[PATCH 03/52] drm: add managed resources tied to drm_device

[Daniel Vetter]

On Wed, Feb 19, 2020 at 2:33 PM Greg Kroah-Hartman
<gregkh at linuxfoundation.org> wrote:
>
> On Wed, Feb 19, 2020 at 03:28:47PM +0200, Laurent Pinchart wrote:
> > Hi Daniel,
> >
> > Thank you for the patch.
> >
> > On Wed, Feb 19, 2020 at 11:20:33AM +0100, Daniel Vetter wrote:
> > > We have lots of these. And the cleanup code tends to be of dubious
> > > quality. The biggest wrong pattern is that developers use devm_, which
> > > ties the release action to the underlying struct device, whereas
> > > all the userspace visible stuff attached to a drm_device can long
> > > outlive that one (e.g. after a hotunplug while userspace has open
> > > files and mmap'ed buffers). Give people what they want, but with more
> > > correctness.
> > >
> > > Mostly copied from devres.c, with types adjusted to fit drm_device and
> > > a few simplifications - I didn't (yet) copy over everything. Since
> > > the types don't match code sharing looked like a hopeless endeavour.
> > >
> > > For now it's only super simplified, no groups, you can't remove
> > > actions (but kfree exists, we'll need that soon). Plus all specific to
> > > drm_device ofc, including the logging. Which I didn't bother to make
> > > compile-time optional, since none of the other drm logging is compile
> > > time optional either.
> > >
> > > One tricky bit here is the chicken&egg between allocating your
> > > drm_device structure and initiliazing it with drm_dev_init. For
> > > perfect onion unwinding we'd need to have the action to kfree the
> > > allocation registered before drm_dev_init registers any of its own
> > > release handlers. But drm_dev_init doesn't know where exactly the
> > > drm_device is emebedded into the overall structure, and by the time it
> > > returns it'll all be too late. And forcing drivers to be able clean up
> > > everything except the one kzalloc is silly.
> > >
> > > Work around this by having a very special final_kfree pointer. This
> > > also avoids troubles with the list head possibly disappearing from
> > > underneath us when we release all resources attached to the
> > > drm_device.
> >
> > This is all a very good idea ! Many subsystems are plagged by drivers
> > using devm_k*alloc to allocate data accessible by userspace. Since the
> > introduction of devm_*, we've likely reduced the number of memory leaks,
> > but I'm pretty sure we've increased the risk of crashes as I've seen
> > some drivers that used .release() callbacks correctly being naively
> > converted to incorrect devm_* usage :-(
> >
> > This leads me to a question: if other subsystems have the same problem,
> > could we turn this implementation into something more generic ? It
> > doesn't have to be done right away and shouldn't block merging this
> > series, but I think it would be very useful.
>
> It shouldn't be that hard to tie this into a drv_m() type of a thing
> (driver_memory?)
>
> And yes, I think it's much better than devm_* for the obvious reasons of
> this being needed here.

There's two reasons I went with copypasta instead of trying to share code:
- Type checking, I definitely don't want people to mix up devm_ with
drmm_. But even if we do a drv_m that subsystems could embed we do
have quite a few different types of component drivers (and with
drm_panel and drm_bridge even standardized), and I don't want people
to be able to pass the wrong kind of struct to e.g. a managed
drmm_connector_init - it really needs to be the drm_device, not a
panel or bridge or something else.

- We could still share the code as a kind of implementation/backend
library. But it's not much, and with embedding I could use the drm
device logging stuff which is kinda nice. But if there's more demand
for this I can definitely see the point in sharing this, as Laurent
pointed out with the tiny optimization with not allocating a NULL void
* that I've done (and screwed up) it's not entirely trivial code.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch