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

[ygardi@codeaurora.org]

>> On Aug 21, 2015 3:10 PM, "Yaniv Gardi" <ygardi@codeaurora.org> 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@codeaurora.org>
>>>
>>> ---
>>>  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?
>
> I'm not sure if there is a measurable difference, but as the Door-Bell
> register is the one that actually responsible for the HW execution of the
> requests, anyhow, it's recommended to its value will be written
> instantly to the memory.
>
> Also, as the Interrupt context reads this register, and compare it to the
> SW mirroring value (hba->outstanding_reqs) in order to realize what
> requests are already completed, it's important to get the correct value
> by reading this register, otherwise we might realize a request completion
> while it was never even submitted.
>
>>
>>>  }
>>>
>>>  /**
>>> @@ -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.)

the barrier here is important and a must.
before it we prepare descriptors.
after it we write to DOOR-BELL.
(and after the DOOR-BELL we have another one.)
if we remove it, we might get the DOOR-BELL written, before the
descriptors written.




>>
>>>         /* 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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>
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