linux-kernel - Re: [PATCH v8 04/17] drm/vkms: Add typedef and documentation for pixel_read and pixel

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Message-ID: <ZpTdmxfqx3ulALk8@louis-chauvet-laptop>
Date: Mon, 15 Jul 2024 10:28:11 +0200
From: Louis Chauvet <louis.chauvet@...tlin.com>
To: Maíra Canal <mcanal@...lia.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
	Melissa Wen <melissa.srw@...il.com>,
	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>, rdunlap@...radead.org,
	arthurgrillo@...eup.net, Jonathan Corbet <corbet@....net>,
	pekka.paalanen@...oniitty.fi, 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,
	Pekka Paalanen <pekka.paalanen@...labora.com>
Subject: Re: [PATCH v8 04/17] drm/vkms: Add typedef and documentation for
 pixel_read and pixel_write functions

Le 13/07/24 - 11:38, Maíra Canal a écrit :
> On 5/16/24 10:04, Louis Chauvet wrote:
> > Introduce two typedefs: pixel_read_t and pixel_write_t. It allows the
> > compiler to check if the passed functions take the correct arguments.
> > Such typedefs will help ensuring consistency across the code base in
> > case of update of these prototypes.
> > 
> > Rename input/output variable in a consistent way between read_line and
> > write_line.
> > 
> > A warn has been added in get_pixel_*_function to alert when an unsupported
> > pixel format is requested. As those formats are checked before
> > atomic_update callbacks, it should never happen.
> > 
> > Document for those typedefs.
> > 
> > Reviewed-by: Pekka Paalanen <pekka.paalanen@...labora.com>
> > Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>

[...]

> >   /**
> > @@ -265,7 +265,7 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
> >    *
> >    * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
> >    */
> > -void *get_pixel_conversion_function(u32 format)
> > +pixel_read_t get_pixel_read_function(u32 format)
> >   {
> >   	switch (format) {
> >   	case DRM_FORMAT_ARGB8888:
> > @@ -279,7 +279,16 @@ void *get_pixel_conversion_function(u32 format)
> >   	case DRM_FORMAT_RGB565:
> >   		return &RGB565_to_argb_u16;
> >   	default:
> > -		return NULL;
> > +		/*
> > +		 * This is a bug in vkms_plane_atomic_check(). All the supported
> > +		 * format must:
> > +		 * - Be listed in vkms_formats in vkms_plane.c
> > +		 * - Have a pixel_read callback defined here
> > +		 */
> > +		WARN(true,
> > +		     "Pixel format %p4cc is not supported by VKMS planes. This is a kernel bug, atomic check must forbid this configuration.\n",
> > +		     &format);
> 
> Well, if this is a kernel bug, shouldn't we use a BUG_ON()/BUG() macro?

I don't know what is the "rule" for this kind of bug. Yes it is a kernel 
bug, but it is recoverable and I can avoid a full kernel crash by using a 
"do nothing" callback. 

If you prefer I can replace this by:

	pr_err("Pixel format %p4cc is [...]", &format);
	BUG();

and drop the next commit. The kernel will crash if there is a mismatch 
between advertised supported formats and formats listed here.

> Best Regards,
> - Maíra
> 
> >    * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
> >    */
> > -void *get_pixel_write_function(u32 format)
> > +pixel_write_t get_pixel_write_function(u32 format)
> >   {
> >   	switch (format) {
> >   	case DRM_FORMAT_ARGB8888:
> > @@ -304,6 +313,15 @@ void *get_pixel_write_function(u32 format)
> >   	case DRM_FORMAT_RGB565:
> >   		return &argb_u16_to_RGB565;
> >   	default:
> > -		return NULL;
> > +		/*
> > +		 * This is a bug in vkms_writeback_atomic_check. All the supported
> > +		 * format must:
> > +		 * - Be listed in vkms_wb_formats in vkms_writeback.c
> > +		 * - Have a pixel_write callback defined here
> > +		 */
> > +		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;

I can do the same here.

Thanks,
Louis Chauvet

[...]

-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com