Re: [PATCH v3 3/5] drm/amd/display: use a separate helper to translate degamma curves

On 2026-08-04 05:10, Melissa Wen wrote:
> 
> 
> On 01/08/2026 03:41, Alex Hung wrote:
>> I tested above on
>>  - DCN35 - issue fiexed
>>  - DCN401 - no issue and no regression.
>>
>> Does it pass DCN321 that you mentioned abouve?
> 
> Yes, with this change the kms_colorop passes on DCN321 and so far I don't see any visual regression on games.
> 
> I also checked that DCN2+ uses different CM helpers for degamma and gamma as well - but with 12 segments, which is the original distribution of _translate_curve_to_degamma_hw_format.
> Just to say that the proposed approach isn't new.
> 
>>
>> DCN301 doesn't use cm3_helper_translate_curve_to_degamma_hw_format() so it wasn't affected.
> 
> Oh my, I wasn't expecting that! Banding was affecting both DCN301 and DCN321.
> It was my fault, I overlooked DCN301, I'll extend the fix when we agree in the proper fix after Harry's review.
> 

The fix looks right to me. Values in non-linear space are essentially optimally distributed
already, so for a "degamma" operation we would want something close to a uniform distribution
of LUT entries. It makes no sense to use a distribution that's weighted toward dark values.
That is something we would want to do for linear to non-linear conversion, like the shaper,
which already uses a different function and presumably a non-uniform segment distribution.

So, in short, I agree this change is correct.

Once you put that in a patch feel free to add my
Reviewed-by: Harry Wentland <[email protected]>

Harry

> Thanks for the feedback,
> 
> Melissa
> 
>>
>> The changes look good to me, but let's ask @Wentland, Harry for review too. 
>