doc: add documentation for mailbox framework
Some explanations with examples of how to write to implement users and providers of the mailbox framework. Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
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Jassi Brar
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The Common Mailbox Framework
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Jassi Brar <jaswinder.singh@linaro.org>
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This document aims to help developers write client and controller
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drivers for the API. But before we start, let us note that the
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client (especially) and controller drivers are likely going to be
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very platform specific because the remote firmware is likely to be
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proprietary and implement non-standard protocol. So even if two
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platforms employ, say, PL320 controller, the client drivers can't
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be shared across them. Even the PL320 driver might need to accommodate
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some platform specific quirks. So the API is meant mainly to avoid
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similar copies of code written for each platform. Having said that,
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nothing prevents the remote f/w to also be Linux based and use the
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same api there. However none of that helps us locally because we only
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ever deal at client's protocol level.
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Some of the choices made during implementation are the result of this
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peculiarity of this "common" framework.
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Part 1 - Controller Driver (See include/linux/mailbox_controller.h)
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Allocate mbox_controller and the array of mbox_chan.
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Populate mbox_chan_ops, except peek_data() all are mandatory.
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The controller driver might know a message has been consumed
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by the remote by getting an IRQ or polling some hardware flag
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or it can never know (the client knows by way of the protocol).
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The method in order of preference is IRQ -> Poll -> None, which
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the controller driver should set via 'txdone_irq' or 'txdone_poll'
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or neither.
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Part 2 - Client Driver (See include/linux/mailbox_client.h)
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The client might want to operate in blocking mode (synchronously
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send a message through before returning) or non-blocking/async mode (submit
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a message and a callback function to the API and return immediately).
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struct demo_client {
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struct mbox_client cl;
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struct mbox_chan *mbox;
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struct completion c;
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bool async;
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/* ... */
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};
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/*
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* This is the handler for data received from remote. The behaviour is purely
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* dependent upon the protocol. This is just an example.
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*/
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static void message_from_remote(struct mbox_client *cl, void *mssg)
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{
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struct demo_client *dc = container_of(mbox_client,
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struct demo_client, cl);
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if (dc->aysnc) {
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if (is_an_ack(mssg)) {
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/* An ACK to our last sample sent */
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return; /* Or do something else here */
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} else { /* A new message from remote */
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queue_req(mssg);
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}
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} else {
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/* Remote f/w sends only ACK packets on this channel */
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return;
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}
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}
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static void sample_sent(struct mbox_client *cl, void *mssg, int r)
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{
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struct demo_client *dc = container_of(mbox_client,
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struct demo_client, cl);
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complete(&dc->c);
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}
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static void client_demo(struct platform_device *pdev)
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{
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struct demo_client *dc_sync, *dc_async;
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/* The controller already knows async_pkt and sync_pkt */
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struct async_pkt ap;
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struct sync_pkt sp;
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dc_sync = kzalloc(sizeof(*dc_sync), GFP_KERNEL);
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dc_async = kzalloc(sizeof(*dc_async), GFP_KERNEL);
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/* Populate non-blocking mode client */
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dc_async->cl.dev = &pdev->dev;
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dc_async->cl.rx_callback = message_from_remote;
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dc_async->cl.tx_done = sample_sent;
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dc_async->cl.tx_block = false;
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dc_async->cl.tx_tout = 0; /* doesn't matter here */
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dc_async->cl.knows_txdone = false; /* depending upon protocol */
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dc_async->async = true;
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init_completion(&dc_async->c);
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/* Populate blocking mode client */
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dc_sync->cl.dev = &pdev->dev;
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dc_sync->cl.rx_callback = message_from_remote;
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dc_sync->cl.tx_done = NULL; /* operate in blocking mode */
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dc_sync->cl.tx_block = true;
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dc_sync->cl.tx_tout = 500; /* by half a second */
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dc_sync->cl.knows_txdone = false; /* depending upon protocol */
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dc_sync->async = false;
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/* ASync mailbox is listed second in 'mboxes' property */
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dc_async->mbox = mbox_request_channel(&dc_async->cl, 1);
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/* Populate data packet */
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/* ap.xxx = 123; etc */
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/* Send async message to remote */
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mbox_send_message(dc_async->mbox, &ap);
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/* Sync mailbox is listed first in 'mboxes' property */
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dc_sync->mbox = mbox_request_channel(&dc_sync->cl, 0);
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/* Populate data packet */
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/* sp.abc = 123; etc */
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/* Send message to remote in blocking mode */
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mbox_send_message(dc_sync->mbox, &sp);
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/* At this point 'sp' has been sent */
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/* Now wait for async chan to be done */
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wait_for_completion(&dc_async->c);
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}
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