Re: [PATCH v1 14/15] scsi: ufs: commit descriptors before setting the doorbell

[Rob Herring]

On Aug 21, 2015 3:10 PM, "Yaniv Gardi" <ygardi@xxxxxxxxxxxxxx> wrote:
>
> Add a write memory barrier to make sure descriptors prepared are actually
> written to memory before ringing the doorbell. We have also added the
> write memory barrier after ringing the doorbell register so that
> controller sees the new request immediately.
>
> Signed-off-by: Yaniv Gardi <ygardi@xxxxxxxxxxxxxx>
>
> ---
>  drivers/scsi/ufs/ufshcd.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index fef0660..876148b 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -833,6 +833,8 @@ void ufshcd_send_command(struct ufs_hba *hba, unsigned int task_tag)
>         ufshcd_clk_scaling_start_busy(hba);
>         __set_bit(task_tag, &hba->outstanding_reqs);
>         ufshcd_writel(hba, 1 << task_tag, REG_UTP_TRANSFER_REQ_DOOR_BELL);
> +       /* Make sure that doorbell is committed immediately */
> +       wmb();

Is this really necessary? Is there a measurable difference?

>  }
>
>  /**
> @@ -1418,6 +1420,8 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
>                 goto out;
>         }
>
> +       /* Make sure descriptors are ready before ringing the doorbell */
> +       wmb();

The writel for the doorbell will do a barrier first. (I didn't check
what exactly ufshcd_writel does, but that is why I don't like these
private access wrappers.)

>         /* issue command to the controller */
>         spin_lock_irqsave(hba->host->host_lock, flags);
>         ufshcd_send_command(hba, tag);
> @@ -1627,6 +1631,8 @@ static int ufshcd_exec_dev_cmd(struct ufs_hba *hba,
>
>         hba->dev_cmd.complete = &wait;
>
> +       /* Make sure descriptors are ready before ringing the doorbell */
> +       wmb();
>         spin_lock_irqsave(hba->host->host_lock, flags);
>         ufshcd_send_command(hba, tag);
>         spin_unlock_irqrestore(hba->host->host_lock, flags);
> --
> 1.8.5.2
>
> --
> QUALCOMM ISRAEL, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
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