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SubjectRe: [PATCHv7 2/5] mailbox: Introduce framework for mailbox
Hi Jassi,

I started using this from v5 and tried briefly to follow previous versions
and discussion. Please forgive me if I ask questions that are already answered.

On 12/06/14 18:01, Jassi Brar wrote:
> Introduce common framework for client/protocol drivers and
> controller drivers of Inter-Processor-Communication (IPC).
>
> Client driver developers should have a look at
> include/linux/mailbox_client.h to understand the part of
> the API exposed to client drivers.
> Similarly controller driver developers should have a look
> at include/linux/mailbox_controller.h
>
> Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
> ---
> drivers/mailbox/Makefile | 4 +
> drivers/mailbox/mailbox.c | 487 +++++++++++++++++++++++++++++++++++++
> include/linux/mailbox_client.h | 45 ++++
> include/linux/mailbox_controller.h | 121 +++++++++
> 4 files changed, 657 insertions(+)
> create mode 100644 drivers/mailbox/mailbox.c
> create mode 100644 include/linux/mailbox_client.h
> create mode 100644 include/linux/mailbox_controller.h
>
> diff --git a/drivers/mailbox/Makefile b/drivers/mailbox/Makefile
> index e0facb3..2fa343a 100644
> --- a/drivers/mailbox/Makefile
> +++ b/drivers/mailbox/Makefile
> @@ -1,3 +1,7 @@
> +# Generic MAILBOX API
> +
> +obj-$(CONFIG_MAILBOX) += mailbox.o
> +
> obj-$(CONFIG_PL320_MBOX) += pl320-ipc.o
>
> obj-$(CONFIG_OMAP_MBOX) += omap-mailbox.o
> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> new file mode 100644
> index 0000000..336fa10
> --- /dev/null
> +++ b/drivers/mailbox/mailbox.c
> @@ -0,0 +1,487 @@
> +/*
> + * Mailbox: Common code for Mailbox controllers and users
> + *
> + * Copyright (C) 2014 Linaro Ltd.
> + * Author: Jassi Brar <jassisinghbrar@gmail.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#include <linux/interrupt.h>
> +#include <linux/spinlock.h>
> +#include <linux/mutex.h>
> +#include <linux/delay.h>
> +#include <linux/slab.h>
> +#include <linux/err.h>
> +#include <linux/module.h>
> +#include <linux/device.h>
> +#include <linux/mailbox_client.h>
> +#include <linux/mailbox_controller.h>
> +
> +#define TXDONE_BY_IRQ (1 << 0) /* controller has remote RTR irq */
> +#define TXDONE_BY_POLL (1 << 1) /* controller can read status of last TX */
> +#define TXDONE_BY_ACK (1 << 2) /* S/W ACK recevied by Client ticks the TX */
> +
> +static LIST_HEAD(mbox_cons);
> +static DEFINE_MUTEX(con_mutex);
> +
> +static int _add_to_rbuf(struct mbox_chan *chan, void *mssg)
> +{
> + int idx;
> + unsigned long flags;
> +
> + spin_lock_irqsave(&chan->lock, flags);
> +
> + /* See if there is any space left */
> + if (chan->msg_count == MBOX_TX_QUEUE_LEN) {
> + spin_unlock_irqrestore(&chan->lock, flags);
> + return -ENOMEM;
> + }
> +

Any particular reason why the standard list implementation can't be used
instead of this. It eliminates limitation of MBOX_TX_QUEUE_LEN and would
remove msg_free/count

