linux-kernel - Re: [PATCH v3 31/37] drm/bridge: Provide pointers to the connector and crtc in bridge state

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Message-ID: <20250218-adamant-translucent-civet-aebcc5@houat>
Date: Tue, 18 Feb 2025 11:23:00 +0100
From: Maxime Ripard <mripard@...nel.org>
To: 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 v3 31/37] drm/bridge: Provide pointers to the connector
 and crtc in bridge state

Hi,

Thanks for your answer

On Mon, Feb 17, 2025 at 05:41:24PM +0100, Simona Vetter wrote:
> On Thu, Feb 13, 2025 at 03:43:50PM +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.
> 
> Eh, this isn't quite how this works. So unless bridges have become very
> dynamic and gained flexible routing the bridge(->bridge->...)->encoder
> chain is static. And the crtc for an encoder you find by walking the
> connector states in a drm_atomic_state, finding the right one that points
> at your encoder, and then return the ->crtc pointer from that connector
> state.
> 
> It's a bit bonkers, but I think it's better to compute this than adding
> more pointers that potentially diverge. Unless there's a grand plan here,
> but then I think we want some safety checks that all the pointers never
> get out of sync here.

That work stemed from this series
https://lore.kernel.org/all/20250210132620.42263-1-herve.codina@bootlin.com/

and in particular:
https://lore.kernel.org/all/20250210132620.42263-5-herve.codina@bootlin.com/

Bridges, outside of the modesetting code path, don't have a way to
access the drm_atomic_state since drm_bridge_state->state is typically
cleared after swap_state. So accessing the connectors and CRTCs don't
work anymore.

In this particular case, we needed to access those from the bridge
interrupt handler.

Maxime

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