Re: [PATCH v5 05/16] drm/vkms: Add dummy pixel_read/pixel_write callbacks to avoid NULL pointers

[Pekka Paalanen]

On Wed, 13 Mar 2024 18:44:59 +0100
Louis Chauvet <louis.chauvet@xxxxxxxxxxx> wrote:

> Introduce two callbacks which does nothing. They are used in replacement
> of NULL and it avoid kernel OOPS if this NULL is called.
> 
> If those callback are used, it means that there is a mismatch between
> what formats are announced by atomic_check and what is realy supported by
> atomic_update.
> 
> Signed-off-by: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/vkms/vkms_formats.c | 43 +++++++++++++++++++++++++++++++------
>  1 file changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> index 55a4365d21a4..b57d85b8b935 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -136,6 +136,21 @@ static void RGB565_to_argb_u16(u8 *in_pixel, struct pixel_argb_u16 *out_pixel)
>  	out_pixel->b = drm_fixp2int_round(drm_fixp_mul(fp_b, fp_rb_ratio));
>  }
>  
> +/**
> + * black_to_argb_u16() - pixel_read callback which always read black
> + *
> + * This callback is used when an invalid format is requested for plane reading.
> + * It is used to avoid null pointer to be used as a function. In theory, this function should
> + * never be called, except if you found a bug in the driver/DRM core.
> + */
> +static void black_to_argb_u16(u8 *in_pixel, struct pixel_argb_u16 *out_pixel)
> +{
> +	out_pixel->a = (u16)0xFFFF;
> +	out_pixel->r = 0;
> +	out_pixel->g = 0;
> +	out_pixel->b = 0;
> +}
> +
>  /**
>   * vkms_compose_row - compose a single row of a plane
>   * @stage_buffer: output line with the composed pixels
> @@ -238,6 +253,16 @@ static void argb_u16_to_RGB565(u8 *out_pixel, struct pixel_argb_u16 *in_pixel)
>  	*pixel = cpu_to_le16(r << 11 | g << 5 | b);
>  }
>  
> +/**
> + * argb_u16_to_nothing() - pixel_write callback with no effect
> + *
> + * This callback is used when an invalid format is requested for writeback.
> + * It is used to avoid null pointer to be used as a function. In theory, this should never
> + * happen, except if there is a bug in the driver
> + */
> +static void argb_u16_to_nothing(u8 *out_pixel, struct pixel_argb_u16 *in_pixel)
> +{}
> +
>  /**
>   * Generic loop for all supported writeback format. It is executed just after the blending to
>   * write a line in the writeback buffer.
> @@ -261,8 +286,8 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
>  
>  /**
>   * Retrieve the correct read_pixel function for a specific format.
> - * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
> - * pointer is valid before using it in a vkms_plane_state.
> + * If the format is not supported by VKMS a warn is emitted and a dummy "always read black"
> + * function is returned.
>   *
>   * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
>   */
> @@ -285,18 +310,21 @@ pixel_read_t get_pixel_read_function(u32 format)
>  		 * format must:
>  		 * - Be listed in vkms_formats in vkms_plane.c
>  		 * - Have a pixel_read callback defined here
> +		 *
> +		 * To avoid kernel crash, a dummy "always read black" function is used It means
> +		 * that during the composition, this plane will always be black.
>  		 */
>  		WARN(true,
>  		     "Pixel format %p4cc is not supported by VKMS planes. This is a kernel bug, atomic check must forbid this configuration.\n",
>  		     &format);
> -		return (pixel_read_t)NULL;
> +		return &black_to_argb_u16;

Hi Louis,

I'm perhaps a bit paranoid in these things, but I'd make this not
black. Maybe something more "screaming" like magenta. There is a slight
chance that black might sometimes be expected, or not affect the
result. After all, blending something into black with pre-multiplied
alpha is equivalent to no-blending (a copy). The kernel warning is
good, the magenta is more like an assurance.

Anyway,

Acked-by: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx>


Thanks,
pq


>  	}
>  }
>  
>  /**
>   * Retrieve the correct write_pixel function for a specific format.
> - * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
> - * pointer is valid before using it in a vkms_writeback_job.
> + * If the format is not supported by VKMS a warn is emitted and a dummy "don't do anything"
> + * function is returned.
>   *
>   * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
>   */
> @@ -319,10 +347,13 @@ pixel_write_t get_pixel_write_function(u32 format)
>  		 * format must:
>  		 * - Be listed in vkms_wb_formats in vkms_writeback.c
>  		 * - Have a pixel_write callback defined here
> +		 *
> +		 * To avoid kernel crash, a dummy "don't do anything" function is used It means
> +		 * that the resulting writeback buffer is not composed and can contains any values.
>  		 */
>  		WARN(true,
>  		     "Pixel format %p4cc is not supported by VKMS writeback. This is a kernel bug, atomic check must forbid this configuration.\n",
>  		     &format);
> -		return (pixel_write_t)NULL;
> +		return &argb_u16_to_nothing;
>  	}
>  }
>

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