> + idx = chan->msg_free;
> + chan->msg_data[idx] = mssg;
> + chan->msg_count++;
> +
> + if (idx == MBOX_TX_QUEUE_LEN - 1)
> + chan->msg_free = 0;
> + else
> + chan->msg_free++;
> +
> + spin_unlock_irqrestore(&chan->lock, flags);
> +
> + return idx;
> +}
> +
> +static void _msg_submit(struct mbox_chan *chan)
> +{
> + unsigned count, idx;
> + unsigned long flags;
> + void *data;
> + int err;
> +
> + spin_lock_irqsave(&chan->lock, flags);
> +
> + if (!chan->msg_count || chan->active_req) {
> + spin_unlock_irqrestore(&chan->lock, flags);
> + return;
> + }
> +
> + count = chan->msg_count;
> + idx = chan->msg_free;
> + if (idx >= count)
> + idx -= count;
> + else
> + idx += MBOX_TX_QUEUE_LEN - count;
> +
> + data = chan->msg_data[idx];
> +
> + /* Try to submit a message to the MBOX controller */
> + err = chan->mbox->ops->send_data(chan, data);
> + if (!err) {
> + chan->active_req = data;
> + chan->msg_count--;
> + }
> +
> + spin_unlock_irqrestore(&chan->lock, flags);
> +}
> +
> +static void tx_tick(struct mbox_chan *chan, int r)
> +{
> + unsigned long flags;
> + void *mssg;
> +
> + spin_lock_irqsave(&chan->lock, flags);
> + mssg = chan->active_req;
> + chan->active_req = NULL;
> + spin_unlock_irqrestore(&chan->lock, flags);
> +
> + /* Submit next message */
> + _msg_submit(chan);
> +
> + /* Notify the client */
> + if (chan->cl->tx_block)
> + complete(&chan->tx_complete);
> + else if (mssg && chan->cl->tx_done)
> + chan->cl->tx_done(chan->cl, mssg, r);
> +}
> +
> +static void poll_txdone(unsigned long data)
> +{
> + struct mbox_controller *mbox = (struct mbox_controller *)data;
> + bool txdone, resched = false;
> + int i;
> +
> + for (i = 0; i < mbox->num_chans; i++) {
> + struct mbox_chan *chan = &mbox->chans[i];
> +
> + if (chan->active_req && chan->cl) {
> + resched = true;
> + txdone = chan->mbox->ops->last_tx_done(chan);
> + if (txdone)
> + tx_tick(chan, 0);
> + }
> + }
> +
> + if (resched)
> + mod_timer(&mbox->poll,
> + jiffies + msecs_to_jiffies(mbox->period));

[Nit] Alignment here and few other places to '('

> +}
> +
> +/**
> + * mbox_chan_received_data - A way for controller driver to push data
> + * received from remote to the upper layer.
> + * @chan: Pointer to the mailbox channel on which RX happened.
> + * @data: Client specific message typecasted as void *
> + *

[Nit] 'data' needs to be changed to 'mssg' or vice-versa

> + * After startup and before shutdown any data received on the chan
> + * is passed on to the API via atomic mbox_chan_received_data().
> + * The controller should ACK the RX only after this call returns.

Does this mean we can't support asynchronous messages from the remote.
One possible scenario I can think is if the remote system power controller
has feature to configure the bounds for thermal sensors and it can send
async interrupt when the bounds are crossed. We can't just block one channel
for this always. Again this might have been discussed before and you might have
solution, I could not gather it with my brief look at older discussions.

> + */
> +void mbox_chan_received_data(struct mbox_chan *chan, void *mssg)
> +{
> + /* No buffering the received data */
> + if (chan->cl->rx_callback)
> + chan->cl->rx_callback(chan->cl, mssg);
> +}
> +EXPORT_SYMBOL_GPL(mbox_chan_received_data);
> +
> +/**
> + * mbox_chan_txdone - A way for controller driver to notify the
> + * framework that the last TX has completed.
> + * @chan: Pointer to the mailbox chan on which TX happened.
> + * @r: Status of last TX - OK or ERROR
> + *
> + * The controller that has IRQ for TX ACK calls this atomic API
> + * to tick the TX state machine. It works only if txdone_irq
> + * is set by the controller.
> + */
> +void mbox_chan_txdone(struct mbox_chan *chan, int r)
> +{
> + if (unlikely(!(chan->txdone_method & TXDONE_BY_IRQ))) {
> + pr_err("Controller can't run the TX ticker\n");
> + return;
> + }
> +
> + tx_tick(chan, r);
> +}
> +EXPORT_SYMBOL_GPL(mbox_chan_txdone);
> +
> +/**
> + * mbox_client_txdone - The way for a client to run the TX state machine.
> + * @chan: Mailbox channel assigned to this client.
> + * @r: Success status of last transmission.
> + *
> + * The client/protocol had received some 'ACK' packet and it notifies
> + * the API that the last packet was sent successfully. This only works
> + * if the controller can't sense TX-Done.
> + */
> +void mbox_client_txdone(struct mbox_chan *chan, int r)
> +{
> + if (unlikely(!(chan->txdone_method & TXDONE_BY_ACK))) {
> + pr_err("Client can't run the TX ticker\n");
> + return;
> + }
> +
> + tx_tick(chan, r);
> +}
> +EXPORT_SYMBOL_GPL(mbox_client_txdone);
> +
> +/**
> + * mbox_client_peek_data - A way for client driver to pull data
> + * received from remote by the controller.
> + * @chan: Mailbox channel assigned to this client.
> + *
> + * A poke to controller driver for any received data.
> + * The data is actually passed onto client via the
> + * mbox_chan_received_data()
> + * The call can be made from atomic context, so the controller's
> + * implementation of peek_data() must not sleep.
> + *
> + * Return: True, if controller has, and is going to push after this,
> + * some data.
> + * False, if controller doesn't have any data to be read.
> + */
> +bool mbox_client_peek_data(struct mbox_chan *chan)
> +{
> + if (chan->mbox->ops->peek_data)
> + return chan->mbox->ops->peek_data(chan);
> +
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(mbox_client_peek_data);

