Re: [PATCH 04/12] drm: Add structures for querying color capabilities

[Damien Lespiau <damien.lespiau@intel.com>]

On Thu, Jul 02, 2015 at 05:20:45PM +0100, Damien Lespiau wrote:
> On Wed, Jul 01, 2015 at 09:18:14PM +0530, Kausal Malladi wrote:
> > From: Kausal Malladi <Kausal.Malladi@intel.com>
> > 
> > The DRM color management framework is targeting various hardware
> > platforms and drivers. Different platforms can have different color
> > correction and enhancement capabilities.
> > 
> > A commom user space application can query these capabilities using the
> > DRM property interface. Each driver can fill this property with its
> > platform's color capabilities.
> > 
> > This patch adds new structures in DRM layer for querying color
> > capabilities. These structures will be used by all user space
> > agents to configure appropriate color configurations.
> 
> As I indicated before, I don't think we should go for a full fledged 
> query API, because, I don't believe we can ever make it good enough to 
> cover future hardware (and probably not today's hardware across 
> vendors). 
>  
> These kinds of query APIs have been frown upon in the past for that 
> exact reason. 
>  
> - Accept configurations that are mostly likely to be working across vendors 
> (256 enties for 8 bits) That should be enough for basic functionality. 
>  
> - To support things that are really hw specific: make sure the kernel API can 
> accept those, put the hw specific knowledge into a user-space HAL where APIs 
> can evolve. What you're trying to do here with queries about per-platform 
> details can go into userspace and still have a generic compositor code using 
> those limits. Let's just not put the limits into the kernel API, we won't be 
> able to get it right. 
>  
> Now maybe there's a middle ground and we can expose basic limits. In this case, 
> maybe a list of supported LUT sizes, but the sampling details don't belong to a 
> kernel interface IMHO. I'm even hesitant putting the hw precision at that 
> level. 

To re-iterate the point with actual examples, the proposed query API
doesn't seem to handle things we'd want to know today:

- What are the extra values are coding for. eg. for 8 bits, the values
  after index 255 are special and we have no description for those. (and
  it can get fiddly to describe them, you may want to add the type of
  interpolation for instance).
- How to you represent capabilities across hardware unit. Eg. split
  gamma lowering the number of LUT entries.

That somewhat demonstrates my point I think. We won't be able to get the
query API right.

-- 
Damien
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