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Date:   Fri, 3 Jun 2022 16:16:06 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        dri-devel <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 <freedreno@lists.freedesktop.org>,
        linux-arm-msm <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>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 3/4] drm/bridge: Add devm_drm_bridge_add()
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On Fri, Jun 03, 2022 at 06:52:05AM -0700, Doug Anderson wrote:
> On Fri, Jun 3, 2022 at 3:19 AM Dmitry Baryshkov
> <dmitry.baryshkov@linaro.org> wrote:
> > On Fri, 3 Jun 2022 at 11:21, Maxime Ripard <maxime@cerno.tech> wrote:
> > >
> > > On Tue, May 31, 2022 at 02:06:34PM -0700, Doug Anderson wrote:
> > > > On Mon, May 23, 2022 at 10:00 AM Doug Anderson <dianders@chromium.o=
rg> wrote:
> > > > > On Sat, May 21, 2022 at 2:17 AM Maxime Ripard <maxime@cerno.tech>=
 wrote:
> > > > > > On Tue, May 10, 2022 at 12:29:43PM -0700, Douglas Anderson wrot=
e:
> > > > > > > This adds a devm managed version of drm_bridge_add(). Like ot=
her
> > > > > > > "devm" function listed in drm_bridge.h, this function takes an
> > > > > > > explicit "dev" to use for the lifetime management. A few note=
s:
> > > > > > > * In general we have a "struct device" for bridges that makes=
 a good
> > > > > > >   candidate for where the lifetime matches exactly what we wa=
nt.
> > > > > > > * The "bridge->dev->dev" device appears to be the encoder
> > > > > > >   device. That's not the right device to use for lifetime man=
agement.
> > > > > > >
> > > > > > > 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 wi=
ser to
> > > > > > introduce them as DRM-managed, and not device managed.
> > > > > >
> > > > > > Otherwise, you'll end up in a weird state when a device has bee=
n removed
> > > > > > but the DRM device is still around.
> > > > >
> > > > > I'm kinda confused. In this case there is no DRM device for the b=
ridge
> > > > > and, as per my CL description, "bridge-dev->dev" appears to be the
> > > > > encoder device. I wasn't personally involved in discussions about=
 it,
> > > > > but I was under the impression that this was expected / normal. T=
hus
> > > > > we can't make this DRM-managed.
> > > >
> > > > Since I didn't hear a reply,
> > >
> > > Gah, I replied but it looks like somehow it never reached the ML...
> > >
> > > Here was my original reply:
> > >
> > > > > > 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 manag=
ement.
> > > > > >
> > > > > > 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 wise=
r 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.
> > > >=3D20
> > > > I'm kinda confused. In this case there is no DRM device for the bri=
dge
> > > > and, as per my CL description, "bridge-dev->dev" appears to be the
> > > > encoder device.
> > >
> > > bridge->dev seems right though?
> > >
> > > > I wasn't personally involved in discussions about it, but I was und=
er
> > > > the impression that this was expected / normal. Thus we can't make
> > > > this DRM-managed.
> > >
> > > Still, I don't think devm is the right solution to this either.
> > >
> > > The underlying issue is two-fold:
> > >
> > >   - Encoders can have a pointer to a bridge through of_drm_find_bridge
> > >     or similar. However, bridges are traditionally tied to their devi=
ce
> > >     lifetime (by calling drm_bridge_add in probe, and drm_bridge_remo=
ve
> > >     in remove). Encoders will typically be tied to the DRM device
> > >     however, and that one sticks around until the last application
> > >     closes it. We can thus very easily end up with a dangling pointer,
> > >     and a use-after-free.
> > >
> > >   - It's not the case yet, but it doesn't seem far fetch to expose
> > >     properties of bridges to the userspace. In that case, the userspa=
ce
> > >     would be likely to still hold references to objects that aren't
> > >     there anymore when the bridge is gone.
> > >
> > > The first is obviously a larger concern, but if we can find a solution
> > > that would accomodate the second it would be great.
> > >
> > > As far as I can see, we should fix in two steps:
> > >
> > >   - in drm_bridge_attach, we should add a device-managed call that wi=
ll
> > >     unregister the main DRM device. We don't allow to probe the main =
DRM
> > >     device when the bridge isn't there yet in most case, so it makes
> > >     sense to remove it once the bridge is no longer there as well.
> >
> > The problem is that I do not see a good way to unregister the main DRM
> > device outside of it's driver code.
> >
> > >
> > >   - When the DRM device is removed, have the core cleanup any bridge
> > >     registered. That will remove the need to have drm_bridge_remove in
> > >     the first place.
> > >
> > > > I'll assume that my response addressed your concerns. Assuming I get
> > > > reviews for the other two patches in this series I'll plan to land
> > > > this with Dmitry's review.
> > >
> > > I still don't think it's a good idea to merge it. It gives an illusion
> > > of being safe, but it's really far from it.
> >
> > It is more of removing the boilerplate code spread over all the
> > drivers rather than about particular safety.
> >
> > I'd propose to land devm_drm_bridge_add (and deprecate calling
> > drm_bridge_remove from the bridge driver at some point) and work on
> > the whole drm_device <-> drm_bridge problem in the meantime.
>=20
> At this point it has been landed in drm-misc-next as per my response
> to the cover letter. If need be we can revert it and rework the ps8640
> driver to stop using it but it wouldn't change the lifetime of the
> bridge. I'm not going to rework the bridge lifetime rules here. If
> nothing else it seems like having the devm function at least would
> make it obvious which drivers need to be fixed whenever the bridge
> lifetime problem gets solved.

Not really, no. The issue exists whether or not the driver would be
using devm. Anyway, what's done is done.

Could you please ping earlier than a few minutes before applying the
patch next time though? We could have easily prevented that situation.

Maxime