Re: [PATCH 04/23] drm: Add drm structures for palette color property

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Sep 16, 2015 at 11:07:01PM +0530, Shashank Sharma wrote:
> From: Kausal Malladi <kausalmalladi@gmail.com>
> 
> This patch adds new structures in DRM layer for Palette color
> correction.These structures will be used by user space agents
> to configure appropriate number of samples and Palette LUT for
> a platform.
> 
> Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
> Signed-off-by: Kausal Malladi <kausalmalladi@gmail.com>
> ---
>  include/uapi/drm/drm.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index e3c642f..f72b916 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -840,6 +840,33 @@ struct drm_palette_caps {
>  	__u32 num_samples_after_ctm;
>  };
>  
> +struct drm_r32g32b32 {
> +	/*
> +	 * Data is in U8.24 fixed point format.
> +	 * All platforms support values within [0, 1.0] range,
> +	 * for Red, Green and Blue colors.
> +	 */
> +	__u32 r32;
> +	__u32 g32;
> +	__u32 b32;

It's not strictly required, but adding a __u32 reserved here to align the
struct to 64 bits seems good imo. Slight overhead but meh about that.

> +};
> +
> +struct drm_palette {
> +	/* Structure version. Should be 1 currently */
> +	__u32 version;

Definitely great practice to take compat into account and definitely
needed for the first design using ioctls but I don't think we need this
here. Properties are already extinsible themselves: We can just greate a
"ctm-v2", "ctm-v3" if the layout changes, and since the actual ctm matrix
is stored in the drm_crtc_state any compat code on the kernel will be
shared.

Aside: For an ioctl the recommended way to handle backwards compat and
extensions in drm is with a flags bitfield. That's more flexible than a
linear version field, and extending the ioctl struct at the end is already
handled by the drm core in a transparent fashion (it 0-fills either kernel
or userspace side).

> +	/*
> +	 * This has to be a supported value during get call.
> +	 * Feature will be disabled if this is 0 while set
> +	 */
> +	__u32 num_samples;

blob properties already have a size, storing it again in the blob is
redundnant. Instead I think a small helper to get the number of samples
for a given gamma table blob would be needed.

Cheers, Daniel

> +	/*
> +	 * Starting of palette LUT in R32G32B32 format.
> +	 * Each of RGB value is in U8.24 fixed point format.
> +	 * Actual number of samples will depend upon num_samples
> +	 */
> +	struct drm_r32g32b32 lut[0];
> +};
> +
>  /* typedef area */
>  #ifndef __KERNEL__
>  typedef struct drm_clip_rect drm_clip_rect_t;
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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