Re: [PATCH RFC v2 03/11] scsi: ufs: qcom: Perform read back after writing testbus config

[Manivannan Sadhasivam <mani@kernel.org>]

On Thu, Dec 21, 2023 at 12:25:20PM -0600, Andrew Halaney wrote:
> Currently, the testbus configuration is written and completed with an
> mb().
> 
> 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: 9c46b8676271 ("scsi: ufs-qcom: dump additional testbus registers")
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
> ---
>  drivers/ufs/host/ufs-qcom.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
> index 4c15c8a1d058..6df2ab3b6f23 100644
> --- a/drivers/ufs/host/ufs-qcom.c
> +++ b/drivers/ufs/host/ufs-qcom.c
> @@ -1332,6 +1332,9 @@ static void ufs_qcom_enable_test_bus(struct ufs_qcom_host *host)
>  	ufshcd_rmwl(host->hba, UFS_REG_TEST_BUS_EN,
>  			UFS_REG_TEST_BUS_EN, REG_UFS_CFG1);
>  	ufshcd_rmwl(host->hba, TEST_BUS_EN, TEST_BUS_EN, REG_UFS_CFG1);
> +
> +	/* dummy read to ensure this has been enabled prior to returning */
> +	ufshcd_readl(host->hba, REG_UFS_CFG1);

In this case, I do not see the necessity to do a read back itself since there is
no delay afterwards nor any dependent operation in an altogether different
domain.

So removing the mb() should be sufficient.

- Mani

>  }
>  
>  static void ufs_qcom_get_default_testbus_cfg(struct ufs_qcom_host *host)
> @@ -1429,11 +1432,6 @@ int ufs_qcom_testbus_config(struct ufs_qcom_host *host)
>  		    (u32)host->testbus.select_minor << offset,
>  		    reg);
>  	ufs_qcom_enable_test_bus(host);
> -	/*
> -	 * Make sure the test bus configuration is
> -	 * committed before returning.
> -	 */
> -	mb();
>  
>  	return 0;
>  }
> 
> -- 
> 2.43.0
> 

-- 
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