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

From: Philippe CORNU
Date: Thu Jun 22 2017 - 05:31:55 EST

On 06/22/2017 10:17 AM, Archit Taneja wrote:
> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <eric@xxxxxxxxxx>:
>>> Archit Taneja <architt@xxxxxxxxxxxxxx> writes:
>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>>>>> Archit Taneja <architt@xxxxxxxxxxxxxx> 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.
> Does anyone have better ideas?
> Thanks,
> Archit
>> + Philippe in copy because we have the same probing issue when adding
>> panel-bridge
>> with the dsi bridge

When adding panel-bridge support to the Synopsys DesignWare DSI bridge,
I came to the conclusion that the only solution to make it works
properly (without patching drm) was to "add the DSI bridge" at the end
of the dw_mipi_dsi_host_attach() (see
ie. defers crtc & encoder probing until the panel-bridge connector is

Before that, I spent a lot of time trying different solutions like
patching panel-bridge as Eric did (drm_connector_register...), adding
bind/unbind() everywhere, debugging around __drm_mode_object_add...
Surely DSI Host & device mechanism + panels add complexity to the
related connector creation...

After reading this thread, I have no good solution to suggest... and
"deferring probing until connector registration" works fine now on my
side and seems to be the way others drivers work...


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