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

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Message-ID: <871sqe7ei0.fsf@eliezer.anholt.net>
Date:   Tue, 20 Jun 2017 10:31:03 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Archit Taneja <architt@...eaurora.org>,
        dri-devel@...ts.freedesktop.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, devicetree@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

Archit Taneja <architt@...eaurora.org> writes:

> 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.

The problem, though, is the panel driver needs the MIPI DSI host to
exist to call mipi_dsi_device_register_full() during the probe process.
The adv7533 driver gets around this by registering the DSI device in the
bridge attach step, but drm_panel doesn't have an attach step.

Another alternative is my original version of the panel driver that was
a mipi_dsi_device driver that registered the panel during the DSI device
probe.  That's why vc4's panel lookup is during the MIPI DSI attach
phase, currently.

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