linux-kernel - Re: [PATCH v1 2/2] drm/tegra: Use drm_dp_aux_register

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Date:   Fri, 12 Nov 2021 17:32:43 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        Lyude Paul <lyude@...hat.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Thomas Graichen <thomas.graichen@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] drm/tegra: Use drm_dp_aux_register_ddc/chardev()
 helpers

12.11.2021 13:52, Thierry Reding пишет:
> On Tue, Nov 09, 2021 at 05:39:02PM +0300, Dmitry Osipenko wrote:
>> 09.11.2021 17:17, Dmitry Osipenko пишет:
>>> 09.11.2021 17:08, Dmitry Osipenko пишет:
>>>>> +static void host1x_drm_dev_deinit(struct host1x_device *dev)
>>>>> +{
>>>>> +	struct drm_device *drm = dev_get_drvdata(&dev->dev);
>>>> And platform_unregister_drivers() should be moved here.
>>>>
>>>
>>> Nah, that should cause deadlock. This ad-hoc is too lame.
>>
>> Actually, there is no problem here as I see now. The host1x driver
>> populates DT nodes after host1x_register() [1], meaning that Host1x DRM
>> will be always inited first.
>>
>> [1]
>> https://elixir.bootlin.com/linux/v5.15/source/drivers/gpu/host1x/dev.c#L475
>>
>> Still I'm not a fan of the ad-hoc solution.
> 
> Could we not fix this by making the panel "hot-pluggable"? I don't think
> there's anything inherent to the driver that would prevent doing so. The
> original reason for why things are as intertwined as they are now is
> because back at the time deferred framebuffer creation didn't exist. In
> fact I added deferred framebuffer support with Daniel's help precisely
> to fix a similar issue for things like HDMI and DP.

I don't understand what do you mean by "hot-pluggable", panel is static.
Please either clarify more or type the patch.

Keep in mind that fix should be simple and portable because stable
kernels are wrecked.

> With HDMI and DP it's slightly less critical, because a lack of mode
> support would've created a 1024x768 framebuffer, which most HDMI/DP
> monitors support. However, with panels we need to ensure that the exact
> mode from the panel is used to create the framebuffer, so it can only be
> created when the panel is available.
> 
> But, given that deferred framebuffer creation works now (which allows
> the framebuffer console to show up at the preferred resolution for HDMI
> and DP), even if a monitor is attached only after the driver has probed
> already, we should be able to make something like that work with panels
> as well.

BTW, I see now that DPAUX I2C transfer helper may access
aux->drm_device. Hence v1 patch isn't correct anyways.

For now I'll try to test more the ad-hoc variant with Thomas and send it
as v2 if we won't have a better solution.