[PATCH 04/15] drm/amd/display: Replace msleep with udelay while read edid return defer.

[Zhuo, Qingqing]

[AMD Official Use Only - Internal Distribution Only]

Am 17.09.20 um 00:18 schrieb Alex Deucher:
>> On Wed, Sep 16, 2020 at 6:16 PM Zhuo, Qingqing <Qingqing.Zhuo at amd.com> wrote:
>>> [AMD Official Use Only - Internal Distribution Only]
>>>
>>>On Wed, Sep 16, 2020 at 3:42 PM Qingqing Zhuo <qingqing.zhuo at amd.com> wrote:
>>>> From: jinlong zhang <jinlong.zhang at amd.com>
>>>>
>>> >[why]
>>>>while read edid return defer, then it enter to msleep, but it 
>>>>actually took more time during msleep, this will cause remaining 
>>>>edid read fail.
>>>>
>>>> [how]
>>>> Replacing msleep with udelay, it will not take any extra time, edid return pass finally.
>>> How long of a delay are we talking about here?  Some platforms don't support long udelays and someone will send a patch to change this to msleep.
>>>
>>> Alex
>>>
>>> ---------------------
>>>
>>> Hi Alex,
>>>
>>> It's between 0-5ms for generic cases, though there exist some dongle workaround cases where we will do 70ms. Would this be a concern?
>> I think ARM has a limit of 2ms for udelay.

> Yeah, there is even a define somewhere for this.

> If you need a delay which is longer than this but still more precise than msleep() then there is the high precision timer sleep as alternative.

> I've forgotten the function name to use here, but there was a LWN article about this a few years ago. You just need to google a bit.

Hi Alex and Christian,

Thanks a lot for the input! Given what's been discussed, I will drop this patch for now.

Regards,
Lillian

>
> Regards,
> Christian.
>>
>> Alex
>>
>>> Thank you,
>>> Lillian
>>>
>>>
>>>> Signed-off-by: jinlong zhang <jinlong.zhang at amd.com>
>>>> Reviewed-by: Wenjing Liu <Wenjing.Liu at amd.com>
>>>> Acked-by: Qingqing Zhuo <qingqing.zhuo at amd.com>
>>>> ---
>>>> drivers/gpu/drm/amd/display/dc/dce/dce_aux.c | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> index 743042d5905a..cdcad82765e0 100644
>>>> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> @@ -653,7 +653,7 @@ bool dce_aux_transfer_with_retries(struct ddc_service *ddc,
>>>>                                          if ((*payload->reply == AUX_TRANSACTION_REPLY_AUX_DEFER) ||
>>>>                                                  (*payload->reply == AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER)) {
>>>>                                                  if (payload->defer_delay > 0)
>>>> -                                                       msleep(payload->defer_delay);
>>>> +
>>>> + udelay(payload->defer_delay * 1000);
>>>>                                          }
>>>>                                  }
>>>>                                  break;
>>>> --
>>>> 2.17.1
>>>>
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