mmc: host: Introduce the request_atomic() for the host

The SD host controller can process one request in the atomic context if
the card is nonremovable, which means we can submit next request in the
irq hard handler when using the MMC host software queue to reduce the
latency. Thus this patch adds a new API request_atomic() for the host
controller, as well as adding support for host software queue to submit
a request by the new request_atomic() API.

Moreover there is an unusual case that the card is busy when trying to
send a command, and we can not polling the card status in interrupt
context by using request_atomic() to dispatch requests. Thus we should
queue a work to try again in the non-atomic context in case the host
releases the busy signal later.
Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/a344e27e506cb2329073cbd5cf65e15cc3cbeba9.1586744073.git.baolin.wang7@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/mmc_hsq.c

@@ -16,11 +16,20 @@
 #define HSQ_NUM_SLOTS	64
 #define HSQ_INVALID_TAG	HSQ_NUM_SLOTS
 
+static void mmc_hsq_retry_handler(struct work_struct *work)
+{
+	struct mmc_hsq *hsq = container_of(work, struct mmc_hsq, retry_work);
+	struct mmc_host *mmc = hsq->mmc;
+
+	mmc->ops->request(mmc, hsq->mrq);
+}
+
 static void mmc_hsq_pump_requests(struct mmc_hsq *hsq)
 {
 	struct mmc_host *mmc = hsq->mmc;
 	struct hsq_slot *slot;
 	unsigned long flags;
+	int ret = 0;
 
 	spin_lock_irqsave(&hsq->lock, flags);
 
@@ -42,7 +51,24 @@ static void mmc_hsq_pump_requests(struct mmc_hsq *hsq)
 
 	spin_unlock_irqrestore(&hsq->lock, flags);
 
-	mmc->ops->request(mmc, hsq->mrq);
+	if (mmc->ops->request_atomic)
+		ret = mmc->ops->request_atomic(mmc, hsq->mrq);
+	else
+		mmc->ops->request(mmc, hsq->mrq);
+
+	/*
+	 * If returning BUSY from request_atomic(), which means the card
+	 * may be busy now, and we should change to non-atomic context to
+	 * try again for this unusual case, to avoid time-consuming operations
+	 * in the atomic context.
+	 *
+	 * Note: we just give a warning for other error cases, since the host
+	 * driver will handle them.
+	 */
+	if (ret == -EBUSY)
+		schedule_work(&hsq->retry_work);
+	else
+		WARN_ON_ONCE(ret);
 }
 
 static void mmc_hsq_update_next_tag(struct mmc_hsq *hsq, int remains)
@@ -325,6 +351,7 @@ int mmc_hsq_init(struct mmc_hsq *hsq, struct mmc_host *mmc)
 	hsq->mmc->cqe_private = hsq;
 	mmc->cqe_ops = &mmc_hsq_ops;
 
+	INIT_WORK(&hsq->retry_work, mmc_hsq_retry_handler);
 	spin_lock_init(&hsq->lock);
 	init_waitqueue_head(&hsq->wait_queue);
 

drivers/mmc/host/mmc_hsq.h

@@ -12,6 +12,7 @@ struct mmc_hsq {
 	wait_queue_head_t wait_queue;
 	struct hsq_slot *slot;
 	spinlock_t lock;
+	struct work_struct retry_work;
 
 	int next_tag;
 	int num_slots;

include/linux/mmc/host.h

@@ -92,6 +92,9 @@ struct mmc_host_ops {
 			    int err);
 	void	(*pre_req)(struct mmc_host *host, struct mmc_request *req);
 	void	(*request)(struct mmc_host *host, struct mmc_request *req);
+	/* Submit one request to host in atomic context. */
+	int	(*request_atomic)(struct mmc_host *host,
+				  struct mmc_request *req);
 
 	/*
 	 * Avoid calling the next three functions too often or in a "fast