-git-send-email-sudeep.holla@arm.com>
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On Fri, 23 Feb 2018 16:23:33 +0000
Sudeep Holla <sudeep.holla@arm.com> wrote:

> The SCMI is intended to allow OSPM to manage various functions that are
> provided by the hardware platform it is running on, including power and
> performance functions. SCMI provides two levels of abstraction, protocols
> and transports. Protocols define individual groups of system control and
> management messages. A protocol specification describes the messages
> that it supports. Transports describe the method by which protocol
> messages are communicated between agents and the platform.
> 
> This patch adds basic infrastructure to manage the message allocation,
> initialisation, packing/unpacking and shared memory management.
> 
Hi Sudeep,

A bit of a drive by review as I was curious and happen to have been looking
at the spec.  Anyhow my main comments in here are about consistency of naming.
In many ways it doesn't matter what naming convention you go with, but it is
good to make sure you then use it consistently.

> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
<snip>
> +
> +#include <linux/completion.h>
> +#include <linux/scmi_protocol.h>
> +#include <linux/types.h>
> +
> +/**
> + * struct scmi_msg_hdr - Message(Tx/Rx) header
> + *
> + * @id: The identifier of the command being sent
> + * @protocol_id: The identifier of the protocol used to send @id command
> + * @seq: The token to identify the message. when a message/command returns,
> + *       the platform returns the whole message header unmodified including
> + *	 the token.
Something looks odd with indenting here...

> + */
> +struct scmi_msg_hdr {
> +	u8 id;
I think this is called message_id in the spec, would it be worth
matching that here?

> +	u8 protocol_id;
> +	u16 seq;
> +	u32 status;
> +	bool poll_completion;
> +};
status and poll completion could do with documenting.

> +
> +/**
> + * struct scmi_msg - Message(Tx/Rx) structure
> + *
> + * @buf: Buffer pointer
> + * @len: Length of data in the Buffer
> + */
> +struct scmi_msg {
> +	void *buf;
> +	size_t len;
> +};
> +
> +/**
> + * struct scmi_xfer - Structure representing a message flow
> + *
> + * @hdr: Transmit message header
> + * @tx: Transmit message
> + * @rx: Receive message, the buffer should be pre-allocated to store
> + *	message. If request-ACK protocol is used, we can reuse the same
> + *	buffer for the rx path as we use for the tx path.
> + * @done: completion event
> + */
> +
No blank line here.
> +struct scmi_xfer {
> +	void *con_priv;
> +	struct scmi_msg_hdr hdr;
> +	struct scmi_msg tx;
> +	struct scmi_msg rx;
> +	struct completion done;
> +};
> +
> +void scmi_one_xfer_put(const struct scmi_handle *h, struct scmi_xfer *xfer);
> +int scmi_do_xfer(const struct scmi_handle *h, struct scmi_xfer *xfer);
> +int scmi_one_xfer_init(const struct scmi_handle *h, u8 msg_id, u8 prot_id,
> +		       size_t tx_size, size_t rx_size, struct scmi_xfer **p);
> +int scmi_handle_put(const struct scmi_handle *handle);
> +struct scmi_handle *scmi_handle_get(struct device *dev);
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> new file mode 100644
> index 000000000000..fa0e9cf31f4c
> --- /dev/null
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -0,0 +1,689 @@
> +/*
> + * System Control and Management Interface (SCMI) Message Protocol driver
> + *
> + * SCMI Message Protocol is used between the System Control Processor(SCP)
> + * and the Application Processors(AP). The Message Handling Unit(MHU)
> + * provides a mechanism for inter-processor communication between SCP's
> + * Cortex M3 and AP.
> + *
> + * SCP offers control and management of the core/cluster power states,
> + * various power domain DVFS including the core/cluster, certain system
> + * clocks configuration, thermal sensors and many others.
> + *
> + * Copyright (C) 2017 ARM Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along
> + * with this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/bitmap.h>
> +#include <linux/export.h>
> +#include <linux/io.h>
> +#include <linux/kernel.h>
> +#include <linux/mailbox_client.h>
> +#include <linux/module.h>
> +#include <linux/of_address.h>
> +#include <linux/of_device.h>
> +#include <linux/semaphore.h>
> +#include <linux/slab.h>
> +
> +#include "common.h"
> +
> +#define MSG_ID_SHIFT		0
> +#define MSG_ID_MASK		0xff
> +#define MSG_TYPE_SHIFT		8
> +#define MSG_TYPE_MASK		0x3

Interesting to note you don't specify type in your header structure
above but do have it here.  I guess that is because it is always 0
when you care about.  Might be nice to be consistent though?

