[PATCH 12/12] drm/modifiers: Enforce consistency between the cap an IN_FORMATS

[Daniel Vetter]

On Tue, Apr 13, 2021 at 02:56:02PM +0300, Pekka Paalanen wrote:
> On Tue, 13 Apr 2021 11:49:03 +0200
> Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> 
> > It's very confusing for userspace to have to deal with inconsistencies
> > here, and some drivers screwed this up a bit. Most just ommitted the
> > format list when they meant to say that only linear modifier is
> > allowed, but some also meant that only implied modifiers are
> > acceptable (because actually none of the planes registered supported
> > modifiers).
> > 
> > Now that this is all done consistently across all drivers, document
> > the rules and enforce it in the drm core.
> > 
> > Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Maxime Ripard <mripard at kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann at suse.de>
> > Cc: David Airlie <airlied at linux.ie>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
> >  include/drm/drm_mode_config.h |  2 ++
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index 0dd43882fe7c..16a7e3e57f7f 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -128,6 +128,11 @@
> >   *     pairs supported by this plane. The blob is a struct
> >   *     drm_format_modifier_blob. Without this property the plane doesn't
> >   *     support buffers with modifiers. Userspace cannot change this property.
> > + *
> > + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> > + *     capability for general modifier support. If this flag is set then every
> > + *     plane will have the IN_FORMATS property, even when it only supports
> > + *     DRM_FORMAT_MOD_LINEAR.
> 
> Ooh, that's even better. But isn't that changing the meaning of the
> cap? Isn't the cap older than IN_FORMATS?

Hm indeed. But also how exactly are you going to user modifiers without
IN_FORMATS ... it's a bit hard. I think this is all because we've enabled
modifiers piece-by-piece and never across the entire thing (e.g. with
compositor and protocols), so the missing pieces only became apparent
later on.

I'm not sure whether compositors really want to support this, I guess
worst case we could disable the cap on these old kernels.

> What about the opposite? Is it allowed to have even a single IN_FORMATS
> if you don't have the cap?

That direction is enforced since 5.1, because some drivers screwed it up
and confusion in userspace ensued.

Should I add a bug that on kernels older than 5.1 the situation is more
murky and there's lots of bugs?

> 
> >   */
> >  
> >  static unsigned int drm_num_planes(struct drm_device *dev)
> > @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >  			format_modifier_count++;
> >  	}
> >  
> > -	if (format_modifier_count)
> > +	/* autoset the cap and check for consistency across all planes */
> > +	if (format_modifier_count) {
> > +		WARN_ON(!config->allow_fb_modifiers &&
> > +			!list_empty(&config->plane_list));
> 
> What does this mean?

If allow_fb_modifiers isn't set yet (we do that in the line below) and we
are _not_ the first plane that gets added to the driver (that's done
towards the end of the function) then that means there's already a plane
registered without modifiers and hence IN_FORMAT. Which we then warn
about.

> 
> >  		config->allow_fb_modifiers = true;
> > +	} else {
> > +		WARN_ON(config->allow_fb_modifiers);

This warning here checks the other case of an earlier plane with
modifiers, but the one we're adding now doesn't have them.
-Daniel

> > +	}
> >  
> >  	plane->modifier_count = format_modifier_count;
> >  	plane->modifiers = kmalloc_array(format_modifier_count,
> > @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >   * drm_universal_plane_init() to let the DRM managed resource infrastructure
> >   * take care of cleanup and deallocation.
> >   *
> > + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> > + * even those that only support DRM_FORMAT_MOD_LINEAR.
> 
> Good.
> 
> > + *
> >   * Returns:
> >   * Zero on success, error code on failure.
> >   */
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index ab424ddd7665..1ddf7783fdf7 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -909,6 +909,8 @@ struct drm_mode_config {
> >  	 * @allow_fb_modifiers:
> >  	 *
> >  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> > +	 * Note that drivers should not set this directly, it is automatically
> > +	 * set in drm_universal_plane_init().
> >  	 *
> >  	 * IMPORTANT:
> >  	 *
> 
> Thanks,
> pq



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch