[PATCH v17 14/14] scsi: ufs: Inform the block layer about write ordering

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

From the UFSHCI 4.0 specification, about the legacy (single queue) mode:
"The host controller always process transfer requests in-order according
to the order submitted to the list. In case of multiple commands with
single doorbell register ringing (batch mode), The dispatch order for
these transfer requests by host controller will base on their index in
the List. A transfer request with lower index value will be executed
before a transfer request with higher index value."

From the UFSHCI 4.0 specification, about the MCQ mode:
"Command Submission
1. Host SW writes an Entry to SQ
2. Host SW updates SQ doorbell tail pointer

Command Processing
3. After fetching the Entry, Host Controller updates SQ doorbell head
   pointer
4. Host controller sends COMMAND UPIU to UFS device"

In other words, for both legacy and MCQ mode, UFS controllers are
required to forward commands to the UFS device in the order these
commands have been received from the host.

Notes:
- For legacy mode this is only correct if the host submits one
  command at a time. The UFS driver does this.
- Also in legacy mode, the command order is not preserved if
  auto-hibernation is enabled in the UFS controller.

This patch improves performance as follows on a test setup with UFSHCI
3.0 controller:
- With the mq-deadline scheduler: 2.5x more IOPS for small writes.
- When not using an I/O scheduler compared to using mq-deadline with
  zone locking: 4x more IOPS for small writes.

Cc: Bao D. Nguyen <quic_nguyenb@quicinc.com>
Cc: Can Guo <quic_cang@quicinc.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/ufs/core/ufshcd.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 3094f3c89e82..08803ba21668 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -5255,6 +5255,13 @@ static int ufshcd_device_configure(struct scsi_device *sdev,
 	struct ufs_hba *hba = shost_priv(sdev->host);
 	struct request_queue *q = sdev->request_queue;
 
+	/*
+	 * With auto-hibernation disabled, the write order is preserved per
+	 * MCQ. Auto-hibernation may cause write reordering that results in
+	 * unaligned write errors. The SCSI core will retry the failed writes.
+	 */
+	lim->driver_preserves_write_order = true;
+
 	lim->dma_pad_mask = PRDT_DATA_BYTE_COUNT_PAD - 1;
 
 	/*