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From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Date: Tue, 11 Jun 2024 14:26:12 +0300
Message-ID: <CAA8EJppcqLm7UbW9n2Y_4mfJv4JFWDkPXJ8=TBL53LiFPADzpQ@mail.gmail.com>
Subject: Re: [PATCH v5 2/9] drm/bridge-connector: switch to using drmm allocations
To: Maxime Ripard <mripard@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>, Andrzej Hajda <andrzej.hajda@intel.com>, 
	Neil Armstrong <neil.armstrong@linaro.org>, Robert Foss <rfoss@kernel.org>, 
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>, Jonas Karlman <jonas@kwiboo.se>, 
	Jernej Skrabec <jernej.skrabec@gmail.com>, 
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>, Thomas Zimmermann <tzimmermann@suse.de>, 
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>, Rob Clark <robdclark@gmail.com>, 
	Abhinav Kumar <quic_abhinavk@quicinc.com>, Sean Paul <sean@poorly.run>, 
	Marijn Suijten <marijn.suijten@somainline.org>, dri-devel@lists.freedesktop.org, 
	linux-arm-msm@vger.kernel.org, freedreno@lists.freedesktop.org, 
	linux-kernel@vger.kernel.org
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On Tue, 11 Jun 2024 at 11:54, Maxime Ripard <mripard@kernel.org> wrote:
>
> On Mon, Jun 10, 2024 at 08:54:09PM GMT, Dmitry Baryshkov wrote:
> > On Mon, Jun 10, 2024 at 02:07:06PM +0200, Maxime Ripard wrote:
> > > Hi,
> > >
> > > +Hans
> > >
> > > On Mon, Jun 10, 2024 at 02:46:03PM GMT, Dmitry Baryshkov wrote:
> > > > On Mon, 10 Jun 2024 at 11:04, Maxime Ripard <mripard@kernel.org> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > On Fri, Jun 07, 2024 at 04:22:59PM GMT, Dmitry Baryshkov wrote:
> > > > > > Turn drm_bridge_connector to using drmm_kzalloc() and
> > > > > > drmm_connector_init() and drop the custom destroy function. The
> > > > > > drm_connector_unregister() and fwnode_handle_put() are already handled
> > > > > > by the drm_connector_cleanup() and so are safe to be dropped.
> > > > > >
> > > > > > Acked-by: Maxime Ripard <mripard@kernel.org>
> > > > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > > > > > ---
> > > > > >  drivers/gpu/drm/drm_bridge_connector.c | 23 +++++------------------
> > > > > >  1 file changed, 5 insertions(+), 18 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_bridge_connector.c b/drivers/gpu/drm/drm_bridge_connector.c
> > > > > > index 982552c9f92c..e093fc8928dc 100644
> > > > > > --- a/drivers/gpu/drm/drm_bridge_connector.c
> > > > > > +++ b/drivers/gpu/drm/drm_bridge_connector.c
> > > > > > @@ -15,6 +15,7 @@
> > > > > >  #include <drm/drm_connector.h>
> > > > > >  #include <drm/drm_device.h>
> > > > > >  #include <drm/drm_edid.h>
> > > > > > +#include <drm/drm_managed.h>
> > > > > >  #include <drm/drm_modeset_helper_vtables.h>
> > > > > >  #include <drm/drm_probe_helper.h>
> > > > > >
> > > > > > @@ -193,19 +194,6 @@ drm_bridge_connector_detect(struct drm_connector *connector, bool force)
> > > > > >       return status;
> > > > > >  }
> > > > > >
> > > > > > -static void drm_bridge_connector_destroy(struct drm_connector *connector)
> > > > > > -{
> > > > > > -     struct drm_bridge_connector *bridge_connector =
> > > > > > -             to_drm_bridge_connector(connector);
> > > > > > -
> > > > > > -     drm_connector_unregister(connector);
> > > > > > -     drm_connector_cleanup(connector);
> > > > > > -
> > > > > > -     fwnode_handle_put(connector->fwnode);
> > > > > > -
> > > > > > -     kfree(bridge_connector);
> > > > > > -}
> > > > > > -
> > > > > >  static void drm_bridge_connector_debugfs_init(struct drm_connector *connector,
> > > > > >                                             struct dentry *root)
> > > > > >  {
> > > > > > @@ -224,7 +212,6 @@ static const struct drm_connector_funcs drm_bridge_connector_funcs = {
> > > > > >       .reset = drm_atomic_helper_connector_reset,
> > > > > >       .detect = drm_bridge_connector_detect,
> > > > > >       .fill_modes = drm_helper_probe_single_connector_modes,
> > > > > > -     .destroy = drm_bridge_connector_destroy,
> > > > > >       .atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
> > > > > >       .atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