I am unable to understand how this API will be used. IIUC when the controller
receives any data from remote, it calls mbox_chan_received_data to push data to
client.

> +
> +/**
> + * mbox_send_message - For client to submit a message to be
> + * sent to the remote.
> + * @chan: Mailbox channel assigned to this client.
> + * @mssg: Client specific message typecasted.
> + *
> + * For client to submit data to the controller destined for a remote
> + * processor. If the client had set 'tx_block', the call will return
> + * either when the remote receives the data or when 'tx_tout' millisecs
> + * run out.
> + * In non-blocking mode, the requests are buffered by the API and a
> + * non-negative token is returned for each queued request. If the request
> + * is not queued, a negative token is returned. Upon failure or successful
> + * TX, the API calls 'tx_done' from atomic context, from which the client
> + * could submit yet another request.
> + * In blocking mode, 'tx_done' is not called, effectively making the
> + * queue length 1.
> + * The pointer to message should be preserved until it is sent
> + * over the chan, i.e, tx_done() is made.
> + * This function could be called from atomic context as it simply
> + * queues the data and returns a token against the request.
> + *
> + * Return: Non-negative integer for successful submission (non-blocking mode)
> + * or transmission over chan (blocking mode).
> + * Negative value denotes failure.
> + */
> +int mbox_send_message(struct mbox_chan *chan, void *mssg)
> +{
> + int t;
> +
> + if (!chan || !chan->cl)
> + return -EINVAL;
> +
> + t = _add_to_rbuf(chan, mssg);
> + if (t < 0) {
> + pr_err("Try increasing MBOX_TX_QUEUE_LEN\n");
> + return t;
> + }
> +
> + _msg_submit(chan);
> +
> + reinit_completion(&chan->tx_complete);
> +
> + if (chan->txdone_method == TXDONE_BY_POLL)
> + poll_txdone((unsigned long)chan->mbox);
> +
> + if (chan->cl->tx_block && chan->active_req) {
> + unsigned long wait;
> + int ret;
> +
> + if (!chan->cl->tx_tout) /* wait for ever */
> + wait = msecs_to_jiffies(3600000);
> + else
> + wait = msecs_to_jiffies(chan->cl->tx_tout);
> +
> + ret = wait_for_completion_timeout(&chan->tx_complete, wait);
> + if (ret == 0) {
> + t = -EIO;
> + tx_tick(chan, -EIO);
> + }
> + }
> +
> + return t;
> +}
> +EXPORT_SYMBOL_GPL(mbox_send_message);
> +
> +/**
> + * mbox_request_channel - Request a mailbox channel.
> + * @cl: Identity of the client requesting the channel.
> + *
> + * The Client specifies its requirements and capabilities while asking for
> + * a mailbox channel. It can't be called from atomic context.
> + * The channel is exclusively allocated and can't be used by another
> + * client before the owner calls mbox_free_channel.
> + * After assignment, any packet received on this channel will be
> + * handed over to the client via the 'rx_callback'.
> + * The framework holds reference to the client, so the mbox_client
> + * structure shouldn't be modified until the mbox_free_channel returns.
> + *
> + * Return: Pointer to the channel assigned to the client if successful.
> + * ERR_PTR for request failure.
> + */
> +struct mbox_chan *mbox_request_channel(const struct mbox_client *cl)
> +{
> + struct device *dev = cl->dev;
> + struct mbox_controller *mbox;
> + struct of_phandle_args spec;
> + struct mbox_chan *chan;
> + unsigned long flags;
> + int count, i, ret;
> +
> + if (!dev || !dev->of_node) {
> + pr_err("%s: No owner device node\n", __func__);
> + return ERR_PTR(-ENODEV);
