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

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Date:   Fri, 17 Feb 2017 16:56:43 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Lionel Landwerlin <lionel.g.landwerlin@...el.com>
Cc:     Brian Starkey <brian.starkey@....com>,
        Daniel Stone <daniel@...ishbar.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v2] drm/color: Document CTM eqations

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.
> >
> > -Brian
> >
> > [1] https://lkml.org/lkml/2017/2/13/304
> >
> > On Wed, Feb 15, 2017 at 11:56:55AM +0000, Daniel Stone wrote:
> >> Hi,
> >>
> >> On 15 February 2017 at 11:39, Ville Syrjälä
> >> <ville.syrjala@...ux.intel.com> 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@...ux.intel.com> 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.
> >>
> >> Cheers,
> >> Daniel
> >
> If we're talking fixed point reprsentation, ChromeOS is using this :
> 
> https://cs.chromium.org/chromium/src/ui/ozone/platform/drm/gpu/drm_device.cc?q=DrmDevice&l=209

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.

-- 
Ville Syrjälä
Intel OTC