linux-kernel - Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

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Date:   Tue, 20 Jun 2017 10:31:19 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Archit Taneja <architt@...eaurora.org>
Cc:     Eric Anholt <eric@...olt.net>, dri-devel@...ts.freedesktop.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

Hi Archit,

On Tuesday 20 Jun 2017 09:18:00 Archit Taneja wrote:
> On 06/16/2017 08:13 PM, Eric Anholt wrote:
> > Archit Taneja <architt@...eaurora.org> writes:
> >> On 06/16/2017 02:11 AM, Eric Anholt wrote:
> >>> If the panel-bridge is being set up after the drm_mode_config_reset(),
> >>> then the connector's state would never get initialized, and we'd
> >>> dereference the NULL in the hotplug path.  We also need to register
> >>> the connector, so that userspace can get at it.
> >> 
> >> Shouldn't the KMS driver make sure the panel-bridge is set up before
> >> drm_mode_config_reset? Is it the case when we're inserting the
> >> panel-bridge driver as a module?
> >> 
> >> 
> >> All the connectors that have been added are registered automatically
> >> when drm_dev_register() is called by the KMS driver. Registering a
> >> connector in the middle of setting up our driver is prone to race
> >> conditions if the userspace decides to use them immediately.
> > 
> > Yeah, this is fixing initializing panel_bridge at DSI host_attach time,
> > which in the case of a panel module that creates the DSI device
> > (adv7533-style, like you said I should use as a reference) will be after
> > drm_mode_config_reset() and drm_dev_register().
> 
> Okay. In the case of the msm kms driver, we defer probe until the
> adv7533 module is inserted, only then we proceed to drm_mode_config_reset()
> and drm_dev_register(). I assumed this was the general practice followed by
> most kms drivers. I.,e the kms driver defers probe until all connector
> related modules are inserted, and only then proceed to create a drm device.

I'd love to see support for a more dynamic approach that would allow 
registering outputs at runtime. Until that's implemented, however, I agree 
with your statement, drivers should wait until all components are available 
before registering the DRM device.

> Feedback from others on this would be appreciated, though.

-- 
Regards,

Laurent Pinchart