Re: [PATCH v2 4/9] drm/vkms: Add typedef and documentation for pixel_read and pixel_write functions

[Louis Chauvet <louis.chauvet@bootlin.com>]

Le 29/02/24 - 11:07, Pekka Paalanen a écrit :
> On Tue, 27 Feb 2024 16:02:13 +0100
> Louis Chauvet <louis.chauvet@bootlin.com> wrote:
> 
> > Le 26/02/24 - 13:36, Pekka Paalanen a écrit :
> > > On Fri, 23 Feb 2024 12:37:24 +0100
> > > Louis Chauvet <louis.chauvet@bootlin.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@bootlin.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"
> 
> Sounds good!
> 
> > 
> > [...]
> > 
> > > > 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).
> 
> My concern here is that does hitting this cause additional breakage of
> the UAPI contract? For example, the UAPI contract expects that the old
> FB is unreffed and the new FB is reffed by the plane in question. If
> this early return causes that FB swap to be skipped, it could cause
> secondary unexpected failures or misbehaviour for userspace later. That
> could mislead debugging to think that there is a userspace bug.
>
> Even if you cannot actually read FB due to an internal bug, it would be
> good to still apply all the state changes that the UAPI contract
> mandates.
> 
> Unless, this is a kernel bug kind of thing which explodes very
> verbosely, but DRM_WARN is not that.

You are right. In this case I maybe can just create a dummy 
"pixel_read" which always return black? (The v4 will have it so you can 
see how it look)

This way, I can:
- keep the check and avoid null function pointers (and OOPS);
- log (WARN, DRM_WARN, DRM_ERROR or whatever suggested by DRM maintainers 
to warn very loudly);
- Apply the requested change from userspace (and don't break the UAPI 
contract).

The resulting behavior will be "the virtual plane is black", which is, I
think, not very important. As the primary usage of VKMS is testing, it
will just broke all the tests, and a quick look at the kernel logs will
show this bug.
 
> > > 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.
> 
> I only know about the UAPI, I'm not familiar with kernel internals.
> 
> We see that atomic_update returns void, so I think it simply cannot
> return errors. To my understanding, atomic_check needs to ensure that
> atomic_update cannot fail. There is even UAPI to exercise atomic_check
> alone: the atomic commit TEST_ONLY flag. Userspace trusts that flag, and
> will not expect an identical atomic commit to fail without TEST_ONLY
> when it succeeded with TEST_ONLY.

That my understanding of the UAPI/DRM internals too, is my suggestion 
above sufficient? It will always succeed, no kernel OOPS.

> > 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.
> 
> Maybe maintainers can comment whether even DRM_ERROR is strong enough.
> 
> As for the message, what you wrote in the comment is the most important
> part that I'd put in the log. It explains what's going on, while that
> "format not supported" is a detail without context.
> 

Is something like this better?

	/*
	 * This is a bug in vkms_plane_atomic_check. All the supported 
	 * format must:
	 * - Be listed in vkms_formats
	 * - Have a pixel_read_line callback
	 */
	WARN(true, "Pixel format %4cc is not supported by VKMS planes. This is a kernel bug. Atomic check must forbid this configuration.\n", fmt)

> Thanks,
> pq

[...]

Kind regards,
Louis Chauvet

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