linux-kernel - Re: [PATCH 23/29] drm/bridge: Provide a helper to retrieve current bridge state

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Message-ID: <Z4jvH9yud8TfXPJ0@phenom.ffwll.local>
Date: Thu, 16 Jan 2025 12:35:59 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Maxime Ripard <mripard@...nel.org>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	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>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Jonas Karlman <jonas@...boo.se>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Douglas Anderson <dianders@...omium.org>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 23/29] drm/bridge: Provide a helper to retrieve current
 bridge state

On Thu, Jan 16, 2025 at 02:43:37AM +0200, Dmitry Baryshkov wrote:
> On Wed, Jan 15, 2025 at 10:05:30PM +0100, Maxime Ripard wrote:
> > The current bridge state is accessible from the drm_bridge structure,
> > but since it's fairly indirect it's not easy to figure out.
> > 
> > Provide a helper to retrieve it.
> > 
> > Signed-off-by: Maxime Ripard <mripard@...nel.org>
> > ---
> >  include/drm/drm_bridge.h | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> > index 8e18130be8bb85fc2463917dde9bf1d281934184..95c5037a6335e4c1be511e6c31308202015c7754 100644
> > --- a/include/drm/drm_bridge.h
> > +++ b/include/drm/drm_bridge.h
> > @@ -955,10 +955,27 @@ static inline struct drm_bridge *of_drm_find_bridge(struct device_node *np)
> >  {
> >  	return NULL;
> >  }
> >  #endif
> >  
> > +/**
> > + * @drm_bridge_get_current_state() - Get the current bridge state
> > + * @bridge: bridge object
> > + *
> > + * RETURNS:
> > + *
> > + * The current bridge state, or NULL if there is none.
> 
> Are there any kind of limitations on when and how this function can be
> used? I don't think we can be accessing the state randomly, as the
> framework can change it at some points. E.g. what if the driver uses
> this state from audio or cec callbacks, while the DRM framework performs
> atomic commit and changes / frees the state right concurrently?

Yeah you can only look at this when either holding the corresponding
modesetlock, or in atomic commit. But in the latter it's a much cleaner
design pattern to instead look up the state from the drm_atomic_state,
since that pointer is the magic thing which guarantees lifetim/ownership
for lockless access in atomic commit code.
-Sima

> 
> > + */
> > +static inline struct drm_bridge_state *
> > +drm_bridge_get_current_state(struct drm_bridge *bridge)
> > +{
> > +	if (!bridge)
> > +		return NULL;
> > +
> > +	return drm_priv_to_bridge_state(bridge->base.state);
> > +}
> > +
> >  /**
> >   * drm_bridge_get_next_bridge() - Get the next bridge in the chain
> >   * @bridge: bridge object
> >   *
> >   * RETURNS:
> > 
> > -- 
> > 2.47.1
> > 
> 
> -- 
> With best wishes
> Dmitry

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