> +#define MSG_PROTOCOL_ID_SHIFT	10
> +#define MSG_PROTOCOL_ID_MASK	0xff
> +#define MSG_TOKEN_ID_SHIFT	18
> +#define MSG_TOKEN_ID_MASK	0x3ff
> +#define MSG_XTRACT_TOKEN(header)	\
> +	(((header) >> MSG_TOKEN_ID_SHIFT) & MSG_TOKEN_ID_MASK)
> +
> +enum scmi_error_codes {
> +	SCMI_SUCCESS = 0,	/* Success */
> +	SCMI_ERR_SUPPORT = -1,	/* Not supported */
> +	SCMI_ERR_PARAMS = -2,	/* Invalid Parameters */
> +	SCMI_ERR_ACCESS = -3,	/* Invalid access/permission denied */
> +	SCMI_ERR_ENTRY = -4,	/* Not found */
> +	SCMI_ERR_RANGE = -5,	/* Value out of range */
> +	SCMI_ERR_BUSY = -6,	/* Device busy */
> +	SCMI_ERR_COMMS = -7,	/* Communication Error */
> +	SCMI_ERR_GENERIC = -8,	/* Generic Error */
> +	SCMI_ERR_HARDWARE = -9,	/* Hardware Error */
> +	SCMI_ERR_PROTOCOL = -10,/* Protocol Error */
> +	SCMI_ERR_MAX
> +};
> +
> +/* List of all  SCMI devices active in system */
> +static LIST_HEAD(scmi_list);
> +/* Protection for the entire list */
> +static DEFINE_MUTEX(scmi_list_mutex);
> +
> +/**
> + * 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
> + */
> +struct scmi_xfers_info {
> +	struct scmi_xfer *xfer_block;
> +	unsigned long *xfer_alloc_table;
> +	/* protect transfer allocation */
This is here as warning suppression as it's clearly documented
above.  Personally I've always just marked those downs as false
positives rather than having the ugliness of documenting it twice.
 
