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

[Archit Taneja <architt-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 06/24/2017 03:20 AM, Eric Anholt wrote:
> Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> writes:
> 
>> On Thu, 22 Jun 2017 13:47:43 +0530
>> Archit Taneja <architt-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org> wrote:
>>
>>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>>>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>:
>>>>> Archit Taneja <architt-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org> writes:
>>>>>   
>>>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>>>>>>> Archit Taneja <architt-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.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.
>>
>> How about making DSI dev registration fully asynchronous, that is, DSI
>> devs declared in the DT under the DSI host node will be
>> registered/attached at probe time, and devs using another control bus
>> (like the adv7533 controller over i2c) will be registered afterwards.
>>
>> That implies moving the drm_brige registration logic outside of the DSI
>> host ->probe() path. The idea would be to check if all devs connected
>> to the DSI bus are ready at dsi_host->attach() time. If they are, we
>> can finally register the XXX -> DSI bridge. If they're not (because
>> some devs connected to the DSI bus have not been probed yet), then we
>> do not register the drm_bridge and wait for the next dsi_host->attach()
>> event.
>>
>> With this solution, I think we can avoid the chicken&egg problem where
>> the adv7533 dev is waiting for the DSI host to be probed to be able to
>> register a DSI dev with mipi_dsi_device_register_full() and the DSI
>> host needs all devs to be registered to not return -EPROBE_DEFER.
> 
> I've now tried having mipi_dsi_device_register_full() succeed early and
> just do the device-add part when the host shows up.  The problem is
> there's still mipi_dsi_attach(), which needs to be delayed until the
> panel driver fills in the rest of mipi_dsi_device's fields.  Why aren't
> those part of the info?

In the past, the only way to create mipi_dsi_devices was to add them as
children DT nodes under the DSI host node. The KMS driver calls
mipi_dsi_host_register(), which resulted in the creation of all the
children mipi_dsi_devices. The DSI host was still unaware of parameters
like number of lanes, mode_flags etc. mipi_dsi_attach() was a way by
which the DSI device could share these with its DSI host. Another use
of mipi_dsi_attach()/detach() is if the panel driver wants to notify
the DSI host an update in one of the device's fields (for example, a
change in the number of lanes).

We've recently added another way of creating mipi_dsi_devices (i.e, by
making a driver call mipi_dsi_device_register_full) for cases where there
isn't a DT representation of the DSI device.

For the second method, the need to call mipi_dsi_attach() separately becomes
sort of redundant. We could consider embedding the other mipi_dsi_device
fields (lanes etc) in mipi_dsi_device_info itself, and invoke the DSI host's
->attach() callback through mipi_dsi_device_register_full() itself. Would
that help resolving your issue?

Archit

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