linux-kernel - Re: [PATCH v3 3/4] drm/bridge: Add devm_drm_bridge

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Message-ID: <YqDFNx0Oim9RBIMf@phenom.ffwll.local>
Date:   Wed, 8 Jun 2022 17:50:15 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Douglas Anderson <dianders@...omium.org>,
        dri-devel@...ts.freedesktop.org,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Philip Chen <philipchen@...omium.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        Robert Foss <robert.foss@...aro.org>,
        freedreno@...ts.freedesktop.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        linux-arm-msm@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] drm/bridge: Add devm_drm_bridge_add()

On Sat, May 21, 2022 at 11:17:51AM +0200, Maxime Ripard wrote:
> Hi,
> 
> On Tue, May 10, 2022 at 12:29:43PM -0700, Douglas Anderson wrote:
> > This adds a devm managed version of drm_bridge_add(). Like other
> > "devm" function listed in drm_bridge.h, this function takes an
> > explicit "dev" to use for the lifetime management. A few notes:
> > * In general we have a "struct device" for bridges that makes a good
> >   candidate for where the lifetime matches exactly what we want.
> > * The "bridge->dev->dev" device appears to be the encoder
> >   device. That's not the right device to use for lifetime management.
> > 
> > Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> 
> If we are to introduce more managed helpers, I think it'd be wiser to
> introduce them as DRM-managed, and not device managed.
> 
> Otherwise, you'll end up in a weird state when a device has been removed
> but the DRM device is still around.

Top-level post since I didn't see any good place to reply in the thread
below:

- devm is for device stuff, which drm_bridge is (it's not uapi visible in
  any way or fasion)

- drmm is for uapi visible stuff (like drm_encoder)

Yes the uapi-visible stuff can outlive the device-related pieces. The way
to handle this is:

- drm_dev_unplug() when the device disappears underneath you (or just a
  part, I guess the infra for that doesn't exist yet and maybe we should
  add it).

- drm_dev_enter/exit wrapped around the device related parts.

Iow, this patch here I think is the right direction, and gets my

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

But also, it's definitely not a complete solution as the discussion in the
thread here points out.

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