linux-kernel - Re: [RFC PATCH v1 01/12] Revert "drm/sysfs: Link DRM connectors to corresponding Type-C connectors"

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Message-ID: <CAA8EJpqUg2-k7LLBL38RHU1sThkXB54ca68xEMd1yMnHQcQ++w@mail.gmail.com>
Date:   Tue, 5 Sep 2023 13:56:59 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        "Bryan O'Donoghue" <bryan.odonoghue@...aro.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Janne Grunau <j@...nau.net>, Simon Ser <contact@...rsion.fr>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        freedreno@...ts.freedesktop.org, Won Chung <wonchung@...gle.com>
Subject: Re: [RFC PATCH v1 01/12] Revert "drm/sysfs: Link DRM connectors to
 corresponding Type-C connectors"

Hi Heikki,

On Tue, 5 Sept 2023 at 11:50, Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
>
> Hi Dmitry,
>
> On Mon, Sep 04, 2023 at 12:41:39AM +0300, Dmitry Baryshkov wrote:
> > The kdev->fwnode pointer is never set in drm_sysfs_connector_add(), so
> > dev_fwnode() checks never succeed, making the respective commit NOP.
>
> That's not true. The dev->fwnode is assigned when the device is
> created on ACPI platforms automatically. If the drm_connector fwnode
> member is assigned before the device is registered, then that fwnode
> is assigned also to the device - see drm_connector_acpi_find_companion().
>
> But please note that even if drm_connector does not have anything in
> its fwnode member, the device may still be assigned fwnode, just based
> on some other logic (maybe in drivers/acpi/acpi_video.c?).
>
> > And if drm_sysfs_connector_add() is modified to set kdev->fwnode, it
> > breaks drivers already using components (as it was pointed at [1]),
> > resulting in a deadlock. Lockdep trace is provided below.
> >
> > Granted these two issues, it seems impractical to fix this commit in any
> > sane way. Revert it instead.
>
> I think there is already user space stuff that relies on these links,
> so I'm not sure you can just remove them like that. If the component
> framework is not the correct tool here, then I think you need to
> suggest some other way of creating them.

The issue (that was pointed out during review) is that having a
component code in the framework code can lead to lockups. With the
patch #2 in place (which is the only logical way to set kdev->fwnode
for non-ACPI systems) probing of drivers which use components and set
drm_connector::fwnode breaks immediately.

Can we move the component part to the respective drivers? With the
patch 2 in place, connector->fwnode will be copied to the created
kdev's fwnode pointer.

Another option might be to make this drm_sysfs component registration optional.

> Side note. The problem you are describing here is a limitation in the
> component framework - right now it's made with the idea that a device
> can represent a single component, but it really should allow a device
> to represent multiple components. I'm not saying that you should try
> to fix the component framework, but I just wanted to make a note about
> this (and this is not the only problem with the component framework).
>
> I like the component framework as a concept, but I think it needs a
> lot of improvements - possibly rewrite.

Yes. There were several attempts to rewrite the component framework,
but none succeeded up to now. Anyway, I consider rewriting components
framework to be a bigger topic compared to drm connector fwnode setup.

--
With best wishes
Dmitry