LinuxLists.cc - [PATCH 0/7] RPi touchscreen as a panel driver again

2017-06-20 03:48:09

by Archit Taneja

[permalink] [raw]

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

On 06/16/2017 08:13 PM, Eric Anholt wrote:
> Archit Taneja <[email protected]> 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.

Feedback from others on this would be appreciated, though.

Thanks,
Archit

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2017-06-20 07:30:47

by Laurent Pinchart

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

Hi Archit,

On Tuesday 20 Jun 2017 09:18:00 Archit Taneja wrote:
> On 06/16/2017 08:13 PM, Eric Anholt wrote:
> > Archit Taneja <[email protected]> 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.

I'd love to see support for a more dynamic approach that would allow
registering outputs at runtime. Until that's implemented, however, I agree
with your statement, drivers should wait until all components are available
before registering the DRM device.

> Feedback from others on this would be appreciated, though.

--
Regards,

Laurent Pinchart

2017-06-20 17:31:08

Archit Taneja <[email protected]> writes:

> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <[email protected]>:
>>> Archit Taneja <[email protected]> writes:
>>>
>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>>>>> Archit Taneja <[email protected]> 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.

Attachments:

signature.asc (832.00 B)

2017-06-23 21:51:04

by Eric Anholt

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

Boris Brezillon <[email protected]> writes:

> On Thu, 22 Jun 2017 13:47:43 +0530
> Archit Taneja <[email protected]> wrote:
>
>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>> > 2017-06-20 19:31 GMT+02:00 Eric Anholt <[email protected]>:
>> >> Archit Taneja <[email protected]> writes:
>> >>
>> >>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>> >>>> Archit Taneja <[email protected]> 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?

Attachments:

signature.asc (832.00 B)

2017-06-27 06:53:42

by Archit Taneja

[permalink] [raw]

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

On 06/24/2017 03:20 AM, Eric Anholt wrote:
> Boris Brezillon <[email protected]> writes:
>
>> On Thu, 22 Jun 2017 13:47:43 +0530
>> Archit Taneja <[email protected]> wrote:
>>
>>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>>>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <[email protected]>:
>>>>> Archit Taneja <[email protected]> writes:
>>>>>
>>>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
>>>>>>> Archit Taneja <[email protected]> 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

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project