linux-kernel - Re: [PATCH v9 0/6] MHDP8546 fixes related to DBANC usecase

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Message-ID: <ujpesz2g2emhukb75yrxdlxzd2cfwbfoejlngtyxurjt45w45p@qjtcvk7in7mz>
Date: Tue, 18 Nov 2025 17:20:54 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Harikrishna Shenoy <h-shenoy@...com>, 
	Laurent.pinchart@...asonboard.com, airlied@...il.com, andrzej.hajda@...el.com, 
	andy.yan@...k-chips.com, aradhya.bhatia@...ux.dev, devarsht@...com, dianders@...omium.org, 
	dri-devel@...ts.freedesktop.org, javierm@...hat.com, jernej.skrabec@...il.com, jonas@...boo.se, 
	linux-kernel@...r.kernel.org, linux@...blig.org, luca.ceresoli@...tlin.com, lumag@...nel.org, 
	lyude@...hat.com, maarten.lankhorst@...ux.intel.com, mordan@...ras.ru, 
	neil.armstrong@...aro.org, rfoss@...nel.org, s-jain1@...com, simona@...ll.ch, 
	tzimmermann@...e.de, u-kumar1@...com
Subject: Re: [PATCH v9 0/6] MHDP8546 fixes related to DBANC usecase

On Tue, Nov 18, 2025 at 04:52:49PM +0200, Tomi Valkeinen wrote:
> Hi,
> 
> On 18/11/2025 14:40, Maxime Ripard wrote:
> > Hi,
> > 
> > On Tue, Nov 18, 2025 at 05:22:49PM +0530, Harikrishna Shenoy wrote:
> >> With the DBANC framework, the connector is no longer initialized in 
> >> bridge_attach() when the display controller sets the 
> >> DRM_BRIDGE_ATTACH_NO_CONNECTOR flag. 
> >> This causes a null pointer dereference in cdns_mhdp_modeset_retry_fn() 
> >> when trying to access &conn->dev->mode_config.mutex. 
> >> Observed on a board where EDID read failed. 
> >> (log: https://gist.github.com/Jayesh2000/233f87f9becdf1e66f1da6fd53f77429)
> >>
> >> Patch 1 adds a connector_ptr which takes care of both 
> >> DBANC and !DBANC case by setting the pointer in appropriate hooks
> >> and checking for pointer validity before accessing the connector.
> >> Patch 2 adds mode validation hook to bridge fucntions.
> >> Patch 3 fixes HDCP to work with both DBANC and !DBANC case by
> >> moving HDCP state handling into the bridge atomic check in line with 
> >> the DBANC model.
> >> Patches 4,5 do necessary cleanup and alignment for using
> >> connector pointer.
> > 
> > It's mentioned several times in your series, and it might be obvious to
> > you, but documenting what is the "DBANC framework" is would be helpful.
> > I have no idea what it's about, and it appears that Google doesn't know
> > either.
> Yes, I was a bit baffled initially. DRM_BRIDGE_ATTACH_NO_CONNECTOR.

Oooooh, thanks

> I think it makes sense to only use "DBANC" if it's first introduced in
> that patch. So don't have a patch that just uses "DBANC", even if the
> previous patch did explain what it means. And if there's just one or two
> "DBANC"s, just spell it out "DRM_BRIDGE_ATTACH_NO_CONNECTOR".

Yeah, I'd go even further. Acronyms are fun but something being obvious
is better still. Use the proper flag name every time.

Maxime

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