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Message-ID: <20240325140542.4fffd42c.pekka.paalanen@collabora.com>
Date: Mon, 25 Mar 2024 14:05:42 +0200
From: Pekka Paalanen <pekka.paalanen@...labora.com>
To: Louis Chauvet <louis.chauvet@...tlin.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>, Melissa Wen
<melissa.srw@...il.com>, Maíra Canal
<mairacanal@...eup.net>, Haneen Mohammed <hamohammed.sa@...il.com>, Daniel
Vetter <daniel@...ll.ch>, Maarten Lankhorst
<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
arthurgrillo@...eup.net, Jonathan Corbet <corbet@....net>,
dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
jeremie.dautheribes@...tlin.com, miquel.raynal@...tlin.com,
thomas.petazzoni@...tlin.com, seanpaul@...gle.com, marcheu@...gle.com,
nicolejadeyee@...gle.com
Subject: Re: [PATCH v5 05/16] drm/vkms: Add dummy pixel_read/pixel_write
callbacks to avoid NULL pointers
On Wed, 13 Mar 2024 18:44:59 +0100
Louis Chauvet <louis.chauvet@...tlin.com> 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@...tlin.com>
> ---
> 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@...labora.com>
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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