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

[Daniel Vetter]

On Fri, May 15, 2020 at 02:55:38PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Apr 28, 2020 at 03:15:12PM +0200, Daniel Vetter wrote:
> > 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 at 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 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.
> > > 
> > > 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 ...
> 
> I owe you a response to this.  I'm going to try to carve out some time
> on Monday to do this, sorry for the delay :(

No worries, I got myself plenty occupied with other fun stuff in graphics
for now. And the part of the series which doesn't need this here is all
landed, so new drivers already look a notch cleaner in their code.

I'd have poked you earlier, but ofc very much appreciated if you can look
into this a bit before the next merge window.

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