Message-ID: <qutg3vvms7whzxpdy57wzvbwciw7zuq5txudy3hn7lmvmjebrt@ccddcrydz57s>
Date: Tue, 23 Sep 2025 18:20:41 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>, Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jyri Sarha <jyri.sarha@....fi>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        Devarsh Thakkar <devarsht@...com>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/29] drm/atomic: Add atomic_state_readout infrastructure

On Tue, Sep 23, 2025 at 02:41:16PM +0300, Laurent Pinchart wrote:
> On Tue, Sep 23, 2025 at 01:43:29PM +0300, Dmitry Baryshkov wrote:
> > On Tue, Sep 23, 2025 at 01:32:23PM +0300, Laurent Pinchart wrote:
> > > On Tue, Sep 23, 2025 at 01:28:57PM +0300, Dmitry Baryshkov wrote:
> > > > On Tue, Sep 23, 2025 at 11:38:17AM +0200, Maxime Ripard wrote:
> > > > > On Mon, Sep 15, 2025 at 09:38:44PM +0300, Dmitry Baryshkov wrote:
> > > > > > On Mon, Sep 15, 2025 at 10:42:22AM +0200, Maxime Ripard wrote:
> > > > > > > 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.
> > > > > > 
> > > > > > DP bridge drivers have a pretty big state (DPCD and all the features).
> > > > > 
> > > > > I meant drm_bridge_state. Subclasses would have their own implementation
> > > > > anyway.
> > > > > 
> > > > > > Other bridge drivers randomly leak state to the non-state structs.
> > > > > 
> > > > > I'm not sure what you mean by that though. Can you expand?
> > > > 
> > > > I think I've seen bridge drivers which stored subclassed drm_bridge
> > > > instead of drm_bridge_state or stored the data in the long-living data
> > > > structures. YEs, that's a bug, which should be fixed on its own.
> > > 
> > > There's one exception to the "all state in state structure" rules if I
> > > understand things correctly (and I'm mentioning it here partly to be
> > > corrected if my understanding is wrong). Active state data that needs to
> > > be accessed from a non-sleepable context need to be copied to the
> > > device-specific structure when the state is applied, as we can't take
> > > the mutex protecting state access there.
> > 
> > I'd even relax this slightly (please correct me if I'm wrong here): the
> > state can be extracted to the device-specific structure in
> > atomic_pre_enable() and cleared in atomic_post_disable().
> 
> Overall I agree. I think this should be minimized when reasonably
> possible to avoid data duplication though.
> 
> > Likewise it's generally fine to store non-state changing data in the
> > private data (e.g. hw params or infoframe from the HDMI codec data).
> 
> Most hardware parameters are a translation of the software state. I
> would store them in the state when possible (e.g. when they need to be
> computed at atomic check time, and when the computation is expensive
> enough to make caching useful), and store in private data only the data
> that needs to also be accessed from a location where state access is not
> easily possible.

I'd agree here.

> 
> > > I'd expect that to be uncommon
> > > for bridges.

-- 
With best wishes
Dmitry

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