Re: [PATCH RFC v3 01/11] scsi: ufs: qcom: Perform read back after writing reset bit

[Can Guo <quic_cang@quicinc.com>]

On 12/22/2023 3:09 AM, Andrew Halaney wrote:
> Currently, the reset bit for the UFS provided reset controller (used by
> its phy) is written to, and then a mb() happens to try and ensure that
> hit the device. Immediately afterwards a usleep_range() occurs.
> 
> 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. By doing so and
> guaranteeing the ordering against the immediately following
> usleep_range(), the mb() can safely be removed.
> 
> Fixes: 81c0fc51b7a7 ("ufs-qcom: add support for Qualcomm Technologies Inc platforms")
> Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
> ---
>   drivers/ufs/host/ufs-qcom.h | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/ufs/host/ufs-qcom.h b/drivers/ufs/host/ufs-qcom.h
> index 9dd9a391ebb7..b9de170983c9 100644
> --- a/drivers/ufs/host/ufs-qcom.h
> +++ b/drivers/ufs/host/ufs-qcom.h
> @@ -151,10 +151,10 @@ static inline void ufs_qcom_assert_reset(struct ufs_hba *hba)
>   	ufshcd_rmwl(hba, UFS_PHY_SOFT_RESET, UFS_PHY_SOFT_RESET, REG_UFS_CFG1);
>   
>   	/*
> -	 * Make sure assertion of ufs phy reset is written to
> -	 * register before returning
> +	 * Dummy read to ensure the write takes effect before doing any sort
> +	 * of delay
>   	 */
> -	mb();
> +	ufshcd_readl(hba, REG_UFS_CFG1);
>   }
>   
>   static inline void ufs_qcom_deassert_reset(struct ufs_hba *hba)
> @@ -162,10 +162,10 @@ static inline void ufs_qcom_deassert_reset(struct ufs_hba *hba)
>   	ufshcd_rmwl(hba, UFS_PHY_SOFT_RESET, 0, REG_UFS_CFG1);
>   
>   	/*
> -	 * Make sure de-assertion of ufs phy reset is written to
> -	 * register before returning
> +	 * Dummy read to ensure the write takes effect before doing any sort
> +	 * of delay
>   	 */
> -	mb();
> +	ufshcd_readl(hba, REG_UFS_CFG1);
>   }
>   
>   /* Host controller hardware version: major.minor.step */
> 
Reviewed-by: Can Guo <quic_cang@quicinc.com>