linux-kernel - Re: [PATCH v2 1/4] media: uapi: hevc: Add scaling matrix control

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Message-ID: <20200108151157.17cf9774@kernel.org>
Date:   Wed, 8 Jan 2020 15:11:57 +0100
From:   Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To:     Jernej Skrabec <jernej.skrabec@...l.net>
Cc:     mripard@...nel.org, paul.kocialkowski@...tlin.com,
        hverkuil@...all.nl, gregkh@...uxfoundation.org, wens@...e.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/4] media: uapi: hevc: Add scaling matrix control

Em Fri, 13 Dec 2019 17:04:25 +0100
Jernej Skrabec <jernej.skrabec@...l.net> escreveu:

> HEVC has a scaling matrix concept. Add support for it.

> +struct v4l2_ctrl_hevc_scaling_matrix {
> +	__u8	scaling_list_4x4[6][16];
> +	__u8	scaling_list_8x8[6][64];
> +	__u8	scaling_list_16x16[6][64];
> +	__u8	scaling_list_32x32[2][64];
> +	__u8	scaling_list_dc_coef_16x16[6];
> +	__u8	scaling_list_dc_coef_32x32[2];
> +};

I never looked at HEVC spec, but the above seems really weird.

Please correct me if I am wrong, but each of the above matrixes
is independent, and the driver will use just one of the above on
any specific time (for a given video output node), right?

If so, why would userspace be forced to update lots of matrixes, if would
likely use just one at a given time?

IMO, the proper way would be, instead, to use an uAPI like:

/*
 * Actually, as this is uAPI, we will use a fixed size integer type, like
 *  unsigned int
 */
enum hevc_scaling_matrix_type {
	HEVC_SCALING_MATRIX_4x4,
	HEVC_SCALING_MATRIX_8x8,
...
	HEVC_SCALING_MATRIX_DC_COEF_32x32,
};

struct v4l2_ctrl_hevc_scaling_matrix {
	__u32	scaling_type 		/* as defined by enum hevc_scaling_matrix_type */

	union {
		__u8	scaling_list_4x4[6][16];
		__u8	scaling_list_8x8[6][64];
		__u8	scaling_list_16x16[6][64];
		__u8	scaling_list_32x32[2][64];
		__u8	scaling_list_dc_coef_16x16[6];
		__u8	scaling_list_dc_coef_32x32[2];
	};
};

And let the core use a default for each scaling matrix, if userspace doesn't
set it.



Cheers,
Mauro