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

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

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@linux.intel.com>
> >> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> >> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >> Signed-off-by: Brian Starkey <brian.starkey@arm.com>
> >> ---
> >>  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