> +	spinlock_t xfer_lock;
> +};
> +
> +/**
> + * struct scmi_desc - Description of SoC integration
> + *
> + * @max_rx_timeout_ms: Timeout for communication with SoC (in Milliseconds)
> + * @max_msg: Maximum number of messages that can be pending
> + *	simultaneously in the system
> + * @max_msg_size: Maximum size of data per message that can be handled.
> + */
> +struct scmi_desc {
> +	int max_rx_timeout_ms;
> +	int max_msg;
> +	int max_msg_size;
> +};
> +
> +/**
> + * struct scmi_info - Structure representing a  SCMI instance
> + *
> + * @dev: Device pointer
> + * @desc: SoC description for this instance
> + * @handle: Instance of SCMI handle to send to clients
> + * @cl: Mailbox Client
> + * @tx_chan: Transmit mailbox channel
> + * @tx_payload: Transmit mailbox channel payload area
> + * @minfo: Message info
> + * @node: list head
Nitpick of the day :) Inconsistent capitalization.  Also
useful to know which list this is for...
> + * @users: Number of users of this instance
> + */
> +struct scmi_info {
> +	struct device *dev;
> +	const struct scmi_desc *desc;
> +	struct scmi_handle handle;
> +	struct mbox_client cl;
> +	struct mbox_chan *tx_chan;
> +	void __iomem *tx_payload;
> +	struct scmi_xfers_info minfo;
> +	struct list_head node;
> +	int users;
> +};
> +
> +#define client_to_scmi_info(c)	container_of(c, struct scmi_info, cl)
> +#define handle_to_scmi_info(h)	container_of(h, struct scmi_info, handle)
> +
> +/*
> + * SCMI specification requires all parameters, message headers, return
> + * arguments or any protocol data to be expressed in little endian
> + * format only.
> + */
> +struct scmi_shared_mem {
> +	__le32 reserved;
> +	__le32 channel_status;
> +#define SCMI_SHMEM_CHAN_STAT_CHANNEL_ERROR	BIT(1)
> +#define SCMI_SHMEM_CHAN_STAT_CHANNEL_FREE	BIT(0)
> +	__le32 reserved1[2];
> +	__le32 flags;
> +#define SCMI_SHMEM_FLAG_INTR_ENABLED	BIT(0)
> +	__le32 length;
> +	__le32 msg_header;
> +	u8 msg_payload[0];
> +};
> +
> +static const int scmi_linux_errmap[] = {
> +	/* better than switch case as long as return value is continuous */
> +	0,			/* SCMI_SUCCESS */
> +	-EOPNOTSUPP,		/* SCMI_ERR_SUPPORT */
> +	-EINVAL,		/* SCMI_ERR_PARAM */
> +	-EACCES,		/* SCMI_ERR_ACCESS */
> +	-ENOENT,		/* SCMI_ERR_ENTRY */
> +	-ERANGE,		/* SCMI_ERR_RANGE */
> +	-EBUSY,			/* SCMI_ERR_BUSY */
> +	-ECOMM,			/* SCMI_ERR_COMMS */
> +	-EIO,			/* SCMI_ERR_GENERIC */
> +	-EREMOTEIO,		/* SCMI_ERR_HARDWARE */
> +	-EPROTO,		/* SCMI_ERR_PROTOCOL */
> +};
> +
> +static inline int scmi_to_linux_errno(int errno)
> +{
> +	if (errno < SCMI_SUCCESS && errno > SCMI_ERR_MAX)
> +		return scmi_linux_errmap[-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, "Command ID: %x Sequence ID: %x Protocol: %x\n",
> +		hdr->id, hdr->seq, hdr->protocol_id);
> +}
> +
> +static void scmi_fetch_response(struct scmi_xfer *xfer,
> +				struct scmi_shared_mem __iomem *mem)
> +{
> +	xfer->hdr.status = ioread32(mem->msg_payload);
> +	/* Skip the length of header and statues in payload area i.e 8 bytes*/
> +	xfer->rx.len = min_t(size_t, xfer->rx.len, ioread32(&mem->length) - 8);
> +
> +	/* Take a copy to the rx buffer.. */
> +	memcpy_fromio(xfer->rx.buf, mem->msg_payload + 4, xfer->rx.len);
> +}
> +
> +/**
> + * scmi_rx_callback() - mailbox client callback for receive messages
> + *
> + * @cl: client pointer
> + * @m: mailbox message
> + *
> + * Processes one received message to appropriate transfer information and
> + * signals completion of the transfer.
> + *
> + * NOTE: This function will be invoked in IRQ context, hence should be
> + * as optimal as possible.
> + */
> +static void scmi_rx_callback(struct mbox_client *cl, void *m)
> +{
> +	u16 xfer_id;
> +	struct scmi_xfer *xfer;
> +	struct scmi_info *info = client_to_scmi_info(cl);
> +	struct scmi_xfers_info *minfo = &info->minfo;
> +	struct device *dev = info->dev;
> +	struct scmi_shared_mem __iomem *mem = info->tx_payload;
> +
> +	xfer_id = MSG_XTRACT_TOKEN(ioread32(&mem->msg_header));
> +
> +	/*
> +	 * Are we even expecting this?
> +	 */
Single line comment syntax probably better here.  Also the error text
makes it obvious anyway so not sure this comment adds much...

