[PATCH v2 1/2] drm: Introduce plane SIZE_HINTS property

[Ville Syrjälä]

On Thu, Feb 09, 2023 at 01:58:55PM +0200, Pekka Paalanen wrote:
> On Wed,  8 Feb 2023 23:10:16 +0200
> Ville Syrjala <ville.syrjala at linux.intel.com> wrote:
> 
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > Add a new immutable plane property by which a plane can advertise
> > a handful of recommended plane sizes. This would be mostly exposed
> > by cursor planes as a slightly more capable replacement for
> > the DRM_CAP_CURSOR_WIDTH/HEIGHT caps, which can only declare
> > a one size fits all limit for the whole device.
> > 
> > Currently eg. amdgpu/i915/nouveau just advertize the max cursor
> > size via the cursor size caps. But always using the max sized
> > cursor can waste a surprising amount of power, so a better
> > stragey is desirable.
> > 
> > Most other drivers don't specify any cursor size at all, in
> > which case the ioctl code just claims that 64x64 is a great
> > choice. Whether that is actually true is debatable.
> > 
> > A poll of various compositor developers informs us that
> > blindly probing with setcursor/atomic ioctl to determine
> > suitable cursor sizes is not acceptable, thus the
> > introduction of the new property to supplant the cursor
> > size caps. The compositor will now be free to select a
> > more optimal cursor size from the short list of options.
> > 
> > Note that the reported sizes (either via the property or the
> > caps) make no claims about things such as plane scaling. So
> > these things should only really be consulted for simple
> > "cursor like" use cases.
> > 
> > v2: Try to add some docs
> > 
> > Cc: Simon Ser <contact at emersion.fr>
> > Cc: Jonas Ådahl <jadahl at redhat.com>
> > Cc: Daniel Stone <daniel at fooishbar.org>
> > Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> > Acked-by: Harry Wentland <harry.wentland at amd.com>
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/drm_mode_config.c |  7 +++++
> >  drivers/gpu/drm/drm_plane.c       | 48 +++++++++++++++++++++++++++++++
> >  include/drm/drm_mode_config.h     |  5 ++++
> >  include/drm/drm_plane.h           |  4 +++
> >  include/uapi/drm/drm_mode.h       | 11 +++++++
> >  5 files changed, 75 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> > index 87eb591fe9b5..21860f94a18c 100644
> > --- a/drivers/gpu/drm/drm_mode_config.c
> > +++ b/drivers/gpu/drm/drm_mode_config.c
> > @@ -374,6 +374,13 @@ static int drm_mode_create_standard_properties(struct drm_device *dev)
> >  		return -ENOMEM;
> >  	dev->mode_config.modifiers_property = prop;
> >  
> > +	prop = drm_property_create(dev,
> > +				   DRM_MODE_PROP_IMMUTABLE | DRM_MODE_PROP_BLOB,
> > +				   "SIZE_HINTS", 0);
> > +	if (!prop)
> > +		return -ENOMEM;
> > +	dev->mode_config.size_hints_property = prop;
> > +
> >  	return 0;
> >  }
> >  
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index 24e7998d1731..ae51b1f83755 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -140,6 +140,21 @@
> >   *     DRM_FORMAT_MOD_LINEAR. Before linux kernel release v5.1 there have been
> >   *     various bugs in this area with inconsistencies between the capability
> >   *     flag and per-plane properties.
> > + *
> > + * SIZE_HINTS:
> > + *     Blob property which contains the set of recommended plane size
> > + *     which can used for simple "cursor like" use cases (eg. no scaling).
> > + *     Using these hints frees userspace from extensive probing of
> > + *     supported plane sizes through atomic/setcursor ioctls.
> > + *
> > + *     For optimal usage userspace should pick the smallest size
> > + *     that satisfies its own requirements.
> > + *
> > + *     The blob contains an array of struct drm_plane_size_hint.
> > + *
> > + *     Drivers should only attach this property to planes that
> > + *     support a very limited set of sizes (eg. cursor planes
> > + *     on typical hardware).
> 
> Hi Ville,
> 
> sounds good. Maybe a minor nit about "typical hardware". Would e.g.
> "legacy PC hardware" be more accurate?

"legacy" doesn't feel quite right for current and upcoming hardware.

-- 
Ville Syrjälä
Intel