> + }
> +
> + count = of_property_count_strings(dev->of_node, "mbox-names");
> + if (count < 0) {
> + pr_err("%s: mbox-names property of node '%s' missing\n",
> + __func__, dev->of_node->full_name);
> + return ERR_PTR(-ENODEV);
> + }
> +
> + mutex_lock(&con_mutex);
> +
> + ret = -ENODEV;
> + for (i = 0; i < count; i++) {
> + const char *s;
> +
> + if (of_property_read_string_index(dev->of_node,
> + "mbox-names", i, &s))
> + continue;
> +
> + if (strcmp(cl->chan_name, s))
> + continue;
> +
> + if (of_parse_phandle_with_args(dev->of_node,
> + "mbox", "#mbox-cells", i, &spec))
> + continue;
> +
> + chan = NULL;
> + list_for_each_entry(mbox, &mbox_cons, node)
> + if (mbox->dev->of_node == spec.np) {
> + chan = mbox->of_xlate(mbox, &spec);
> + break;
> + }
> +
> + of_node_put(spec.np);
> +
> + if (!chan)
> + continue;
> +
> + ret = -EBUSY;
> + if (!chan->cl && try_module_get(mbox->dev->driver->owner))
> + break;
> + }
> +
> + if (i == count) {
> + mutex_unlock(&con_mutex);
> + return ERR_PTR(ret);
> + }
> +
> + spin_lock_irqsave(&chan->lock, flags);
> + chan->msg_free = 0;
> + chan->msg_count = 0;
> + chan->active_req = NULL;
> + chan->cl = cl;
> + init_completion(&chan->tx_complete);
> +
> + if (chan->txdone_method == TXDONE_BY_POLL
> + && cl->knows_txdone)
> + chan->txdone_method |= TXDONE_BY_ACK;
> + spin_unlock_irqrestore(&chan->lock, flags);
> +
> + ret = chan->mbox->ops->startup(chan);
> + if (ret) {
> + pr_err("Unable to startup the chan (%d)\n", ret);
> + mbox_free_channel(chan);
> + chan = ERR_PTR(ret);
> + }
> +
> + mutex_unlock(&con_mutex);
> + return chan;
> +}
> +EXPORT_SYMBOL_GPL(mbox_request_channel);
> +
> +/**
> + * mbox_free_channel - The client relinquishes control of a mailbox
> + * channel by this call.
> + * @chan: The mailbox channel to be freed.
> + */
> +void mbox_free_channel(struct mbox_chan *chan)
> +{
> + unsigned long flags;
> +
> + if (!chan || !chan->cl)
> + return;
> +
> + chan->mbox->ops->shutdown(chan);
> +
> + /* The queued TX requests are simply aborted, no callbacks are made */
> + spin_lock_irqsave(&chan->lock, flags);
> + chan->cl = NULL;
> + chan->active_req = NULL;
> + if (chan->txdone_method == (TXDONE_BY_POLL | TXDONE_BY_ACK))
> + chan->txdone_method = TXDONE_BY_POLL;
> +
> + module_put(chan->mbox->dev->driver->owner);
> + spin_unlock_irqrestore(&chan->lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(mbox_free_channel);
> +
> +static struct mbox_chan *
> +of_mbox_index_xlate(struct mbox_controller *mbox,
> + const struct of_phandle_args *sp)
> +{
> + int ind = sp->args[0];
> +
> + if (ind >= mbox->num_chans)
> + return NULL;
> +
> + return &mbox->chans[ind];
> +}
> +
> +/**
> + * mbox_controller_register - Register the mailbox controller
> + * @mbox: Pointer to the mailbox controller.
> + *
> + * The controller driver registers its communication chans
> + */
> +int mbox_controller_register(struct mbox_controller *mbox)
> +{
> + int i, txdone;
> +
> + /* Sanity check */
> + if (!mbox || !mbox->dev || !mbox->ops || !mbox->num_chans)
> + return -EINVAL;
> +
> + if (mbox->txdone_irq)
> + txdone = TXDONE_BY_IRQ;
> + else if (mbox->txdone_poll)
> + txdone = TXDONE_BY_POLL;
> + else /* It has to be ACK then */
> + txdone = TXDONE_BY_ACK;
> +
> + if (txdone == TXDONE_BY_POLL) {
> + mbox->poll.function = &poll_txdone;
> + mbox->poll.data = (unsigned long)mbox;
> + init_timer(&mbox->poll);
> + }
> +
> + for (i = 0; i < mbox->num_chans; i++) {
> + struct mbox_chan *chan = &mbox->chans[i];

