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

[Greg Kroah-Hartman]

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 at 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.

Doing this in the middle of the merge window is a bit rough :)

thanks,

greg k-h