Re: [PATCH RFC v3 10/11] scsi: ufs: core: Perform read back to commit doorbell

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On 12/22/2023 3:09 AM, Andrew Halaney wrote:
Currently, the doorbell is written to and a wmb() is used to commit it

wmb() ensures that the write completes before following writes occur,
but completion doesn't mean that it isn't stored in a buffer somewhere.
The recommendation for ensuring this bit has taken effect on the device
is to perform a read back to force it to make it all the way to the
device. This is documented in device-io.rst and a talk by Will Deacon on
this can be seen over here:

Let's do that to ensure the bit hits the device. Because the wmb()'s
purpose wasn't to add extra ordering (on top of the ordering guaranteed
by writel()/readl()), it can safely be removed.

Fixes: ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the doorbell")
Signed-off-by: Andrew Halaney <ahalaney@xxxxxxxxxx>
  drivers/ufs/core/ufshcd.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 75a03ee9a1ba..caebd589e08c 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -7050,7 +7050,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,
ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
  	/* Make sure that doorbell is committed immediately */
-	wmb();
+	ufshcd_readl(hba, REG_UTP_TASK_REQ_DOOR_BELL);
spin_unlock_irqrestore(host->host_lock, flags);
Reviewed-by: Can Guo <quic_cang@xxxxxxxxxxx>

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