[PATCH 01/35] drm/amd/display: Use udelay when waiting between aux retries

[Wentland, Harry]

On 2019-02-01 5:55 p.m., sylvain.bertrand at gmail.com wrote:
> On Fri, Feb 01, 2019 at 09:20:56PM +0000, Wentland, Harry wrote:
>> DRM's AUX code uses usleep_range in drm_dp_dpcd_access.
> 
> My bad, forgot about the usleep_range switch. That said AUX_RETRY_INTERVAL is
> 500 us, with a usleep_range top bound of 600 us.
> 
> Then, it would mean DC DP timeout retries would happen after ~1ms, and drm
> ~600us. Additionaly, it seems that the number of retries are 3 in drm code and
> 7 in DC code. 
> 

DRM actually bumped this to 32 due to an issue with a Dell 4k display.

> I may be wrong, but it seems DC code is much more "insisting" on auxchannel
> (edid retrieval) and much more forgiving on monitor ability to timeout in
> time (~1ms).
> 
> If I did read the code the right way, it may be more reasonable to have similar
> behavior in drm code than in DC code, right?
> 

It depends. I wouldn't call one or the other more correct. I seem to remember that the DP spec is quite vague on these retries but I could be mistaken. Since our driver hasn't show any problems with the DRM code and I believe others (such as i915) also pass DP compliance without issues I wouldn't proactively change this.

Harry