Re: [PATCH 01/44] drivers/base: Always release devres on device_del

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Apr 06, 2020 at 03:55:28PM +0200, Daniel Vetter wrote:
> On Mon, Apr 6, 2020 at 3:38 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Mon, Apr 06, 2020 at 02:32:51PM +0200, Daniel Vetter wrote:
> > > On Fri, Apr 3, 2020 at 3:58 PM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > > >
> > > > In drm we've added nice drm_device (the main gpu driver thing, which
> > > > also represents the userspace interfaces and has everything else
> > > > dangling off it) init functions using devres, devm_drm_dev_init and
> > > > soon devm_drm_dev_alloc (this patch series adds that).
> > > >
> > > > A slight trouble is that drm_device itself holds a reference on the
> > > > struct device it's sitting on top (for sysfs links and dmesg debug and
> > > > lots of other things), so there's a reference loop. For real drivers
> > > > this is broken at remove/unplug time, where all devres resources are
> > > > released device_release_driver(), before the final device reference is
> > > > dropped. So far so good.
> > > >
> > > > There's 2 exceptions:
> > > > - drm/vkms|vgem: Virtual drivers for which we create a fake/virtual
> > > >   platform device to make them look more like normal devices to
> > > >   userspace. These aren't drivers in the driver model sense, we simple
> > > >   create a platform_device and register it.
> > > >
> > > > - drm/i915/selftests, where we create minimal mock devices, and again
> > > >   the selftests aren't proper drivers in the driver model sense.
> > > >
> > > > For these two cases the reference loop isn't broken, because devres is
> > > > only cleaned up when the last device reference is dropped. But that's
> > > > not happening, because the drm_device holds that last struct device
> > > > reference.
> > > >
> > > > Thus far this wasn't a problem since the above cases simply
> > > > hand-rolled their cleanup code. But I want to convert all drivers over
> > > > to the devm_ versions, hence it would be really nice if these
> > > > virtual/fake/mock uses-cases could also be managed with devres
> > > > cleanup.
> > > >
> > > > I see three possible approaches:
> > >
> > > Restarting this at the top level, because the discussion thus far just
> > > ended in a long "you're doing it wrong", despite that I think we're
> > > doing what v4l is doing (plus/minus that we can't do an exact matching
> > > handling in drm because our uapi has a lot more warts, which we can't
> > > change because no breaking userspace).
> > >
> > > So which one of the three below is the right approach?
> > >
> > > Aside, looking at the v4l solution I think there's also a confusion
> > > about struct device representing a char device (which v4l directly
> > > uses as its userspace interface refcounted thing, and which drm does
> > > _not_ directly). And a struct device embedded into something like
> > > platform_device or a virtual device, where a driver can bind to. My
> > > question here is about the former, I don't care how cdev struct device
> > > are cleaned up one bit. Now if other subsystems relies on the devres
> > > cleanup behaviour we currently have because of such cdev usage, then
> > > yeah first approach doesn't work (and I have a big surprised that use
> > > case, but hey would actually learn something).
> > >
> > > End of aside, since again I want to figure out which of the tree
> > > approaches it the right one. Not about how wrong one of them is,
> > > ignoring the other three I laid out. And maybe there's even more
> > > options for this.
> >
> > Sorry, been swamped with other things, give me a few days to get back to
> > this, I need to dig into how you all are dealing with the virtual
> > drivers.
> 
> Sure, no problem.
> 
> > Doing this in the middle of the merge window is a bit rough :)
> 
> Ah I always forget ... we freeze drm at -rc6, so merge window is
> actually my most relaxed time since everyone is busy and no one has
> time to report drm bugs :-)

Hi Greg,

Since -rc3 is out, had any to ponder this? Otherwise we'll be right back
in the next merge window ...

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