linux-kernel - Re: [PATCH v4] drm/color: Document CTM eqations

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Date:   Sun, 26 Feb 2017 20:48:42 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Brian Starkey <brian.starkey@....com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>, mihail.atanassov@....com,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH v4] drm/color: Document CTM eqations

On Tue, Jan 31, 2017 at 05:43:48PM +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@...ux.intel.com>
> Cc: Lionel Landwerlin <lionel.g.landwerlin@...el.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Brian Starkey <brian.starkey@....com>

Ugh, but well that's the current uabi. Applied to drm-misc-next, thanks
for your patch. One idea below.

> ---
>  drivers/gpu/drm/drm_color_mgmt.c |   13 +++++++++++++
>  include/uapi/drm/drm_mode.h      |   14 +++++++++++++-
>  2 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> index 789b4c65cd69..307f7b0545b6 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..3f179b8867e5 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -525,7 +525,19 @@ struct drm_mode_crtc_lut {
>  };
>  
>  struct drm_color_ctm {
> -	/* Conversion matrix in S31.32 format. */
> +	/*
> +	 * Conversion matrix in row-major form:
> +	 *
> +	 * | matrix[0] matrix[1] matrix[2] |
> +	 * | matrix[3] matrix[4] matrix[5] |
> +	 * | matrix[6] matrix[7] matrix[8] |
> +	 *
> +	 * Each element is in a sign + magnitude format: S31.32, with the sign
> +	 * in bit 63, and the magnitude represented as fixed-point 31.32.
> +	 *
> +	 * i.e. +1.25 = 0x00000001.40000000
> +	 *      -1.25 = 0x80000001.40000000
> +	 */

This is great, except none of our uabi headers are part of the kernel-doc
build :( If you have time, would be real awesome to include them, and at
least give this struct here a proper kernel-doc. Then you could also
reference it in the above kerneldoc comment in drm_color_mgmt.c.

We already have a drm-uapi.rst file, adding a new section there for
userspace abi headers seems like the right place for the include stanza.

Cheers, Daniel

>  	__s64 matrix[9];
>  };
>  
> -- 
> 1.7.9.5
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch