Re: [PATCH [RESEND] v9 02/20] drm: Define histogram structures exposed to user

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 02 Dec 2025, Arun R Murthy <arun.r.murthy@intel.com> wrote:
> Display Histogram is an array of bins and can be generated in many ways
> referred to as modes.
> Ex: HSV max(RGB), Wighted RGB etc.
>
> Understanding the histogram data format(Ex: HSV max(RGB))
> Histogram is just the pixel count.
> For a maximum resolution of 10k (10240 x 4320 = 44236800)
> 25 bits should be sufficient to represent this along with a buffer of 7
> bits(future use) u32 is being considered.
> max(RGB) can be 255 i.e 0xFF 8 bit, considering the most significant 5
> bits, hence 32 bins.
> Below mentioned algorithm illustrates the histogram generation in
> hardware.
>
> hist[32] = {0};
> for (i = 0; i < resolution; i++) {
> 	bin = max(RGB[i]);
> 	bin = bin >> 3;	/* consider the most significant bits */
> 	hist[bin]++;
> }
> If the entire image is Red color then max(255,0,0) is 255 so the pixel
> count of each pixels will be placed in the last bin. Hence except
> hist[31] all other bins will have a value zero.
> Generated histogram in this case would be hist[32] = {0,0,....44236800}
>
> Description of the structures, properties defined are documented in the
> header file include/uapi/drm/drm_mode.h
>
> v8: Added doc for HDR planes, removed reserved variables (Dmitry)
>
> Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
> ---
>  include/uapi/drm/drm_mode.h | 65 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
>
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index cbbbfc1dfe2b806c641c720b0215e825e350bd03..aa24fef15da708f3e6804d07ffae5b0312dfdce0 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -1545,6 +1545,71 @@ struct drm_mode_closefb {
>  	__u32 pad;
>  };
>  
> +/**
> + * enum drm_mode_histogram
> + *
> + * @DRM_MODE_HISTOGRAM_HSV_MAX_RGB:
> + * Maximum resolution at present 10k, 10240x4320 = 44236800
> + * can be denoted in 25bits. With an additional 7 bits in buffer each bin
> + * can be a u32 value.
> + * For SDL, Maximum value of max(RGB) is 255, so max 255 bins.
> + * If the most significant 5 bits are considered, then bins = 2^5
> + * will be 32 bins.
> + * For HDR, maximum value of max(RGB) is 65535, so max 65535 bins.
> + * For illustration consider a full RED image of 10k resolution considering all
> + * 8 bits histogram would look like hist[255] = {0,0,....44236800} with SDR
> + * plane similarly with HDR the same would look like hist[65535] =
> + * {0,0,0,....44236800}
> + */
> +enum drm_mode_histogram {
> +	DRM_MODE_HISTOGRAM_HSV_MAX_RGB = 0x01,
> +};
> +
> +/**
> + * struct drm_histogram_caps
> + *
> + * @histogram_mode: histogram generation modes, defined in the
> + *		    enum drm_mode_histogram
> + * @bins_count: number of bins for a chosen histogram mode. For illustration
> + *		refer the above defined histogram mode.
> + */
> +struct drm_histogram_caps {
> +	__u32 histogram_mode;
> +	__u32 bins_count;
> +};
> +
> +/**
> + * struct drm_histogram_config
> + *
> + * @hist_mode_data: address to the histogram mode specific data if any
> + * @nr_hist_mode_data: number of elements pointed by the address in
> + *		       hist_mode_data
> + * @hist_mode: histogram mode(HSV max(RGB), RGB, LUMA etc)
> + * @enable: flag to enable/disable histogram
> + */
> +struct drm_histogram_config {
> +	__u64 hist_mode_data;
> +	__u32 nr_hist_mode_data;
> +	enum drm_mode_histogram hist_mode;
> +	bool enable;

enums for sure can't be used in UABI structs, and I don't think bools
either.

> +};
> +
> +/**
> + * struct drm_histogram
> + *
> + * @config: histogram configuration data pointed by struct drm_histogram_config
> + * @data_ptr: pointer to the array of histogram.
> + *	      Histogram is an array of bins. Data format for each bin depends
> + *	      on the histogram mode. Refer to the above histogram modes for
> + *	      more information.
> + * @nr_elements: number of bins in the histogram.
> + */
> +struct drm_histogram {
> +	struct drm_histogram_config config;
> +	__u64 data_ptr;
> +	__u32 nr_elements;
> +};
> +
>  #if defined(__cplusplus)
>  }
>  #endif

-- 
Jani Nikula, Intel