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

[Archit Taneja]

On 06/24/2017 03:20 AM, Eric Anholt wrote:
> Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> writes:
>
> > On Thu, 22 Jun 2017 13:47:43 +0530
> > Archit Taneja <architt@xxxxxxxxxxxxxx> 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.
> >
> > 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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