Hi Tohmas,
On Tue, Sep 02, 2025 at 03:44:54PM +0200, Thomas Zimmermann wrote:
> > +/**
> > + * drm_atomic_build_readout_state - Creates an initial state from the
> > hardware
> > + * @dev: DRM device to build the state for
> > + *
> > + * This function allocates a &struct drm_atomic_state, calls the
> > + * atomic_readout_state callbacks, and fills the global state old states
> > + * by what the callbacks returned.
> > + *
> > + * Returns:
> > + *
> > + * A partially initialized &struct drm_atomic_state on success, an error
> > + * pointer otherwise.
> > + */
> > +static struct drm_atomic_state *
> > +drm_atomic_build_readout_state(struct drm_device *dev)
> > +{
> > + struct drm_connector_list_iter conn_iter;
> > + struct drm_atomic_state *state;
> > + struct drm_mode_config *config =
> > + &dev->mode_config;
> > + struct drm_connector *connector;
> > + struct drm_printer p =
> > + drm_info_printer(dev->dev);
> > + struct drm_encoder *encoder;
> > + struct drm_plane *plane;
> > + struct drm_crtc *crtc;
> > + int ret;
> > +
> > + drm_dbg_kms(dev, "Starting to build atomic state from hardware
> > state.\n");
> > +
> > + state = drm_atomic_state_alloc(dev);
> > + if (WARN_ON(!state))
> > + return ERR_PTR(-ENOMEM);
> > +
> > + state->connectors = kcalloc(config->num_connector,
> > sizeof(*state->connectors), GFP_KERNEL);
> > + if (WARN_ON(!state->connectors)) {
> > + ret = -ENOMEM;
> > + goto err_state_put;
> > + }
> > +
> > + state->private_objs = kcalloc(count_private_obj(dev),
> > sizeof(*state->private_objs), GFP_KERNEL);
> > + if (WARN_ON(!state->private_objs)) {
> > + ret = -ENOMEM;
> > + goto err_state_put;
> > + }
> > +
> > + drm_for_each_crtc(crtc, dev) {
> > + const struct drm_crtc_funcs *crtc_funcs =
> > + crtc->funcs;
> > + struct drm_crtc_state *crtc_state;
> > +
> > + drm_dbg_kms(dev, "Initializing CRTC %s state.\n", crtc->name);
> > +
> > + if (crtc_funcs->atomic_readout_state) {
> > + crtc_state = crtc_funcs->atomic_readout_state(crtc);
> > + } else if (crtc_funcs->reset) {
> > + crtc_funcs->reset(crtc);
> > +
> > + /*
> > + * We don't want to set crtc->state field yet. Let's
> > save and clear it up.
> > + */
> > + crtc_state = crtc->state;
> > + crtc->state = NULL;
>
> Chancing the crtc->state pointer behind the back of the reset callback seems
> fragile. We never how if some other piece of the driver refers to it
> (although illegally).
I agree that it's clunky. I'm not sure who would use it at this point
though: we're in the middle of the drm_mode_config_reset(), so the
drivers' involvement is pretty minimal.
I did wonder if changing reset to return the object instead of setting
$OBJECT->state would be a better interface?
> For now, wouldn't it be better to require a read-out helper for all elements
> of the driver's mode-setting pipeline? The trivial implementation would
> copy the existing reset function and keep crtc->state to NULL.
I also considered that, but I'm not sure we can expect bridges to have
readout hooks filled for every configuration in the wild.
But maybe we can look during drm_mode_config_reset() at whether all the
objects have their hook filled, and if not fall back on reset for
everything.
It would make the implementation easier, but missing bridges
implementations would trigger a mode change when it might actually work
just fine since bridge state is pretty minimal.
Idk.
> > --- a/include/drm/drm_bridge.h
> > +++ b/include/drm/drm_bridge.h
> > @@ -490,10 +490,31 @@ struct drm_bridge_funcs {
> > * The @atomic_post_disable callback is optional.
> > */
> > void (*atomic_post_disable)(struct drm_bridge *bridge,
> > struct drm_atomic_state *state);
> > + /**
> > + * @atomic_readout_state:
> > + *
> > + * Initializes,this bridge atomic state.
> > + *
> > + * It's meant to be used by drivers that wants to implement fast
>
> 'want'
>
> > + * / flicker-free boot and allows to initialize the atomic state
>
> I think we should only call it flicker-free boot. Fast boot is misleading.
I agree, but it's also how it's been called by the only implementation
of it we have so far (i915), and the name of the module parameter that
controls it.
Maxime
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