On Thu, Feb 13, 2025 at 03:43:20PM +0100, Maxime Ripard wrote: > After some discussions on the mailing-list for an earlier revision of > the series, it was suggested to document the evolution of > drm_atomic_state and its use by drivers to explain some of the confusion > one might still encounter when reading the framework code. > > Suggested-by: Simona Vetter <simona.vetter@xxxxxxxx> > Link: https://lore.kernel.org/dri-devel/Z4jtKHY4qN3RNZNG@phenom.ffwll.local/ > Signed-off-by: Maxime Ripard <mripard@xxxxxxxxxx> Thanks for documenting that little bit of lore! Reviewed-by: Simona Vetter <simona.vetter@xxxxxxxx> Cheers, Sima > --- > include/drm/drm_atomic.h | 31 +++++++++++++++++++++++++++++++ > 1 file changed, 31 insertions(+) > > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h > index 1ded9a8d4e84d7d9879d7f60a876ba9d69785766..4c673f0698fef6b60f77db980378d5e88e0e250e 100644 > --- a/include/drm/drm_atomic.h > +++ b/include/drm/drm_atomic.h > @@ -355,10 +355,41 @@ struct __drm_private_objs_state { > * these. > * > * States are added to an atomic update by calling drm_atomic_get_crtc_state(), > * drm_atomic_get_plane_state(), drm_atomic_get_connector_state(), or for > * private state structures, drm_atomic_get_private_obj_state(). > + * > + * NOTE: struct drm_atomic_state first started as a single collection of > + * entities state pointers (drm_plane_state, drm_crtc_state, etc.). > + * > + * At atomic_check time, you could get the state about to be committed > + * from drm_atomic_state, and the one currently running from the > + * entities state pointer (drm_crtc.state, for example). After the call > + * to drm_atomic_helper_swap_state(), the entities state pointer would > + * contain the state previously checked, and the drm_atomic_state > + * structure the old state. > + * > + * Over time, and in order to avoid confusion, drm_atomic_state has > + * grown to have both the old state (ie, the state we replace) and the > + * new state (ie, the state we want to apply). Those names are stable > + * during the commit process, which makes it easier to reason about. > + * > + * You can still find some traces of that evolution through some hooks > + * or callbacks taking a drm_atomic_state parameter called names like > + * "old_state". This doesn't necessarily mean that the previous > + * drm_atomic_state is passed, but rather that this used to be the state > + * collection we were replacing after drm_atomic_helper_swap_state(), > + * but the variable name was never updated. > + * > + * Some atomic operations implementations followed a similar process. We > + * first started to pass the entity state only. However, it was pretty > + * cumbersome for drivers, and especially CRTCs, to retrieve the states > + * of other components. Thus, we switched to passing the whole > + * drm_atomic_state as a parameter to those operations. Similarly, the > + * transition isn't complete yet, and one might still find atomic > + * operations taking a drm_atomic_state pointer, or a component state > + * pointer. The former is the preferred form. > */ > struct drm_atomic_state { > /** > * @ref: > * > > -- > 2.48.0 > -- Simona Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
