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

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Date:   Tue, 9 Nov 2021 10:19:29 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        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

On Mon, Nov 08, 2021 at 09:16:07PM +0300, Dmitry Osipenko wrote:
> 08.11.2021 18:17, Daniel Vetter пишет:
> > On Mon, Nov 08, 2021 at 02:08:21AM +0300, Dmitry Osipenko wrote:
> >> Use drm_dp_aux_register_ddc/chardev() helpers that allow to register I2C
> >> adapter separately from the character device. This fixes broken display
> >> panel driver of Acer Chromebook CB5-311 that fails to probe starting with
> >> v5.13 kernel when DP AUX registration order was changed. Tegra SOR driver
> >> is never probed now using the new registration order because tegra-output
> >> always fails with -EPROBE_DEFER due to missing display panel that requires
> >> DP AUX DDC to be registered first. The offending commit made DDC to be
> >> registered after SOR's output, which can't ever happen. Use new helpers
> >> to restore the registration order and revive display panel.
> > 
> > This feels a bit backward, I think the clean solution would be to untangle
> > the SOR loading from the panel driver loading, and then only block
> > registering the overall drm_device on both drivers having loaded.
> 
> Sounds impossible.
> 
> 1. DRM device can be created only when all components are ready, panel
> is one of the components.

Nope. drm_device can be instantiated whenever you feel like.
drm_dev_register can only be called when it's all fully set up. Absolutely
nothing would work if drm_device wouldn't support this two-stage setup.

So sequence:

1. drm_dev_init

2. bind sor driver

3. create dp aux ddc

4. bind panel

5. yay we have everything, drm_dev_register

This should work, and it's designed to work like this actually. You
couldn't write big complex drivers otherwise.
-Daniel

> 
> 2. SOR driver is controlling panel and programs h/w based on panel presence.
> 
> 3. Panel can't become ready until DP AUX DDC is created.
> 
> 4. DP AUX DDC can't be created until DRM device is created.
> 
> 5. Go to 1.
> 
> Even if there is an option to somehow rewrite Tegra DRM driver to
> accommodate it to the desired driver model, it won't be something
> portable to stable kernels.
> 
> > This here at least feels like a game of whack-a-mole, if like every driver
> > needs its own careful staging of everything.
> 
> That is inevitable because each hardware design is individual.

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch