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

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Date:   Thu, 22 Jun 2017 09:50:10 +0200
From:   Benjamin Gaignard <benjamin.gaignard@...aro.org>
To:     Eric Anholt <eric@...olt.net>
Cc:     Archit Taneja <architt@...eaurora.org>,
        "dri-devel@...ts.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>
Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

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.
>
> 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.

+ Philippe in copy because we have the same probing issue when adding
panel-bridge
with the dsi bridge

>
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