Re: [RFC][PATCH] drm: kirin: Restrict modes to known good mode clocks

[Daniel Vetter]

On Tue, Jul 11, 2017 at 5:05 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>>> > be even better if you could calculate whether the mode is valid, but I didn't
>>> > spend enough time to figure out if this is possible.
>>>
>>> Theoretically that might be possible, checking if the requested freq
>>> matches the calculated freq, and I've tried that but so far I've not
>>> been able to get it to work, as in some cases the freq on the current
>>> whitelist don't exactly match but do work on the large majority of
>>> monitors tested (while other freq have a similar error but don't work
>>> on most monitors).
>>>
>>> I'd like to spend some more time to try to refine and tune this, but
>>> having the current whitelist works fairly well, so I'm not sure its
>>> worth sitting on (this is basically the last functional patch
>>> outstanding for HiKey to fully work upstream - except the mali gpu of
>>> course), while I try to tinker and tune it.
>>>
>>> Thanks so much for the feedback!
>>
>> Yeah the proper approach is to compute your pll/clock settings and bail
>> out if those don't work. That's generally a magic spreadsheet supplied by
>> the hw validation engineers, and I honestly don't want to know how they
>> create it. Explicit modelist in the kernel sounds like a very bad hack.
>
> So without such a magic spreadsheet, how would you suggest I move this forward?
> Not having the whitelist hack and picking modes the device can't
> generate is a fairly major usability issue.

I guess if the whitelist is the only thing I'd do 2 things differently:
- Whitelist the clocks, not modes, since that's what seems to matter here.
- Put it as close as possible to the code that comuptes the clock
settings (yet if you use the clock subsystem that's a bit hard, but
for an atomic driver this should be where this is done ...).

Whitelist of modes looks super-hacky.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch