Re: [PATCH v3 3/4] drm/bridge: Add devm_drm_bridge_add()

From: Daniel Vetter <daniel@ffwll.ch>
To: Maxime Ripard <maxime@cerno.tech>
Cc: Douglas Anderson <dianders@chromium.org>,
	dri-devel@lists.freedesktop.org,
	Hsin-Yi Wang <hsinyi@chromium.org>,
	Abhinav Kumar <quic_abhinavk@quicinc.com>,
	Philip Chen <philipchen@chromium.org>,
	Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
	Robert Foss <robert.foss@linaro.org>,
	freedreno@lists.freedesktop.org,
	Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	linux-arm-msm@vger.kernel.org, Stephen Boyd <swboyd@chromium.org>,
	Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/4] drm/bridge: Add devm_drm_bridge_add()
Date: Wed, 8 Jun 2022 17:50:15 +0200	[thread overview]
Message-ID: <YqDFNx0Oim9RBIMf@phenom.ffwll.local> (raw)
In-Reply-To: <20220521091751.opeiqbmc5c2okdq6@houat>

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@linaro.org>
> > Signed-off-by: Douglas Anderson <dianders@chromium.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@ffwll.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