linux-kernel - Re: [PATCH v3 01/10] drm/fourcc: Add drm_format_info

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Date:   Wed, 10 Aug 2022 17:59:03 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-m68k@...r.kernel.org, linux-kernel@...r.kernel.org,
        Javier Martinez Canillas <javierm@...hat.com>
Subject: Re: [PATCH v3 01/10] drm/fourcc: Add drm_format_info_bpp() helper

On Fri, Jul 08, 2022 at 08:20:46PM +0200, Geert Uytterhoeven wrote:
> Add a helper to retrieve the actual number of bits per pixel for a
> plane, taking into account the number of characters and pixels per
> block for tiled formats.
> 
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> v3:
>   - Add Reviewed-by,
> 
> v2:
>   - Move up.
> ---
>  drivers/gpu/drm/drm_fourcc.c | 19 +++++++++++++++++++
>  include/drm/drm_fourcc.h     |  1 +
>  2 files changed, 20 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 07741b678798b0f1..cf48ea0b2cb70ba8 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -370,6 +370,25 @@ unsigned int drm_format_info_block_height(const struct drm_format_info *info,
>  }
>  EXPORT_SYMBOL(drm_format_info_block_height);
>  
> +/**
> + * drm_format_info_bpp - number of bits per pixel
> + * @info: pixel format info
> + * @plane: plane index
> + *
> + * Returns:
> + * The actual number of bits per pixel, depending on the plane index.
> + */
> +unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane)
> +{
> +	if (!info || plane < 0 || plane >= info->num_planes)
> +		return 0;
> +
> +	return info->char_per_block[plane] * 8 /
> +	       (drm_format_info_block_width(info, plane) *
> +		drm_format_info_block_height(info, plane));

Do we really needs this for blocky formats where this is potentially
ill-defined? I think if there's no need then this should also return 0
when block_width/height != 1, it doesn't make much sense to compute bpp
when it's not really bits per _pixel_.

Minimally this needs to check whether the division actually makes sense or
whether there's a reminder, and if there's  reminder, then fail. But that
feels like a bad hack and I think we should avoid it if it's not
absolutely necessary.

Otherwise lgtm.
-Daniel

> +}
> +EXPORT_SYMBOL(drm_format_info_bpp);
> +
>  /**
>   * drm_format_info_min_pitch - computes the minimum required pitch in bytes
>   * @info: pixel format info
> diff --git a/include/drm/drm_fourcc.h b/include/drm/drm_fourcc.h
> index 22aa64d07c7905e2..3800a7ad7f0cda7a 100644
> --- a/include/drm/drm_fourcc.h
> +++ b/include/drm/drm_fourcc.h
> @@ -313,6 +313,7 @@ unsigned int drm_format_info_block_width(const struct drm_format_info *info,
>  					 int plane);
>  unsigned int drm_format_info_block_height(const struct drm_format_info *info,
>  					  int plane);
> +unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane);
>  uint64_t drm_format_info_min_pitch(const struct drm_format_info *info,
>  				   int plane, unsigned int buffer_width);
>  
> -- 
> 2.25.1
> 

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