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

[Philippe CORNU <philippe.cornu@st.com>]

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@anholt.net>:
>>> Archit Taneja <architt@codeaurora.org> writes:
>>>
>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>>>>> Archit Taneja <architt@codeaurora.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.
> 
> 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 
https://lists.freedesktop.org/archives/dri-devel/2017-June/144717.html) 
ie. defers crtc & encoder probing until the panel-bridge connector is 
created.

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...

Philippe

>>>
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>