Re: [PATCH v8 01/14] drm: Define histogram structures exposed to user

[Pekka Paalanen <pekka.paalanen@haloniitty.fi>]

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Hi Arun,

this whole series seems to be missing all the UAPI docs for the DRM
ReST files, e.g. drm-kms.rst. The UAPI header doc comments are not a
replacement for them, I would assume both are a requirement.

Without the ReST docs it is really difficult to see how this new UAPI
should be used.


On Tue, 28 Jan 2025 21:21:07 +0530
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 c082810c08a8b234ef2672ecf54fc8c05ddc2bd3..b8b7b18843ae7224263a9c61b20ac6cbf5df69e9 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -1355,6 +1355,71 @@ 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.

I assume s/SDL/SDR/.

This assumption seems false. SDR can be also 10-bit and probably even
more.

> + * 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.

Does this mean that the histogram is computed on the pixel values
emitted to the cable? What if the cable format is YUV?

> + * 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}

This SDR vs. HDR is a false dichotomy. I presume the meaningful
difference is bits-per-channel, not the dynamic range.

It would be good to have the pseudocode snippet here instead of the
commit message. The commit message should not contain any UAPI notes
that are not in the UAPI docs. OTOH, repeating UAPI docs in the commit
message is probably not very useful, as the more interesting questions
are why this exists and what it can be used for.

> + */
> +enum drm_mode_histogram {
> +	DRM_MODE_HISTOGRAM_HSV_MAX_RGB = 0x01,

What does the HSV stand for?

When talking about pixel values, my first impression is
hue-saturation-value. But there are no hue-saturation-value
computations at all?

> +};
> +
> +/**
> + * 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;
> +};

Patch 3 says:

+ * Property HISTOGRAM_CAPS is a blob pointing to the struct drm_histogram_caps
+ * Description of the structure is in include/uapi/drm/drm_mode.h

This is a read-only property, right?

The blob contains one struct drm_histogram_caps. What if more than one
mode is supported?

If the bin count depends on the bits-per-channel of something which
depends on set video mode or other things, how does updating work?
What if userspace uses a stale count? How does userspace ensure it uses
the current count?

> +
> +/**
> + * struct drm_histogram_config
> + *
> + * @hist_mode_data: address to the histogram mode specific data if any

Do I understand correctly that the KMS blob will contain a userspace
virtual memory address (a user pointer)? How does that work? What are
the lifetime requirements for that memory?

I do not remember any precedent of this, and I suspect it's not a good
design. I believe all the data should be contained in the blobs, e.g.
how IN_FORMATS does it. I'm not sure what would be the best UAPI here
for returning histogram data to userspace, but at least all the data
sent to the kernel should be contained in the blob itself since it
seems to be quite small. Variable length is ok for blobs.

> + * @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;

Don't enum and bool have non-fixed sizes? Hence inappropriate as UABI,
if architecture, build options, or the contents of the enum change the
ABI.

> +};
> +
> +/**
> + * struct drm_histogram
> + *
> + * @config: histogram configuration data pointed by struct drm_histogram_config

s/pointed by/defined by/ I presume? That much is obvious from the field
type. What does it mean? Why is struct drm_histogram_config a separate
struct?

> + * @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.

Another userspace virtual address stored in a KMS blob?

> + * @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
> 

Thanks,
pq

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