linux-kernel - Re: [PATCH] drm/amd/display: Use msleep instead of udelay for 8ms wait

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Date:   Tue, 25 Jun 2019 16:12:15 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Harry Wentland <harry.wentland@....com>, airlied@...il.com,
        natechancellor@...il.com
Cc:     sunpeng.li@....com, Anthony.Koo@....com,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, alexander.deucher@....com,
        Jun.Lei@....com, Bhawanpreet.Lakha@....com,
        christian.koenig@....com
Subject: Re: [PATCH] drm/amd/display: Use msleep instead of udelay for 8ms
 wait

Hi Harry,

Am Dienstag, den 25.06.2019, 10:00 -0400 schrieb Harry Wentland:
> arm32's udelay only allows values up to 2000 microseconds. msleep
> does the trick for us here as there is no problem if this isn't
> microsecond accurate and takes a tad longer.

A "tad" longer in this case means likely double the intended wait.
Please see "SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms)" in
Documentation/timers/timers-howto.txt.

The sleep here should use usleep_range. In general the DC code seems to
use quite a lot of the udelay busy waits. I doubt that many of those
occurrences are in atomic context, so could easily use a sleeping wait.

Digging further this seems to apply across amdgpu, not only DC.

Regards,
Lucas

> Signed-off-by: Harry Wentland <harry.wentland@....com>
> ---
>  drivers/gpu/drm/amd/display/dc/core/dc_link.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> index 4c31930f1cdf..f5d02f89b3f9 100644
> --- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> +++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> @@ -548,7 +548,7 @@ static void
> read_edp_current_link_settings_on_detect(struct dc_link *link)
>  			break;
>  		}
>  
> -		udelay(8000);
> +		msleep(8);
>  	}
>  
>  	ASSERT(status == DC_OK);