> > > > > >       .debugfs_init = drm_bridge_connector_debugfs_init,
> > > > > > @@ -328,7 +315,7 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
> > > > > >       int connector_type;
> > > > > >       int ret;
> > > > > >
> > > > > > -     bridge_connector = kzalloc(sizeof(*bridge_connector), GFP_KERNEL);
> > > > > > +     bridge_connector = drmm_kzalloc(drm, sizeof(*bridge_connector), GFP_KERNEL);
> > > > >
> > > > > So you make destroy's kfree call unnecessary here ...
> > > > >
> > > > > >       if (!bridge_connector)
> > > > > >               return ERR_PTR(-ENOMEM);
> > > > > >
> > > > > > @@ -383,9 +370,9 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
> > > > > >               return ERR_PTR(-EINVAL);
> > > > > >       }
> > > > > >
> > > > > > -     ret = drm_connector_init_with_ddc(drm, connector,
> > > > > > -                                       &drm_bridge_connector_funcs,
> > > > > > -                                       connector_type, ddc);
> > > > > > +     ret = drmm_connector_init(drm, connector,
> > > > > > +                               &drm_bridge_connector_funcs,
> > > > > > +                               connector_type, ddc);
> > > > >
> > > > > ... and here of drm_connector_cleanup.
> > > > >
> > > > > drm_connector_unregister wasn't needed, so can ignore it, but you leak a reference to
> > > > > connector->fwnode since you don't call fwnode_handle_put anymore.
> > > > >
> > > > > We should register a drmm action right below the call to fwnode_handle_get too.
> > > >
> > > > But drm_connector_cleanup() already contains
> > > > fwnode_handle_put(connector->fwnode). Isn't that enough?
> > >
> > > It does, but now I'm confused.
> > >
> > > drm_bridge_connector_init takes a reference, drm_connector_init doesn't.
> > > It will call drm_bridge_connector_destroy() that gives back its
> > > reference (which makes sense to me), but then why do
> > > drm_connector_cleanup() does? None of the drm_connector code even took
> > > that reference, and we end up with a double-put.
> > >
> > > It looks like it was introduced by commit 48c429c6d18d ("drm/connector:
> > > Add a fwnode pointer to drm_connector and register with ACPI (v2)") from
> > > Hans, which does call put, but never gets that reference.
> >
> > The mentioned patch documents that pretty clearly:
> >
> > * Drivers can set this to associate a fwnode with a connector, drivers
> > * are expected to get a reference on the fwnode when setting this.
> > * drm_connector_cleanup() will call fwnode_handle_put() on this.
> >
> > This is logical. Whoever sets the drm_connector::fwnode pointer, should
> > get reference. This way drm_connector_init() doesn't need to play with
> > the reference counting. The cleanup code drops the reference (so the
> > driver doesn't need to), because cleanup might be assynchronous..
>
> Right, but it's the cleanup part that isn't logical. It makes total
> sense to have the connector that sets connector->fwnode get the
> reference itself. It doesn't make sense to have the core give that
> reference instead of the driver.
>
> It's confusing, because if the driver is supposed to handle its
> reference itself, then it should handle all of it itself. This bug is
> the testament for that: the natural approach is buggy.

I'd say this is the 'transfer of the ownership'. The base driver gets
the reference, and then gives it away to the drm_connecter. But indeed
this is not very intuitive.

I have looked at the original series by Hans/Heikky, but I don't seem
to be able to find a good way to solve that. The fwnode can be set
after initialising the drm_connector. And at the same time it doesn't
make so much sense to put that burden onto the driver. One option
might be to add drm_connector_set_fwnode() that will get the reference
internally, but that looks a bit like an overkill.

>
> > The drm_bridge_connector follows this approach: it sets
> > drm_connector->fwnode, so it gets the reference. It uses
> > drm_connector_cleanup(), so it doesn't need to put it.
>
> Yet, it calls fwnode_handle_put in its destroy path, because it grabbed
> a reference before.
>
> Maxime


-- 
With best wishes
Dmitry