linux-kernel - Re: [PATCH v5 2/9] drm/bridge-connector: switch to using drmm allocations

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Date: Mon, 10 Jun 2024 20:54:09 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: Hans de Goede <hdegoede@...hat.com>, 
	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>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Rob Clark <robdclark@...il.com>, 
	Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul <sean@...rly.run>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org, 
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/9] drm/bridge-connector: switch to using drmm
 allocations

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@...nel.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@...nel.org>
> > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.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..

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.

> 
> It has nothing to do with this series anymore, but that's super fishy to
> me, and the source of bugs as we can see here.

-- 
With best wishes
Dmitry