Re: [PATCH RFC v3 09/11] scsi: ufs: core: Perform read back after disabling UIC_COMMAND_COMPL

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On 12/21/23 11:09, Andrew Halaney wrote:
Currently, the UIC_COMMAND_COMPL interrupt is disabled and a wmb() is
used to complete the register write before any following writes.

wmb() ensures the writes complete in that order, 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: d75f7fe495cf ("scsi: ufs: reduce the interrupts for power mode change requests")
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 bb603769b029..75a03ee9a1ba 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -4240,7 +4240,7 @@ static int ufshcd_uic_pwr_ctrl(struct ufs_hba *hba, struct uic_command *cmd)
  		 * Make sure UIC command completion interrupt is disabled before
  		 * issuing UIC command.
-		wmb();
+		ufshcd_readl(hba, REG_INTERRUPT_ENABLE);
  		reenable_intr = true;
  	spin_unlock_irqrestore(hba->host->host_lock, flags);

Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>

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