Re: [Intel-gfx] [PATCH 1/4] drm/uapi: The ctm matrix uses sign-magnitude representation

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Feb 23, 2018 at 04:04:10PM +0200, Ville Syrjälä wrote:
> On Fri, Feb 23, 2018 at 01:52:22PM +0000, Brian Starkey wrote:
> > Hi Ville,
> > 
> > On Thu, Feb 22, 2018 at 11:42:29PM +0200, Ville Syrjala wrote:
> > >From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >
> > >The documentation for the ctm matrix suggests a two's complement
> > >format, but at least the i915 implementation is using sign-magnitude
> > >instead. And looks like malidp is doing the same. Change the docs
> > >to match the current implementation, and change the type from __s64
> > >to __u64 to drive the point home.
> > 
> > I totally agree that this is a good idea, but...
> > 
> > >
> > >Cc: dri-devel@lists.freedesktop.org
> > >Cc: Mihail Atanassov <mihail.atanassov@arm.com>
> > >Cc: Liviu Dudau <liviu.dudau@arm.com>
> > >Cc: Brian Starkey <brian.starkey@arm.com>
> > >Cc: Mali DP Maintainers <malidp@foss.arm.com>
> > >Cc: Johnson Lin <johnson.lin@intel.com>
> > >Cc: Uma Shankar <uma.shankar@intel.com>
> > >Cc: Shashank Sharma <shashank.sharma@intel.com>
> > >Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >---
> > > include/uapi/drm/drm_mode.h | 7 +++++--
> > > 1 file changed, 5 insertions(+), 2 deletions(-)
> > >
> > >diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> > >index 2c575794fb52..b5d7d9e0eff5 100644
> > >--- a/include/uapi/drm/drm_mode.h
> > >+++ b/include/uapi/drm/drm_mode.h
> > >@@ -598,8 +598,11 @@ struct drm_mode_crtc_lut {
> > > };
> > >
> > > struct drm_color_ctm {
> > >-	/* Conversion matrix in S31.32 format. */
> > >-	__s64 matrix[9];
> > >+	/*
> > >+	 * Conversion matrix in S31.32 sign-magnitude
> > >+	 * (not two's complement!) format.
> > >+	 */
> > >+	__u64 matrix[9];
> > 
> > Isn't changing the type liable to break something for someone?
> 
> I hope not. Renaming the member would be a no no, but just changing the
> type should be mostly safe I think. I suppose if someone is building
> something with very strict compiler -W flags and -Werror it might cause
> a build failure, so I guess one might label it as a minor api break but
> not an abi break.
> 
> If people think that's a serious concern I guess we can keep the
> __s64, but I'd rather not give people that much rope to hang
> themselves by interpreting it as 2's complement.

OK, no one complained loudly so I've gone and pushed this to
drm-misc-next. Now we wait and see whether I can dodge the egg...

-- 
Ville Syrjälä
Intel OTC
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