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@xxxxxxxxxxxxxxx>
> >
> >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@xxxxxxxxxxxxxxxxxxxxx
> >Cc: Mihail Atanassov <mihail.atanassov@xxxxxxx>
> >Cc: Liviu Dudau <liviu.dudau@xxxxxxx>
> >Cc: Brian Starkey <brian.starkey@xxxxxxx>
> >Cc: Mali DP Maintainers <malidp@xxxxxxxxxxxx>
> >Cc: Johnson Lin <johnson.lin@xxxxxxxxx>
> >Cc: Uma Shankar <uma.shankar@xxxxxxxxx>
> >Cc: Shashank Sharma <shashank.sharma@xxxxxxxxx>
> >Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> >---
> > 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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