linux-kernel - Re: [PATCH 25/29] drm/bridge: Provide pointers to the connector and crtc in bridge state

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Message-ID: <rqem5pqoufet66verjldsb5ihdwnfwcn6clwouj5l6p53f3ik6@sx52djr475yk>
Date: Thu, 16 Jan 2025 11:53:39 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: 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 25/29] drm/bridge: Provide pointers to the connector and
 crtc in bridge state

On Thu, Jan 16, 2025 at 09:42:54AM +0100, Maxime Ripard wrote:
> Hi,
> 
> On Thu, Jan 16, 2025 at 03:04:19AM +0200, Dmitry Baryshkov wrote:
> > On Wed, Jan 15, 2025 at 10:05:32PM +0100, Maxime Ripard wrote:
> > > Now that connectors are no longer necessarily created by the bridges
> > > drivers themselves but might be created by drm_bridge_connector, it's
> > > pretty hard for bridge drivers to retrieve pointers to the connector and
> > > CRTC they are attached to.
> > > 
> > > Indeed, the only way to retrieve the CRTC is to follow the drm_bridge
> > > encoder field, and then the drm_encoder crtc field, both of them being
> > > deprecated.
> > > 
> > > And for the connector, since we can have multiple connectors attached to
> > > a CRTC, we don't really have a reliable way to get it.
> > > 
> > > Let's provide both pointers in the drm_bridge_state structure so we
> > > don't have to follow deprecated, non-atomic, pointers, and be more
> > > consistent with the other KMS entities.
> > > 
> > > Signed-off-by: Maxime Ripard <mripard@...nel.org>
> > > ---
> > >  drivers/gpu/drm/drm_atomic_state_helper.c |  5 +++++
> > >  drivers/gpu/drm/drm_bridge.c              | 21 +++++++++++++--------
> > >  include/drm/drm_atomic.h                  | 14 ++++++++++++++
> > >  3 files changed, 32 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c b/drivers/gpu/drm/drm_atomic_state_helper.c
> > > index 519228eb109533d2596e899a57b571fa0995824f..66661dca077215b78dffca7bc1712f56d35e3918 100644
> > > --- a/drivers/gpu/drm/drm_atomic_state_helper.c
> > > +++ b/drivers/gpu/drm/drm_atomic_state_helper.c
> > > @@ -777,10 +777,15 @@ EXPORT_SYMBOL(drm_atomic_helper_bridge_duplicate_state);
> > >   * that don't subclass the bridge state.
> > >   */
> > >  void drm_atomic_helper_bridge_destroy_state(struct drm_bridge *bridge,
> > >  					    struct drm_bridge_state *state)
> > >  {
> > > +	if (state->connector) {
> > > +		drm_connector_put(state->connector);
> > > +		state->connector = NULL;
> > > +	}
> > > +
> > >  	kfree(state);
> > >  }
> > >  EXPORT_SYMBOL(drm_atomic_helper_bridge_destroy_state);
> > >  
> > >  /**
> > > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> > > index c937980d6591fd98e33e37d799ebf84e7e6c5529..069c105aa59636c64caffbefcf482133b0db97d9 100644
> > > --- a/drivers/gpu/drm/drm_bridge.c
> > > +++ b/drivers/gpu/drm/drm_bridge.c
> > > @@ -829,19 +829,24 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_enable);
> > >  
> > >  static int drm_atomic_bridge_check(struct drm_bridge *bridge,
> > >  				   struct drm_crtc_state *crtc_state,
> > >  				   struct drm_connector_state *conn_state)
> > >  {
> > > +	struct drm_bridge_state *bridge_state;
> > > +	int ret;
> > > +
> > > +	bridge_state = drm_atomic_get_new_bridge_state(crtc_state->state,
> > > +						       bridge);
> > 
> > It felt like an error to me to call this function for a non-atomic
> > bridges, until I fully followed the code path to find that it will
> > return NULL if the bridge isn't registered as a private object.
> 
> Yeah.. I wasn't too sure what to do about this one either. I think it
> would be more consistent to always have a state properly filled, even if
> we have !atomic drivers. It's what happens with the rest of the
> framework.

Well... Unlike other parts of the framework there is no state for
non-atomic bridges. Of course we can probably fix that by using default
helpers if the callbacks are not provided.

> But also, I have no idea what the side-effects might be.
> 
> One thing though: a driver having an atomic_check callback is not an
> indication of whether it supports atomic mode-setting or not.
> atomic_check is optional, so we can have atomic drivers without
> atomic_check.

Yeah. The framework uses the presence of the .atomic_reset() callback in
order to register the bridge as a private object, maning state
management & co.

-- 
With best wishes
Dmitry