[PATCH v3 16/17] scsi: ufs: Optimize the command queueing code

[Bart Van Assche <bvanassche@acm.org>]

Remove the clock scaling lock from ufshcd_queuecommand() since it is a
performance bottleneck. Instead, use synchronize_rcu_expedited() to wait
for ongoing ufshcd_queuecommand() calls.

Cc: Asutosh Das (asd) <asutoshd@codeaurora.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/ufs/ufshcd.c | 12 +++++++-----
 drivers/scsi/ufs/ufshcd.h |  1 +
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index c4cf5c4b4893..3e4c62c6f9d2 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -1196,6 +1196,13 @@ static int ufshcd_clock_scaling_prepare(struct ufs_hba *hba)
 	/* let's not get into low power until clock scaling is completed */
 	ufshcd_hold(hba, false);
 
+	/*
+	 * Wait for ongoing ufshcd_queuecommand() calls. Calling
+	 * synchronize_rcu_expedited() instead of synchronize_rcu() reduces the
+	 * waiting time from milliseconds to microseconds.
+	 */
+	synchronize_rcu_expedited();
+
 out:
 	return ret;
 }
@@ -2681,9 +2688,6 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
 
 	WARN_ONCE(tag < 0, "Invalid tag %d\n", tag);
 
-	if (!down_read_trylock(&hba->clk_scaling_lock))
-		return SCSI_MLQUEUE_HOST_BUSY;
-
 	/*
 	 * Allows the UFS error handler to wait for prior ufshcd_queuecommand()
 	 * calls.
@@ -2772,8 +2776,6 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
 out:
 	rcu_read_unlock();
 
-	up_read(&hba->clk_scaling_lock);
-
 	if (ufs_trigger_eh()) {
 		unsigned long flags;
 
diff --git a/drivers/scsi/ufs/ufshcd.h b/drivers/scsi/ufs/ufshcd.h
index c3c2792f309f..411c6015bbfe 100644
--- a/drivers/scsi/ufs/ufshcd.h
+++ b/drivers/scsi/ufs/ufshcd.h
@@ -779,6 +779,7 @@ struct ufs_hba_monitor {
  * @clk_list_head: UFS host controller clocks list node head
  * @pwr_info: holds current power mode
  * @max_pwr_info: keeps the device max valid pwm
+ * @clk_scaling_lock: used to serialize device commands and clock scaling
  * @desc_size: descriptor sizes reported by device
  * @urgent_bkops_lvl: keeps track of urgent bkops level for device
  * @is_urgent_bkops_lvl_checked: keeps track if the urgent bkops level for