linux-kernel - Re: [PATCH v2 4/9] drm/vkms: Add typedef and documentation for pixel_read and pixel

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Message-ID: <Zd35ddrlHduMq3_Y@localhost.localdomain>
Date: Tue, 27 Feb 2024 16:02:13 +0100
From: Louis Chauvet <louis.chauvet@...tlin.com>
To: Pekka Paalanen <pekka.paalanen@...labora.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
Subject: Re: [PATCH v2 4/9] drm/vkms: Add typedef and documentation for
 pixel_read and pixel_write functions

Le 26/02/24 - 13:36, Pekka Paalanen a écrit :
> On Fri, 23 Feb 2024 12:37:24 +0100
> Louis Chauvet <louis.chauvet@...tlin.com> 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.
> > 
> > Introduce a check around the get_pixel_*_functions to avoid using a
> > nullptr as a function.
> > 
> > Document for those typedefs.
> > 
> > Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
> > ---
> >  drivers/gpu/drm/vkms/vkms_drv.h       | 23 +++++++++++++++++++++--
> >  drivers/gpu/drm/vkms/vkms_formats.c   |  8 ++++----
> >  drivers/gpu/drm/vkms/vkms_formats.h   |  4 ++--
> >  drivers/gpu/drm/vkms/vkms_plane.c     |  9 ++++++++-
> >  drivers/gpu/drm/vkms/vkms_writeback.c |  9 ++++++++-
> >  5 files changed, 43 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> > index 18086423a3a7..886c885c8cf5 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.h
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> > @@ -53,12 +53,31 @@ struct line_buffer {
> >  	struct pixel_argb_u16 *pixels;
> >  };
> >  
> > +/**
> > + * typedef pixel_write_t - These functions are used to read a pixel from a
> > + * `struct pixel_argb_u16*`, convert it in a specific format and write it in the @dst_pixels
> > + * buffer.
> > + *
> > + * @dst_pixel: destination address to write the pixel
> > + * @in_pixel: pixel to write
> > + */
> > +typedef void (*pixel_write_t)(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel);
> 
> There are some inconsistencies in pixel_write_t and pixel_read_t. At
> this point of the series they still operate on a single pixel, but you
> use dst_pixels and src_pixels, plural. Yet the documentation correctly
> talks about processing a single pixel.

I will fix this for v4.
 
> I would also expect the source to be always const, but that's a whole
> another patch to change.

The v4 will contains a new patch "drm/vkms: Use const pointer for 
pixel_read and pixel_write functions"

[...]

> > diff --git a/drivers/gpu/drm/vkms/vkms_plane.c b/drivers/gpu/drm/vkms/vkms_plane.c
> > index d5203f531d96..f68b1b03d632 100644
> > --- a/drivers/gpu/drm/vkms/vkms_plane.c
> > +++ b/drivers/gpu/drm/vkms/vkms_plane.c
> > @@ -106,6 +106,13 @@ static void vkms_plane_atomic_update(struct drm_plane *plane,
> >  		return;
> >  
> >  	fmt = fb->format->format;
> > +	pixel_read_t pixel_read = get_pixel_read_function(fmt);
> > +
> > +	if (!pixel_read) {
> > +		DRM_WARN("Pixel format is not supported by VKMS planes. State is inchanged\n");
> 
> DRM_WARN() is the kernel equivalent to userspace assert(), right?

For the DRM_WARN it is just a standard prinkt(KERN_WARN, ...) (hidden 
behind drm internal macros).

> In that failing the check means an internal invariant was violated,
> which means a code bug in kernel?
>
> Maybe this could be more specific about what invariant was violated?
> E.g. atomic check should have rejected this attempt already.

I'm not an expert (yet) in DRM, so please correct me:
When atomic_update is called, the new state is always validated by 
atomic_check before? There is no way to pass something not validated by 
atomic_check to atomic_update? If this is the case, yes, it should be an 
ERROR and not a WARN as an invalid format passed the atomic_check 
verification.

If so, is this better?

	if (!pixel_read) {
		/*
		 * This is a bug as the vkms_plane_atomic_check must forbid all unsupported formats.
		 */
		DRM_ERROR("Pixel format %4cc is not supported by VKMS planes.\n", fmt);
		return;
	}

I will put the same code in vkms_writeback.c.

[...]

Kind regards,
Louis Chauvet


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