[Nit] Newline to separate declarations

> + chan->cl = NULL;
> + chan->mbox = mbox;
> + chan->txdone_method = txdone;
> + spin_lock_init(&chan->lock);
> + }
> +
> + if (!mbox->of_xlate)
> + mbox->of_xlate = of_mbox_index_xlate;
> +
> + mutex_lock(&con_mutex);
> + list_add_tail(&mbox->node, &mbox_cons);
> + mutex_unlock(&con_mutex);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(mbox_controller_register);
> +
> +/**
> + * mbox_controller_unregister - UnRegister the mailbox controller
> + * @mbox: Pointer to the mailbox controller.
> + */
> +void mbox_controller_unregister(struct mbox_controller *mbox)
> +{
> + int i;
> +
> + if (!mbox)
> + return;
> +
> + mutex_lock(&con_mutex);
> +
> + list_del(&mbox->node);
> +
> + for (i = 0; i < mbox->num_chans; i++)
> + mbox_free_channel(&mbox->chans[i]);
> +
> + del_timer_sync(&mbox->poll);
> +
> + mutex_unlock(&con_mutex);
> +}
> +EXPORT_SYMBOL_GPL(mbox_controller_unregister);
> diff --git a/include/linux/mailbox_client.h b/include/linux/mailbox_client.h
> new file mode 100644
> index 0000000..53eb078
> --- /dev/null
> +++ b/include/linux/mailbox_client.h
> @@ -0,0 +1,45 @@
> +/*
> + * Copyright (C) 2014 Linaro Ltd.
> + * Author: Jassi Brar <jassisinghbrar@gmail.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#ifndef __MAILBOX_CLIENT_H
> +#define __MAILBOX_CLIENT_H
> +
> +#include <linux/of.h>
> +
> +struct mbox_chan;
> +
> +/**
> + * struct mbox_client - User of a mailbox
> + * @dev: The client device
> + * @chan_name: The "controller:channel" this client wants
> + * @rx_callback: Atomic callback to provide client the data received
> + * @tx_done: Atomic callback to tell client of data transmission
> + * @tx_block: If the mbox_send_message should block until data is
> + * transmitted.
> + * @tx_tout: Max block period in ms before TX is assumed failure
> + * @knows_txdone: if the client could run the TX state machine. Usually
> + * if the client receives some ACK packet for transmission.
> + * Unused if the controller already has TX_Done/RTR IRQ.
> + */
> +struct mbox_client {
> + struct device *dev;
> + const char *chan_name;
> + void (*rx_callback)(struct mbox_client *cl, void *mssg);
> + void (*tx_done)(struct mbox_client *cl, void *mssg, int r);
> + bool tx_block;
> + unsigned long tx_tout;
> + bool knows_txdone;
> +};
> +
> +struct mbox_chan *mbox_request_channel(const struct mbox_client *cl);
> +int mbox_send_message(struct mbox_chan *chan, void *mssg);
> +void mbox_client_txdone(struct mbox_chan *chan, int r);
> +void mbox_free_channel(struct mbox_chan *chan);
> +
> +#endif /* __MAILBOX_CLIENT_H */
> diff --git a/include/linux/mailbox_controller.h b/include/linux/mailbox_controller.h
> new file mode 100644
> index 0000000..5d1915b
> --- /dev/null
> +++ b/include/linux/mailbox_controller.h
> @@ -0,0 +1,121 @@
> +/*
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#ifndef __MAILBOX_CONTROLLER_H
> +#define __MAILBOX_CONTROLLER_H
> +
> +#include <linux/of.h>
> +
> +struct mbox_chan;
> +
> +/**
> + * struct mbox_chan_ops - s/w representation of a communication chan
> + * @send_data: The API asks the MBOX controller driver, in atomic
> + * context try to transmit a message on the bus. Returns 0 if
> + * data is accepted for transmission, -EBUSY while rejecting
> + * if the remote hasn't yet read the last data sent. Actual
> + * transmission of data is reported by the controller via
> + * mbox_chan_txdone (if it has some TX ACK irq). It must not
> + * block.
> + * @startup: Called when a client requests the chan. The controller
> + * could ask clients for additional parameters of communication
> + * to be provided via client's chan_data. This call may
> + * block. After this call the Controller must forward any
> + * data received on the chan by calling mbox_chan_received_data.
> + * @shutdown: Called when a client relinquishes control of a chan.
> + * This call may block too. The controller must not forwared
> + * any received data anymore.
> + * @last_tx_done: If the controller sets 'txdone_poll', the API calls
> + * this to poll status of last TX. The controller must
> + * give priority to IRQ method over polling and never
> + * set both txdone_poll and txdone_irq. Only in polling
> + * mode 'send_data' is expected to return -EBUSY.
> + * Used only if txdone_poll:=true && txdone_irq:=false
> + * @peek_data: Atomic check for any received data. Return true if controller
> + * has some data to push to the client. False otherwise.
> + */
> +struct mbox_chan_ops {
> + int (*send_data)(struct mbox_chan *chan, void *data);
> + int (*startup)(struct mbox_chan *chan);
> + void (*shutdown)(struct mbox_chan *chan);
> + bool (*last_tx_done)(struct mbox_chan *chan);
> + bool (*peek_data)(struct mbox_chan *chan);
> +};
> +
> +/**
> + * struct mbox_controller - Controller of a class of communication chans
> + * @dev: Device backing this controller
> + * @controller_name: Literal name of the controller.