> +	if (!test_bit(xfer_id, minfo->xfer_alloc_table)) {
> +		dev_err(dev, "message for %d is not expected!\n", xfer_id);
> +		return;
> +	}
> +
> +	xfer = &minfo->xfer_block[xfer_id];
> +
> +	scmi_dump_header_dbg(dev, &xfer->hdr);
> +	/* Is the message of valid length? */
> +	if (xfer->rx.len > info->desc->max_msg_size) {
> +		dev_err(dev, "unable to handle %zu xfer(max %d)\n",
> +			xfer->rx.len, info->desc->max_msg_size);
> +		return;
> +	}
> +
> +	scmi_fetch_response(xfer, mem);
> +	complete(&xfer->done);
> +}
> +
> +/**
> + * 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.
> + */
> +static inline u32 pack_scmi_header(struct scmi_msg_hdr *hdr)
> +{
> +	return ((hdr->id & MSG_ID_MASK) << MSG_ID_SHIFT) |
> +	   ((hdr->seq & MSG_TOKEN_ID_MASK) << MSG_TOKEN_ID_SHIFT) |
> +	   ((hdr->protocol_id & MSG_PROTOCOL_ID_MASK) << MSG_PROTOCOL_ID_SHIFT);
> +}
> +
> +/**
> + * scmi_tx_prepare() - mailbox client callback to prepare for the transfer
> + *
> + * @cl: client pointer
> + * @m: mailbox message
> + *
> + * This function prepares the shared memory which contains the header and the
> + * payload.
> + */
> +static void scmi_tx_prepare(struct mbox_client *cl, void *m)
> +{
> +	struct scmi_xfer *t = m;
> +	struct scmi_info *info = client_to_scmi_info(cl);
> +	struct scmi_shared_mem __iomem *mem = info->tx_payload;
> +
> +	/* Mark channel busy + clear error */
> +	iowrite32(0x0, &mem->channel_status);
> +	iowrite32(t->hdr.poll_completion ? 0 : SCMI_SHMEM_FLAG_INTR_ENABLED,
> +		  &mem->flags);
> +	iowrite32(sizeof(mem->msg_header) + t->tx.len, &mem->length);
> +	iowrite32(pack_scmi_header(&t->hdr), &mem->msg_header);
> +	if (t->tx.buf)
> +		memcpy_toio(mem->msg_payload, t->tx.buf, t->tx.len);
> +}
> +
> +/**
> + * scmi_one_xfer_get() - Allocate one message
> + *
> + * @handle: SCMI entity handle
> + *
> + * Helper function which is used by various command 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.
> + *
> + * Return: 0 if all went fine, else corresponding error.
> + */
> +static struct scmi_xfer *scmi_one_xfer_get(const struct scmi_handle *handle)
Maybe it's just me, but I think this would be more clearly named as
scmi_xfer_alloc.

I'd assume we were dealing with one anyway as it's not called scmi_xfers_get
and the get to my mind implies a reference counter rather than allocating
an xfer for use...

> +{
> +	u16 xfer_id;
> +	struct scmi_xfer *xfer;
> +	unsigned long flags, bit_pos;
> +	struct scmi_info *info = handle_to_scmi_info(handle);
> +	struct scmi_xfers_info *minfo = &info->minfo;
> +
> +	/* 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) {
> +		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;
> +
> +	xfer = &minfo->xfer_block[xfer_id];
> +	xfer->hdr.seq = xfer_id;
> +	reinit_completion(&xfer->done);
> +
> +	return xfer;
> +}
> +
> +/**
> + * scmi_one_xfer_put() - Release a message
> + *
> + * @minfo: transfer info pointer
> + * @xfer: message that was reserved by scmi_one_xfer_get
> + *
> + * This holds a spinlock to maintain integrity of internal data structures.
> + */
> +void scmi_one_xfer_put(const struct scmi_handle *handle, struct scmi_xfer *xfer)
> +{
> +	unsigned long flags;
> +	struct scmi_info *info = handle_to_scmi_info(handle);
> +	struct scmi_xfers_info *minfo = &info->minfo;
> +
> +	/*
> +	 * 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);
> +	spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> +}
> +
> +/**
> + * scmi_do_xfer() - Do one transfer
This kind of makes my point above about no need for _one_ in naming.
You never put it here!

> + *
> + * @info: Pointer to SCMI entity information
> + * @xfer: Transfer to initiate and wait for response
> + *
> + * Return: -ETIMEDOUT in case of no response, if transmit error,
> + *   return corresponding error, else if all goes well,
> + *   return 0.
> + */
> +int scmi_do_xfer(const struct scmi_handle *handle, struct scmi_xfer *xfer)
> +{
> +	int ret;
> +	int timeout;
> +	struct scmi_info *info = handle_to_scmi_info(handle);
> +	struct device *dev = info->dev;
> +
> +	ret = mbox_send_message(info->tx_chan, xfer);
> +	if (ret < 0) {
> +		dev_dbg(dev, "mbox send fail %d\n", ret);
> +		return ret;
> +	}
> +
> +	/* mbox_send_message returns non-negative value on success, so reset */
> +	ret = 0;
> +
> +	/* And we wait for the response. */
> +	timeout = msecs_to_jiffies(info->desc->max_rx_timeout_ms);
> +	if (!wait_for_completion_timeout(&xfer->done, timeout)) {
> +		dev_err(dev, "mbox timed out in resp(caller: %pS)\n",
> +			(void *)_RET_IP_);
> +		ret = -ETIMEDOUT;
> +	} else if (xfer->hdr.status) {
> +		ret = scmi_to_linux_errno(xfer->hdr.status);
> +	}
> +	/*
> +	 * NOTE: we might prefer not to need the mailbox ticker to manage the
> +	 * transfer queueing since the protocol layer queues things by itself.
> +	 * Unfortunately, we have to kick the mailbox framework after we have
> +	 * received our message.
> +	 */
> +	mbox_client_txdone(info->tx_chan, ret);
> +
> +	return ret;
> +}
> +
> +/**
> + * scmi_one_xfer_init() - Allocate and initialise one message
Could perhaps make the alloc part of this clear in the name?

