linux-kernel - Re: [PATCH 2/4 v3] drm: Introduce drm_connector_register

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Date:	Tue, 29 Mar 2016 10:19:35 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:	"daniel@...ll.ch" <daniel@...ll.ch>,
	"dh.herrmann@...il.com" <dh.herrmann@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"airlied@...ux.ie" <airlied@...ux.ie>
Subject: Re: [PATCH 2/4 v3] drm: Introduce drm_connector_register_all() helper

On Wed, Mar 23, 2016 at 01:41:05PM +0000, Alexey Brodkin wrote:
> Hi David,
> 
> On Wed, 2016-03-23 at 12:13 +0100, David Herrmann wrote:
> > Hi
> > 
> > On Wed, Mar 23, 2016 at 9:42 AM, Alexey Brodkin
> > <Alexey.Brodkin@...opsys.com> wrote:
> > > 
> > > As a pair to already existing drm_connector_unregister_all() we're adding
> > > generic implementation of what is already done in some drivers.
> > > 
> > > Once this helper is implemented we'll be ready to switch existing
> > > driver-specific implementations with the generic one.
> > > 
> > > Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> > > Cc: Daniel Vetter <daniel@...ll.ch>
> > > Cc: David Airlie <airlied@...ux.ie>
> > > ---
> > > 
> > > Changes v2 -> v3:
> > >  * Updated title with capital after colon
> > >  * Simplified failure path with direct and unconditional invocation of
> > >    unregister_all()
> > >  * Updated kerneldoc description of the drm_connector_register_all()
> > > 
> > > Changes v1 -> v2:
> > >  * Rename drm_connector_unplug_all() to drm_connector_unregister_all()
> > >  * Use drm_for_each_connector() instead of list_for_each_entry()
> > >  * Updated kerneldoc for drm_dev_register()
> > > 
> > >  drivers/gpu/drm/drm_crtc.c | 43 +++++++++++++++++++++++++++++++++++++++++++
> > >  drivers/gpu/drm/drm_drv.c  |  6 +++++-
> > >  include/drm/drm_crtc.h     |  3 ++-
> > >  3 files changed, 50 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> > > index 65488a6..21eea11 100644
> > > --- a/drivers/gpu/drm/drm_crtc.c
> > > +++ b/drivers/gpu/drm/drm_crtc.c
> > > @@ -1081,6 +1081,49 @@ void drm_connector_unregister(struct drm_connector *connector)
> > >  EXPORT_SYMBOL(drm_connector_unregister);
> > > 
> > >  /**
> > > + * drm_connector_register_all - register all connectors
> > > + * @dev: drm device
> > > + *
> > > + * This function registers all connectors in sysfs and other places so that
> > > + * userspace can start to access them. Drivers can call it after calling
> > > + * drm_dev_register() to complete the device registration, if they don't call
> > > + * drm_connector_register() on each connector individually.
> > > + *
> > > + * When a device is unplugged and should be removed from userspace access,
> > > + * call drm_connector_unregister_all(), which is the inverse of this
> > > + * function.
> > > + *
> > > + * Returns:
> > > + * Zero on success, error code on failure.
> > > + */
> > > +int drm_connector_register_all(struct drm_device *dev)
> > > +{
> > > +       struct drm_connector *connector;
> > > +       int ret;
> > > +
> > > +       mutex_lock(&dev->mode_config.mutex);
> > > +
> > > +       drm_for_each_connector(connector, dev) {
> > > +               ret = drm_connector_register(connector);
> > > +               if (ret) {
> > > +                       /*
> > > +                        * We may safely call unregister_all() here within
> > > +                        * area locked with mutex because unregister_all()
> > > +                        * doesn't use locks inside (see a comment in that
> > > +                        * function).
> > > +                        */
> > Ugh? unregister_all() says:
> > 
> > /* FIXME: taking the mode config mutex ends up in a clash with sysfs */
> > 
> > This strongly contradicts your comment. Anyway, regardless how you
> > want to fix it: You better unlock the mode-config mutex before
> > returning below.
> 
> So good catch.
> But what I really meant since we didn't get any further after registering
> all "good" connections (see we're not releasing mutex still) the will be
> no clashes with sysfs.
> 
> Still I;d like Daniel to comment on that separately.

I think doing the unregister_all outside of the loop&locked section is
better for future-proofing. My long-term plan for connector lifetimes and
the connector list is:
- refcounting for connectors (Dave has wip patches already).
- separate lock for the connector list (and only that).

Doing it entirely separate would make things easier and more robust.

I merged patch 1 meanwhile to drm-misc.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch