Re: [PATCH v2 1/5] drm: add legacy support for using degamma for gamma

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

On 30/11/2020 16:10, Daniel Vetter wrote:

> The thing is, the legacy helpers should be able to pull off what userspace
> needs to do when it's using atomic anyway. Hard-coding information in the
> kernel means we have a gap here. Hence imo legacy helpers doing the right
> thing in all reasonable cases is imo better.
> 
> In many cases I think we should even go further, and ditch driver ability
> to overwrite legacy helper hooks like this. I thought we'd need that
> flexibility for legacy userspace being incompatible in awkward ways, but
> wasn't ever really needed. Worse, many drivers forget to wire up the
> compat hooks.
> 
> tldr, imo right thing to do here:
> - move legacy gamma function from helpers into core code
> - call it unconditionally for all atomic drivers (if there's no legacy
>   drivers using the hook left then we can outright remove it)
> - make sure it dtrt in all cases

There are atomic drivers which have their custom gamma_set function. I guess they don't support
atomic color mgmt, but do support (legacy) gamma.

We could make the core code call the gamma legacy helper automatically for atomic drivers that don't
have gamma_set defined but do have GAMMA_LUT or DEGAMMA_LUT. But the gamma_set function is called
also in a few places from drm_fb_helper.c, so this code wouldn't be fully inside drm_color_mgmt.c.

Or we could just change drm_atomic_helper_legacy_gamma_set() to do the right thing, depending on
GAMMA_LUT & DEGAMMA_LUT existence.

 Tomi

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