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[RFC,v2,04/11] scsi: ufs: qcom: Perform read back after writing unipro mode

Message ID 20231221-ufs-reset-ensure-effect-before-delay-v2-4-75af2a9bae51@redhat.com
State New
Headers
Series scsi: ufs: Remove overzealous memory barriers |

Commit Message

Andrew Halaney Dec. 21, 2023, 6:25 p.m. UTC 
  Currently, the QUNIPRO_SEL bit is written to and then an mb() is used to
ensure that completes before continuing.

mb() ensure that the write completes, 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:

    https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678

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

Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
---
 drivers/ufs/host/ufs-qcom.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Manivannan Sadhasivam Dec. 27, 2023, 6:01 a.m. UTC | #1 
On Thu, Dec 21, 2023 at 12:25:21PM -0600, Andrew Halaney wrote:
> Currently, the QUNIPRO_SEL bit is written to and then an mb() is used to
> ensure that completes before continuing.
> 
> mb() ensure that the write completes, 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:
> 
>     https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678
> 
> Let's do that to ensure the bit hits the device. Because the mb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.
> 
> Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
> ---
>  drivers/ufs/host/ufs-qcom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
> index 6df2ab3b6f23..ab1ff7432d11 100644
> --- a/drivers/ufs/host/ufs-qcom.c
> +++ b/drivers/ufs/host/ufs-qcom.c
> @@ -280,7 +280,7 @@ static void ufs_qcom_select_unipro_mode(struct ufs_qcom_host *host)
>  		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);
>  
>  	/* make sure above configuration is applied before we return */
> -	mb();
> +	ufshcd_readl(host->hba, REG_UFS_CFG1);

Same comment as patch 03/11.

- Mani

>  }
>  
>  /*
> 
> -- 
> 2.43.0
>
diff mbox series

Patch

diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
index 6df2ab3b6f23..ab1ff7432d11 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -280,7 +280,7 @@  static void ufs_qcom_select_unipro_mode(struct ufs_qcom_host *host)
 		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);
 
 	/* make sure above configuration is applied before we return */
-	mb();
+	ufshcd_readl(host->hba, REG_UFS_CFG1);
 }
 
 /*