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

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Brian Starkey <brian.starkey@arm.com>
Cc: intel-gfx@lists.freedesktop.org,
	Liviu Dudau <liviu.dudau@arm.com>,
	dri-devel@lists.freedesktop.org,
	Uma Shankar <uma.shankar@intel.com>,
	Mali DP Maintainers <malidp@foss.arm.com>,
	Mihail Atanassov <mihail.atanassov@arm.com>,
	Johnson Lin <johnson.lin@intel.com>
Subject: Re: [PATCH 1/4] drm/uapi: The ctm matrix uses sign-magnitude representation
Date: Fri, 23 Feb 2018 16:04:10 +0200	[thread overview]
Message-ID: <20180223140410.GJ5453@intel.com> (raw)
In-Reply-To: <20180223135222.GA9478@e107564-lin.cambridge.arm.com>

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.

-- 
Ville Syrjälä
Intel OTC
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