[PATCH 39/43] drm/amd/display: Use udelay to avoid context switch

[Harry Wentland]

On 2020-01-10 1:47 p.m., Liu, Zhan wrote:
> 
> 
>> -----Original Message-----
>> From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> On Behalf Of
>> Christian König
>> Sent: 2020/January/10, Friday 10:02 AM
>> To: Siqueira, Rodrigo <Rodrigo.Siqueira at amd.com>; amd-
>> gfx at lists.freedesktop.org
>> Cc: Li, Sun peng (Leo) <Sunpeng.Li at amd.com>; Cheng, Tony
>> <Tony.Cheng at amd.com>; Tsai, Martin <Martin.Tsai at amd.com>; Lakha,
>> Bhawanpreet <Bhawanpreet.Lakha at amd.com>; Wentland, Harry
>> <Harry.Wentland at amd.com>
>> Subject: Re: [PATCH 39/43] drm/amd/display: Use udelay to avoid context
>> switch
>>
>> Am 10.01.20 um 15:46 schrieb Rodrigo Siqueira:
>>> From: Martin Tsai <martin.tsai at amd.com>
>>>
>>> [why]
>>> The rapid msleep operation causes the white line garbage when DAL
>>> check flip pending status in SetVidPnSourceVisibility.
>>> To execute this msleep will induce context switch, and longer delay
>>> could cause worse garbage situation.
>>>
>>> [how]
>>> To replace msleep with udelay.
>>>
>>> Signed-off-by: Martin Tsai <martin.tsai at amd.com>
>>> Reviewed-by: Tony Cheng <Tony.Cheng at amd.com>
>>> Acked-by: Harry Wentland <harry.wentland at amd.com>
>>> Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira at amd.com>
>>> ---
>>>   drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
>>> b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
>>> index 89920924a154..0dc652e76848 100644
>>> --- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
>>> +++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
>>> @@ -1642,9 +1642,9 @@ void dcn20_program_front_end_for_ctx(
>>>   			struct hubp *hubp = pipe->plane_res.hubp;
>>>   			int j = 0;
>>>
>>> -			for (j = 0; j < TIMEOUT_FOR_PIPE_ENABLE_MS
>>> +			for (j = 0; j < TIMEOUT_FOR_PIPE_ENABLE_MS*1000
>>>   					&& hubp->funcs-
>>> hubp_is_flip_pending(hubp); j++)
>>> -				msleep(1);
>>> +				udelay(1);
>>
>> Why not using mdelay() here?
> 
> As far as I know, mdelay() is only defined on Linux side.
> 
> This piece of code is shared by both Linux and Windows, so we have to use a function that's available on both platforms.
> 

It was used here before so we definitely have it defined for Windows as
well.

>From the commit description it sounds like msleep wasn't tight enough
and longer delays lead to issues here, at least on Windows.

Martin, Tony, do you have more details about this?

Thanks,
Harry

> Zhan
> 
>>
>> Christian.
>>
>>>   		}
>>>   	}
>>>
>>
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