Re: [PATCH v2] drm/color: Document CTM eqations

From: Lionel Landwerlin
Date: Fri Feb 17 2017 - 10:05:37 EST

On 17/02/17 14:56, Ville Syrjälä wrote:
On Fri, Feb 17, 2017 at 02:42:26PM +0000, Lionel Landwerlin wrote:
On 17/02/17 13:54, Brian Starkey wrote:
What's the verdict? We've got [1] which is about to become another
(driver) implementation - better to change before that merges than
after I guess.



On Wed, Feb 15, 2017 at 11:56:55AM +0000, Daniel Stone wrote:

On 15 February 2017 at 11:39, Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
On Tue, Jan 31, 2017 at 06:46:39PM +0100, Daniel Vetter wrote:
On Tue, Jan 31, 2017 at 6:22 PM, Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
Hmm. Two's complement is what I was thinking it is. Which shows that
I never managed to read the code in any detail. Definitely needs to
be documented properly.
That sounds supremely backwards. I guess we can't fix this anymore?
I have no idea. Anyone else?
I don't know of any implementation using this; maybe closed Intel
Android stuff? Certainly GitHub showed no-one using it, and neither X
nor Weston/Mutter are using it yet.

If we're talking fixed point reprsentation, ChromeOS is using this :
So it's already using the sign+magnitude stuff. Which presumably
means we can't change it to two's complement anymore :( Maybe we add a
CTM2 property ;)

Using sign+magnitude definitely looks rather inefficient since there's
a branch inside the loop. With two's complement you wouldn't need that
thing slowing you down.

If you're seriously considering that, you might also want to bump struct drm_color_lut to use 32bits fields.
It seems some people have concerned about HDR.