On Tue, Jan 31, 2017 at 03:39:29PM +0000, Brian Starkey wrote:
> Hi Ville,
>
> On Tue, Jan 31, 2017 at 05:18:28PM +0200, Ville Syrjälä wrote:
> >On Tue, Jan 31, 2017 at 10:48:34AM +0000, Brian Starkey wrote:
> >> Explicitly state the expected CTM equations in the kerneldoc for the CTM
> >> property, and the form of the matrix on struct drm_color_ctm.
> >>
> >> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> >> Cc: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
> >> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >> Signed-off-by: Brian Starkey <brian.starkey@xxxxxxx>
> >> ---
> >> drivers/gpu/drm/drm_color_mgmt.c | 13 +++++++++++++
> >> include/uapi/drm/drm_mode.h | 8 +++++++-
> >> 2 files changed, 20 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> >> index 789b4c65cd69..7573ca4b6ea6 100644
> >> --- a/drivers/gpu/drm/drm_color_mgmt.c
> >> +++ b/drivers/gpu/drm/drm_color_mgmt.c
> >> @@ -62,6 +62,19 @@
> >> * unit/pass-thru matrix should be used. This is generally the driver
> >> * boot-up state too.
> >> *
> >> + * The output vector is related to the input vector as below:
> >> + *
> >> + * | ``out[0] = matrix[0] * in[0] + matrix[1] * in[1] + matrix[2] * in[2];``
> >> + * | ``out[1] = matrix[3] * in[0] + matrix[4] * in[1] + matrix[5] * in[2];``
> >> + * | ``out[2] = matrix[6] * in[0] + matrix[7] * in[1] + matrix[8] * in[2];``
> >> + *
> >> + * The component order in the input/output vectors is assumed to be
> >> + * { R, G, B }.
> >> + *
> >> + * The color-space of the input vector must not be confused with the
> >> + * color-space implied by a framebuffer pixel format, which may be the same
> >> + * or different.
> >> + *
> >> * “GAMMA_LUT”:
> >> * Blob property to set the gamma lookup table (LUT) mapping pixel data
> >> * after the transformation matrix to data sent to the connector. The
> >> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> >> index ce7efe2e8a5e..3401637caf8e 100644
> >> --- a/include/uapi/drm/drm_mode.h
> >> +++ b/include/uapi/drm/drm_mode.h
> >> @@ -525,7 +525,13 @@ struct drm_mode_crtc_lut {
> >> };
> >>
> >> struct drm_color_ctm {
> >> - /* Conversion matrix in S31.32 format. */
> >> + /*
> >> + * Conversion matrix in S31.32 format, in row-major form:
> >
> >s32.32 is how I'd state that (to match the regular s32 and whatnot
> >types).
> >
>
> Can you explain a bit more what exactly you mean by s32.32? e.g. what
> would be the bitfield representing the most negative number?
>
> I understand the S31.32 here as a sign + magnitude format (which makes
> it rather odd to store it in a signed variable, but never mind). This
> also appears to be what igt does in set_ctm() in kms_pipe_color.c:
>
> for (i = 0; i < ARRAY_SIZE(ctm.matrix); i++) {
> if (coefficients[i] < 0) {
> ctm.matrix[i] =
> (int64_t) (-coefficients[i] * ((int64_t) 1L << 32));
> ctm.matrix[i] |= 1ULL << 63;
> } else
> ctm.matrix[i] =
> (int64_t) (coefficients[i] * ((int64_t) 1L << 32));
> }
>
> If that's what you meant as well, then I don't think s32.32 is a good
> way to describe it, because the integer part has only 31 bits
> available.
>
> If you meant a regular two's-complement fixed-point number, where the
> most negative number would be 0x10000000.00000000, then yeah that's
> what I thought it meant too originally. Clarifying the docs here
> sounds like a great plan.
>
> I guess the igt implementation means that it's a sign + magnitude
> number, and the fact that it's stored in an s64 is a bizarre quirk
> that we just live with.
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.
--
Ville Syrjälä
Intel OTC
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