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

[Daniel Vetter]

On Tue, Apr 13, 2021 at 01:54:00PM +0200, Lucas Stach wrote:
> Am Dienstag, dem 13.04.2021 um 11:49 +0200 schrieb Daniel Vetter:
> > 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).
> 
> For a lot of the embedded display drivers that never had any out-of-
> band tiling meta shared with the GPU part, the implied modifier is
> actually DRM_FORMAT_MOD_LINEAR, so maybe that's where some of the
> confusion about needing to specify the modifier list comes from.

Yeah that entire discussion last few days is why I wanted to audit all the
drivers and make sure that going forward we're more explicit&consistent
with all this. There's huge confusion around implied modifier vs "no
IN_FORMATS blob property" and what that exactly means.
-Daniel

> > Now that this is all done consistently across all drivers, document
> > the rules and enforce it in the drm core.
> 
> This clarification looks good to me.
> 
> Reviewed-by: Lucas Stach <l.stach at pengutronix.de>
> 
> > 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.
> >   */
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  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));
> >  		config->allow_fb_modifiers = true;
> > +	} else {
> > +		WARN_ON(config->allow_fb_modifiers);
> > +	}
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  	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.
> > + *
> >   * 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:
> >  	 *
> 
> 

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