blk-mq: Document functions for sending request

Add or improve documentation for function regarding creating and sending
IO requests to the hardware.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3c71d52..a12b176 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -641,6 +641,14 @@ bool blk_mq_complete_request(struct request *rq)
 }
 EXPORT_SYMBOL(blk_mq_complete_request);
 
+/**
+ * blk_mq_start_request - Start processing a request
+ * @rq: Pointer to request to be started
+ *
+ * Function used by device drivers to notify the block layer that a request
+ * is going to be processed now, so blk layer can do proper initializations
+ * such as starting the timeout timer.
+ */
 void blk_mq_start_request(struct request *rq)
 {
        struct request_queue *q = rq->q;
@@ -1327,6 +1335,12 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
        return (queued + errors) != 0;
 }
 
+/**
+ * __blk_mq_run_hw_queue - Run a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ *
+ * Send pending requests to the hardware.
+ */
 static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
        int srcu_idx;
@@ -1424,6 +1438,15 @@ select_cpu:
        return next_cpu;
 }
 
+/**
+ * __blk_mq_delay_run_hw_queue - Run (or schedule to run) a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ * @async: If we want to run the queue asynchronously.
+ * @msecs: Microseconds of delay to wait before running the queue.
+ *
+ * If !@async, try to run the queue now. Else, run the queue asynchronously and
+ * with a delay of @msecs.
+ */
 static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
                                        unsigned long msecs)
 {
@@ -1445,12 +1468,28 @@ static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
                                    msecs_to_jiffies(msecs));
 }
 
+/**
+ * blk_mq_delay_run_hw_queue - Run a hardware queue asynchronously.
+ * @hctx: Pointer to the hardware queue to run.
+ * @msecs: Microseconds of delay to wait before running the queue.
+ *
+ * Run a hardware queue asynchronously with a delay of @msecs.
+ */
 void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
 {
        __blk_mq_delay_run_hw_queue(hctx, true, msecs);
 }
 EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
 
+/**
+ * blk_mq_run_hw_queue - Start to run a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ * @async: If we want to run the queue asynchronously.
+ *
+ * Check if the request queue is not in a quiesced state and if there are
+ * pending requests to be sent. If this is true, run the queue to send requests
+ * to hardware.
+ */
 void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 {
        int srcu_idx;
@@ -1474,6 +1513,11 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 }
 EXPORT_SYMBOL(blk_mq_run_hw_queue);
 
+/**
+ * blk_mq_run_hw_queue - Run all hardware queues in a request queue.
+ * @q: Pointer to the request queue to run.
+ * @async: If we want to run the queue asynchronously.
+ */
 void blk_mq_run_hw_queues(struct request_queue *q, bool async)
 {
        struct blk_mq_hw_ctx *hctx;
@@ -1625,7 +1669,11 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
        blk_mq_hctx_mark_pending(hctx, ctx);
 }
 
-/*
+/**
+ * blk_mq_request_bypass_insert - Insert a request at dispatch list.
+ * @rq: Pointer to request to be inserted.
+ * @run_queue: If we should run the hardware queue after inserting the request.
+ *
  * Should only be used carefully, when the caller knows we want to
  * bypass a potential IO scheduler on the target device.
  */
@@ -1805,6 +1853,17 @@ insert:
        return BLK_STS_OK;
 }
 
+/**
+ * blk_mq_try_issue_directly - Try to send a request directly to device driver.
+ * @hctx: Pointer of the associated hardware queue.
+ * @rq: Pointer to request to be sent.
+ * @cookie: Request queue cookie.
+ *
+ * If the device has enough resources to accept a new request now, send the
+ * request directly to device driver. Else, insert at hctx->dispatch queue, so
+ * we can try send it another time in the future. Requests inserted at this
+ * queue have higher priority.
+ */
 static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
                struct request *rq, blk_qc_t *cookie)
 {
@@ -1882,6 +1941,22 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
        }
 }
 
+/**
+ * blk_mq_make_request - Create and send a request to block device.
+ * @q: Request queue pointer.
+ * @bio: Bio pointer.
+ *
+ * Builds up a request structure from @q and @bio and send to the device. The
+ * request may not be queued directly to hardware if:
+ * * This request can be merged with another one
+ * * We want to place request at plug queue for possible future merging
+ * * There is an IO scheduler active at this queue
+ *
+ * It will not queue the request if there is an error with the bio, or at the
+ * request creation.
+ *
+ * Returns: Request queue cookie.
+ */
 static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
        const int is_sync = op_is_sync(bio->bi_opf);
@@ -1927,7 +2002,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
        plug = blk_mq_plug(q, bio);
        if (unlikely(is_flush_fua)) {
-               /* bypass scheduler for flush rq */
+               /* Bypass scheduler for flush requests */
                blk_insert_flush(rq);
                blk_mq_run_hw_queue(data.hctx, true);
        } else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs ||
@@ -1955,6 +2030,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
                blk_add_rq_to_plug(plug, rq);
        } else if (q->elevator) {
+               /* Insert the request at the IO scheduler queue */
                blk_mq_sched_insert_request(rq, false, true, true);
        } else if (plug && !blk_queue_nomerges(q)) {
                /*
@@ -1981,8 +2057,13 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
                }
        } else if ((q->nr_hw_queues > 1 && is_sync) ||
                        !data.hctx->dispatch_busy) {
+               /*
+                * There is no scheduler and we can try to send directly
+                * to the hardware.
+                */
                blk_mq_try_issue_directly(data.hctx, rq, &cookie);
        } else {
+               /* Default case. */
                blk_mq_sched_insert_request(rq, false, true, true);
        }