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

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Date:   Fri, 23 Jun 2017 14:36:20 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Archit Taneja <architt@...eaurora.org>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>
Cc:     "dri-devel\@lists.freedesktop.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,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Philippe Cornu <philippe.cornu@...com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Andrzej Hajda <a.hajda@...sung.com>
Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

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

> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <eric@...olt.net>:
>>> 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.
>
> I'm not sure how we can get around this. We had discussion about this on irc
> recently, but couldn't come up with a good conclusion. We could come up with a
> panel_attach() callback to make it similar to bridges, but that's just us avoiding
> the real issue.
>
>>>
>>> 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.
>> 
>
> This would require you to have a DSI device node in DT, rather than an i2c
> node, right? I don't know if we should do that because of a limitation in
> our drm_mipi_dsi and drm_panel frameworks.

All versions of this patch have had one of those, because either that's
where you attach the driver (the first, no-core-changes version of the
driver) or you need it for the of-graph connections anyway.  These later
versions just haven't had a compatible string on the DSI device node.

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