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

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Message-ID: <87o9te1igt.fsf@eliezer.anholt.net>
Date:   Fri, 23 Jun 2017 14:50:58 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Archit Taneja <architt@...eaurora.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Cc:     Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "dri-devel\@lists.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Philippe Cornu <philippe.cornu@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.

Boris Brezillon <boris.brezillon@...e-electrons.com> writes:

> On Thu, 22 Jun 2017 13:47:43 +0530
> Archit Taneja <architt@...eaurora.org> wrote:
>
>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
>> > 2017-06-20 19:31 GMT+02:00 Eric Anholt <eric@...olt.net>:  
>> >> Archit Taneja <architt@...eaurora.org> writes:
>> >>  
>> >>> On 06/16/2017 08:13 PM, Eric Anholt wrote:  
>> >>>> Archit Taneja <architt@...eaurora.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?

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