[PATCH] drm/i915: Only dither on 6bpc panels

[Jani Nikula]

On Thu, 13 Aug 2015, Mario Kleiner <mario.kleiner.de at gmail.com> wrote:
> Thanks for the quick fix! Comments below...
>
> On 08/12/2015 11:43 AM, Daniel Vetter wrote:
>> In
>>
>> commit d328c9d78d64ca11e744fe227096990430a88477
>> Author: Daniel Vetter <daniel.vetter at ffwll.ch>
>> Date:   Fri Apr 10 16:22:37 2015 +0200
>>
>>      drm/i915: Select starting pipe bpp irrespective or the primary plane
>>
>> we started to select the pipe bpp from sink capabilities and not from
>> the primary framebuffer - that one might change (and we don't want to
>> incur a modeset) and sprites might contain higher bpp content too.
>>
>> Problem is that now if you have a 10bpc screen and display 24bpp rgb
>> primary then we select dithering, and apparently that mangles the high
>> 8 bits even (even thought you'd expect dithering only to affect how
>> 12bpc gets mapped into 10bpc). And that mangling upsets certain users.
>>
>
> Probably doesn't matter, but your explanation of the former problem here 
> is slightly off. We also selected dithering on a 8 bpc screen displaying 
> a 24bpp rgb primary, because pipe_bpp is 24 for such a typical 8 bpc 
> sink, but since the commit mentioned above, base_bpp is always the 
> absolute maximum supported by the hardware, e.g., 36 bpp on my Ironlake 
> chip. Iow. the only way to not get dithering would have been to connect 
> a deep color 12 bpc display, so pipe_bpp == 36 == base_bpp.
>
>> Hence only enable dithering on 6bpc screens where we difinitely and
>> always want it.
>>
>
> Other than that, i tested the patch on both 8 bpc output with my 
> measurement equipment and on the internal laptop 6 bpc panel, and 
> everything is fine now - No banding on the 6 bpc panel, no banding or 
> equipment failure on the external 8 bpc output. Life is good again :)
>
> Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de at gmail.com>

Pushed to drm-intel-fixes, thanks for the patch and review and testing.

BR,
Jani.

>
> thanks,
> -mario
>
>> Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
>> Reported-by: Mario Kleiner <mario.kleiner.de at gmail.com>
>> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>> ---
>>   drivers/gpu/drm/i915/intel_display.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
>> index 9a2f229a1c3a..128462e0a0b5 100644
>> --- a/drivers/gpu/drm/i915/intel_display.c
>> +++ b/drivers/gpu/drm/i915/intel_display.c
>> @@ -12186,7 +12186,9 @@ encoder_retry:
>>   		goto encoder_retry;
>>   	}
>>
>> -	pipe_config->dither = pipe_config->pipe_bpp != base_bpp;
>> +	/* Dithering seems to not pass-through bits correctly when it should, so
>> +	 * only enable it on 6bpc panels. */
>> +	pipe_config->dither = pipe_config->pipe_bpp == 6*3;
>>   	DRM_DEBUG_KMS("plane bpp: %i, pipe bpp: %i, dithering: %i\n",
>>   		      base_bpp, pipe_config->pipe_bpp, pipe_config->dither);
>>
>>
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-- 
Jani Nikula, Intel Open Source Technology Center