> + *
> + * @handle: SCMI entity handle
> + * @msg_id: Message identifier
> + * @msg_prot_id: Protocol identifier for the message
It's called prot_id.  Run the kernel-doc script on this an it'll probably
point out little inconsistencies like this.

> + * @tx_size: transmit message size
> + * @rx_size: receive message size
> + * @p: pointer to the allocated and initialised message
This is a pointer we want to set to this, it's not a pointer to it when
first called.

> + *
> + * This function allocates the message using @scmi_one_xfer_get and
> + * initialise the header.
If we are describing it, should describe everything - also sets the
lengths.

> + *
> + * Return: 0 if all went fine with @p pointing to message, else
> + *	corresponding error.
> + */
> +int scmi_one_xfer_init(const struct scmi_handle *handle, u8 msg_id, u8 prot_id,
> +		       size_t tx_size, size_t rx_size, struct scmi_xfer **p)
> +{
> +	int ret;
> +	struct scmi_xfer *xfer;
> +	struct scmi_info *info = handle_to_scmi_info(handle);
> +	struct device *dev = info->dev;
> +
> +	/* Ensure we have sane transfer sizes */
> +	if (rx_size > info->desc->max_msg_size ||
> +	    tx_size > info->desc->max_msg_size)
> +		return -ERANGE;
> +
> +	xfer = scmi_one_xfer_get(handle);
> +	if (IS_ERR(xfer)) {
> +		ret = PTR_ERR(xfer);
> +		dev_err(dev, "failed to get free message slot(%d)\n", ret);
> +		return ret;
> +	}
> +
> +	xfer->tx.len = tx_size;
> +	xfer->rx.len = rx_size ? : info->desc->max_msg_size;
> +	xfer->hdr.id = msg_id;
> +	xfer->hdr.protocol_id = prot_id;
> +	xfer->hdr.poll_completion = false;
> +
> +	*p = xfer;
blank line here perhaps.

> +	return 0;
> +}
> +
> +/**
> + * scmi_handle_get() - Get the  SCMI handle for a device
Spacing before SCMI is odd.

BTW this is what I'd expect a _get function to be doing - it's
retrieving the thing in question and incrementing a reference
counter to keep it around.

> + *
> + * @dev: pointer to device for which we want SCMI handle
> + *
> + * NOTE: The function does not track individual clients of the framework
> + * and is expected to be maintained by caller of  SCMI protocol library.
> + * scmi_handle_put must be balanced with successful scmi_handle_get
> + *
> + * Return: pointer to handle if successful, NULL on error
> + */
> +struct scmi_handle *scmi_handle_get(struct device *dev)
> +{
> +	struct list_head *p;
> +	struct scmi_info *info;
> +	struct scmi_handle *handle = NULL;
> +
> +	mutex_lock(&scmi_list_mutex);
> +	list_for_each(p, &scmi_list) {
> +		info = list_entry(p, struct scmi_info, node);
> +		if (dev->parent == info->dev) {
> +			handle = &info->handle;
> +			info->users++;
> +			break;
> +		}
> +	}
> +	mutex_unlock(&scmi_list_mutex);
> +
> +	return handle;
> +}
> +
> +/**
> + * scmi_handle_put() - Release the handle acquired by scmi_handle_get
> + *
> + * @handle: handle acquired by scmi_handle_get
> + *
> + * NOTE: The function does not track individual clients of the framework
> + * and is expected to be maintained by caller of  SCMI protocol library.
Again, odd spacing before SCMI..

> + * scmi_handle_put must be balanced with successful scmi_handle_get
> + *
> + * Return: 0 is successfully released
> + *	if null was passed, it returns -EINVAL;
> + */
> +int scmi_handle_put(const struct scmi_handle *handle)
> +{
> +	struct scmi_info *info;
> +
> +	if (!handle)
> +		return -EINVAL;
> +
> +	info = handle_to_scmi_info(handle);
> +	mutex_lock(&scmi_list_mutex);
> +	if (!WARN_ON(!info->users))
> +		info->users--;
> +	mutex_unlock(&scmi_list_mutex);
> +
> +	return 0;
> +}
> +
> +static const struct scmi_desc scmi_generic_desc = {
> +	.max_rx_timeout_ms = 30,	/* we may increase this if required */
Inconsistent capitalization of comment. Doesn't really matter which but looks
a bit odd here with it different on two lines next to each other.

> +	.max_msg = 20,		/* Limited by MBOX_TX_QUEUE_LEN */
> +	.max_msg_size = 128,
> +};
> +
> +/* Each compatible listed below must have descriptor associated with it */
> +static const struct of_device_id scmi_of_match[] = {
> +	{ .compatible = "arm,scmi", .data = &scmi_generic_desc },
> +	{ /* Sentinel */ },
> +};
> +
> +MODULE_DEVICE_TABLE(of, scmi_of_match);
> +
> +static int scmi_xfer_info_init(struct scmi_info *sinfo)
> +{
> +	int i;
> +	struct scmi_xfer *xfer;
> +	struct device *dev = sinfo->dev;
> +	const struct scmi_desc *desc = sinfo->desc;
> +	struct scmi_xfers_info *info = &sinfo->minfo;
> +
> +	/* Pre-allocated messages, no more than what hdr.seq can support */
> +	if (WARN_ON(desc->max_msg >= (MSG_TOKEN_ID_MASK + 1))) {
> +		dev_err(dev, "Maximum message of %d exceeds supported %d\n",
> +			desc->max_msg, MSG_TOKEN_ID_MASK + 1);
> +		return -EINVAL;
> +	}
> +
> +	info->xfer_block = devm_kcalloc(dev, desc->max_msg,
> +					sizeof(*info->xfer_block), GFP_KERNEL);
> +	if (!info->xfer_block)
> +		return -ENOMEM;
> +
> +	info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(desc->max_msg),
> +					      sizeof(long), GFP_KERNEL);
> +	if (!info->xfer_alloc_table)
> +		return -ENOMEM;
Hmm. I wonder if it is worth adding a devm_bitmap_alloc?

> +
> +	bitmap_zero(info->xfer_alloc_table, desc->max_msg);
kcalloc zeros the memory.

> +
> +	/* Pre-initialize the buffer pointer to pre-allocated buffers */
> +	for (i = 0, xfer = info->xfer_block; i < desc->max_msg; i++, xfer++) {
> +		xfer->rx.buf = devm_kcalloc(dev, sizeof(u8), desc->max_msg_size,
> +					    GFP_KERNEL);
> +		if (!xfer->rx.buf)
> +			return -ENOMEM;
> +
> +		xfer->tx.buf = xfer->rx.buf;
> +		init_completion(&xfer->done);
> +	}
> +
> +	spin_lock_init(&info->xfer_lock);
> +
> +	return 0;
> +}
> +
> +static int scmi_mailbox_check(struct device_node *np)
> +{
> +	struct of_phandle_args arg;
> +
> +	return of_parse_phandle_with_args(np, "mboxes", "#mbox-cells", 0, &arg);
> +}
> +
> +static int scmi_mbox_free_channel(struct scmi_info *info)
Some of the naming in here could do with being adjusted to be obviously
'balanced'.  The moment I see a free I expect a matched alloc but in this
case the alloc is done in scmi_mbox_chan_setup which at very least
should be scmi_mbox_setup_channel and should really imply that it is
doing allocation as well.

> +{
> +	if (!IS_ERR_OR_NULL(info->tx_chan)) {
> +		mbox_free_channel(info->tx_chan);
> +		info->tx_chan = NULL;
> +	}
> +
> +	return 0;
> +}
> +
> +static int scmi_remove(struct platform_device *pdev)
> +{
> +	int ret = 0;
> +	struct scmi_info *info = platform_get_drvdata(pdev);
> +
> +	mutex_lock(&scmi_list_mutex);
> +	if (info->users)
> +		ret = -EBUSY;
> +	else
> +		list_del(&info->node);
> +	mutex_unlock(&scmi_list_mutex);
> +
> +	if (!ret)
This would have a more readable flow if you just did
if (ret)
	return ret;

return sci_mbox_free_channel(info)

