[Intel-gfx] [PATCH 1/3] drm/i915: dp: fix order of dp aux i2c device cleanup

[Imre Deak]

On Mon, 2014-02-10 at 10:23 +0100, Daniel Vetter wrote:
> On Sat, Feb 08, 2014 at 02:52:11AM +0200, Imre Deak wrote:
> > Atm we set the parent of the dp i2c device to be the correspondig
> > connector device. During driver cleanup we first remove the connector
> > device through intel_modeset_cleanup()->drm_sysfs_connector_remove() and
> > only after that the i2c device through the encoder's destroy callback.
> > This order is not supported by the device core and we'll get a warning,
> > see the below bugzilla ticket. The proper order is to remove first any
> > child device and only then the parent device.
> > 
> > The first part of the fix changes the i2c device's parent to be the drm
> > device. Its logical owner is not the connector anyway, but the encoder.
> > Since the encoder doesn't have a device object, the next best choice is
> > the drm device. This is the same what we do in the case of the sdvo i2c
> > device and what the nouveau driver does.
> > 
> > The second part creates a symlink in the connector's sysfs directory
> > pointing to the i2c device. This is so, that we keep the current ABI,
> > which also makes sense in case someone wants to look up the i2c device
> > belonging to a specific connector. To remove this symlink a new
> > unregister callback is added to the connector. In general this should
> > remove any custom interfaces we added, through which the connector can
> > be accessed.
> > 
> > I also thought of moving the connector sysfs cleanup into its destroy
> > callback, but that would mean allowing user space access to race with
> > the destruction of the encoders for example. The following comment
> > for drm_mode_config_cleanup() also tells me that we have to make sure
> > there is no pending or new access to the connectors before we call it:
> > 
> > """
> > Note that since this /should/ happen single-threaded at driver/device
> > teardown time, no locking is required. It's the driver's job to
> > ensure that this guarantee actually holds true.
> > """
> > 
> > Reference: http://lists.freedesktop.org/archives/intel-gfx/2014-January/038782.html
> > Reference: http://lists.freedesktop.org/archives/intel-gfx/2014-February/039427.html
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70523
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c |  5 +++++
> >  drivers/gpu/drm/i915/intel_dp.c      | 24 +++++++++++++++++++++++-
> >  drivers/gpu/drm/i915/intel_drv.h     |  7 +++++++
> >  3 files changed, 35 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index 4d4a0d9..5b5ec8a 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -11472,6 +11472,11 @@ void intel_modeset_cleanup(struct drm_device *dev)
> >  
> >  	/* destroy the backlight and sysfs files before encoders/connectors */
> >  	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
> > +		struct intel_connector *intel_connector;
> > +
> > +		intel_connector = to_intel_connector(connector);
> > +		if (intel_connector->unregister)
> > +			intel_connector->unregister(intel_connector);
> >  		intel_panel_destroy_backlight(connector);
> >  		drm_sysfs_connector_remove(connector);
> >  	}
> 
> I don't see what this new callback buys us compared to moving everything
> into the ->destroy callbacks: The driver is already half torn-down, so
> userspace better not poke at any sysfs/debugfs files.

But the above part makes sure exactly that userspace can't access any
sysfs files. The ->unregister callback is doing the connector specific
part of that. If we move these things into ->destroy userspace will have
access to all these sysfs files and if that happens during the following
encoder/connector ->destroy callbacks things will break. See for
example 

commit d9255d57147e1dbcebdf6670409c2fa0ac3609e6
Author: Paulo Zanoni <paulo.r.zanoni at intel.com>
Date:   Thu Sep 26 20:05:59 2013 -0300

which fixed a bug caused by such a userspace access.

> Fixing this correctly will be much more invasive (and to be able to do it
> we need to start at the drm core), adding new stuff here will only make
> things more complicated once we get around to it.

I think it still make sense to do an early removal of any userspace
interface to the driver before we start tearing down things. It's
similar to how ioctls and other open fds of the driver node prevent the
teardown from starting, by holding a reference on the module.

--Imre