Looks like this is gone now

> + * @ops: Operators that work on each communication chan
> + * @chans: Null terminated array of chans.
> + * @txdone_irq: Indicates if the controller can report to API when
> + * the last transmitted data was read by the remote.
> + * Eg, if it has some TX ACK irq.
> + * @txdone_poll: If the controller can read but not report the TX
> + * done. Ex, some register shows the TX status but
> + * no interrupt rises. Ignored if 'txdone_irq' is set.
> + * @txpoll_period: If 'txdone_poll' is in effect, the API polls for
> + * last TX's status after these many millisecs

Does it makes sense to add description for of_xlate and num_chans ?

> + */
> +struct mbox_controller {
> + struct device *dev;
> + struct mbox_chan_ops *ops;
> + struct mbox_chan *chans;
> + int num_chans;
> + bool txdone_irq;
> + bool txdone_poll;
> + unsigned txpoll_period;
> + struct mbox_chan *(*of_xlate)(struct mbox_controller *mbox,
> + const struct of_phandle_args *sp);
> + /*
> + * If the controller supports only TXDONE_BY_POLL,
> + * this timer polls all the links for txdone.
> + */
> + struct timer_list poll;
> + unsigned period;
> + /* Hook to add to the global controller list */
> + struct list_head node;
> +};
> +
> +/*
> + * The length of circular buffer for queuing messages from a client.
> + * 'msg_count' tracks the number of buffered messages while 'msg_free'
> + * is the index where the next message would be buffered.
> + * We shouldn't need it too big because every transferr is interrupt
> + * triggered and if we have lots of data to transfer, the interrupt
> + * latencies are going to be the bottleneck, not the buffer length.
> + * Besides, mbox_send_message could be called from atomic context and
> + * the client could also queue another message from the notifier 'tx_done'
> + * of the last transfer done.
> + * REVIST: If too many platforms see the "Try increasing MBOX_TX_QUEUE_LEN"
> + * print, it needs to be taken from config option or somesuch.
> + */
> +#define MBOX_TX_QUEUE_LEN 20
> +

As mentioned before IMO using std list might eliminate this.

> +struct mbox_chan {
> + struct mbox_controller *mbox; /* Parent Controller */
> + unsigned txdone_method;
> +
> + /* client */
> + struct mbox_client *cl;
> + struct completion tx_complete;
> +
> + void *active_req;
> + unsigned msg_count, msg_free;
> + void *msg_data[MBOX_TX_QUEUE_LEN];
> + /* Access to the channel */
> + spinlock_t lock;
> +
> + /* Private data for controller */
> + void *con_priv;
> +};
> +
> +int mbox_controller_register(struct mbox_controller *mbox);
> +void mbox_chan_received_data(struct mbox_chan *chan, void *data);
> +void mbox_chan_txdone(struct mbox_chan *chan, int r);
> +void mbox_controller_unregister(struct mbox_controller *mbox);
> +
> +#endif /* __MAILBOX_CONTROLLER_H */
> --
> 1.8.1.2
>
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