Re: [PATCH v8 05/17] drm/vkms: Add dummy pixel_read/pixel_write callbacks to avoid NULL pointers

["Maíra Canal" <mcanal@igalia.com>]

On 5/16/24 10:04, Louis Chauvet wrote:
> Introduce two callbacks which does nothing. They are used in replacement
> of NULL and it avoid kernel OOPS if this NULL is called.

I don't believe we should avoid a reasonable kernel OOPS. As you
noticed, if the user got this kernel OOPS it means that there is a
mismatch between what formats are announced by atomic_check and what is
really supported by atomic_update. This is a driver error.

If this happens, I want the kernel OOPS because it means I need to fix
the driver. Sometimes a kernel OOPS can save you some valuable debugging
time.

I'd probably drop this patch.

Best Regards,
- Maíra

> 
> 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.
> 
> Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> ---
>   drivers/gpu/drm/vkms/vkms_formats.c | 45 ++++++++++++++++++++++++++++++++-----
>   1 file changed, 39 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> index 6b3e17374b19..c28c32b00e39 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -135,6 +135,21 @@ 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));
>   }
>   
> +/**
> + * magenta_to_argb_u16() - pixel_read callback which always read magenta
> + *
> + * 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 magenta_to_argb_u16(u8 *in_pixel, struct pixel_argb_u16 *out_pixel)
> +{
> +	out_pixel->a = (u16)0xFFFF;
> +	out_pixel->r = (u16)0xFFFF;
> +	out_pixel->g = 0;
> +	out_pixel->b = (u16)0xFFFF;
> +}
> +
>   /**
>    * vkms_compose_row - compose a single row of a plane
>    * @stage_buffer: output line with the composed pixels
> @@ -237,6 +252,16 @@ 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)
> +{}
> +
>   /**
>    * vkms_writeback_row() - Generic loop for all supported writeback format. It is executed just
>    * after the blending to write a line in the writeback buffer.
> @@ -260,8 +285,10 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
>   
>   /**
>    * get_pixel_conversion_function() - 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.
> + * format.
> + *
> + * If the format is not supported by VKMS a warning is emitted and a dummy "always read magenta"
> + * function is returned.
>    *
>    * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
>    */
> @@ -284,18 +311,21 @@ 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 magenta" function is used. It means
> +		 * that during the composition, this plane will always be magenta.
>   		 */
>   		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 &magenta_to_argb_u16;
>   	}
>   }
>   
>   /**
>    * get_pixel_write_function() - 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 warning 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])
>    */
> @@ -318,10 +348,13 @@ 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;
>   	}
>   }
>