> +		/* Safe to free channels since no more users */
> +		return scmi_mbox_free_channel(info);
> +
> +	return ret;
> +}
> +
> +static inline int scmi_mbox_chan_setup(struct scmi_info *info)
> +{
> +	int ret;
> +	struct resource res;
> +	resource_size_t size;
> +	struct device *dev = info->dev;
> +	struct device_node *shmem, *np = dev->of_node;
> +	struct mbox_client *cl;
> +
> +	cl = &info->cl;
> +	cl->dev = dev;
> +	cl->rx_callback = scmi_rx_callback;
> +	cl->tx_prepare = scmi_tx_prepare;
> +	cl->tx_block = false;
> +	cl->knows_txdone = true;
> +
> +	shmem = of_parse_phandle(np, "shmem", 0);
> +	ret = of_address_to_resource(shmem, 0, &res);
> +	of_node_put(shmem);
> +	if (ret) {
> +		dev_err(dev, "failed to get SCMI Tx payload mem resource\n");
> +		return ret;
> +	}
> +
> +	size = resource_size(&res);
> +	info->tx_payload = devm_ioremap(dev, res.start, size);
> +	if (!info->tx_payload) {
> +		dev_err(dev, "failed to ioremap SCMI Tx payload\n");
> +		return -EADDRNOTAVAIL;
> +	}
> +
> +	/* Transmit channel is first entry i.e. index 0 */
> +	info->tx_chan = mbox_request_channel(cl, 0);
> +	if (IS_ERR(info->tx_chan)) {
> +		ret = PTR_ERR(info->tx_chan);
> +		if (ret != -EPROBE_DEFER)
> +			dev_err(dev, "failed to request SCMI Tx mailbox\n");
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +static int scmi_probe(struct platform_device *pdev)
> +{
> +	int ret;
> +	struct scmi_handle *handle;
> +	const struct scmi_desc *desc;
> +	struct scmi_info *info;
> +	struct device *dev = &pdev->dev;
> +	struct device_node *np = dev->of_node;
> +
> +	/* Only mailbox method supported, check for the presence of one */
> +	if (scmi_mailbox_check(np)) {
> +		dev_err(dev, "no mailbox found in %pOF\n", np);
> +		return -EINVAL;
> +	}
> +
> +	desc = of_match_device(scmi_of_match, dev)->data;
> +
> +	info = devm_kzalloc(dev, sizeof(*info), GFP_KERNEL);
> +	if (!info)
> +		return -ENOMEM;
> +
> +	info->dev = dev;
> +	info->desc = desc;
> +	INIT_LIST_HEAD(&info->node);
> +
> +	ret = scmi_xfer_info_init(info);
> +	if (ret)
> +		return ret;
> +
> +	platform_set_drvdata(pdev, info);
> +
> +	handle = &info->handle;
> +	handle->dev = info->dev;
> +
> +	ret = scmi_mbox_chan_setup(info);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&scmi_list_mutex);
> +	list_add_tail(&info->node, &scmi_list);
> +	mutex_unlock(&scmi_list_mutex);
> +
> +	return 0;
> +}
> +
> +static struct platform_driver scmi_driver = {
> +	.driver = {
> +		   .name = "arm-scmi",
> +		   .of_match_table = scmi_of_match,
> +		   },
> +	.probe = scmi_probe,
> +	.remove = scmi_remove,
> +};
> +
> +module_platform_driver(scmi_driver);
> +
> +MODULE_ALIAS("platform: arm-scmi");
> +MODULE_AUTHOR("Sudeep Holla <sudeep.holla@arm.com>");
> +MODULE_DESCRIPTION("ARM SCMI protocol driver");
> +MODULE_LICENSE("GPL v2");
> diff --git a/include/linux/scmi_protocol.h b/include/linux/scmi_protocol.h
> new file mode 100644
> index 000000000000..854ed2479993
> --- /dev/null
> +++ b/include/linux/scmi_protocol.h
> @@ -0,0 +1,27 @@
> +/*
> + * SCMI Message Protocol driver header
> + *
> + * Copyright (C) 2017 ARM Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along with
> + * this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +#include <linux/types.h>
> +
> +/**
> + * struct scmi_handle - Handle returned to ARM SCMI clients for usage.
> + *
> + * @dev: pointer to the SCMI device
> + */
> +struct scmi_handle {
> +	struct device *dev;
> +};