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

[Archit Taneja]

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
>
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