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

Wed Feb 19 19:57:52 UTC 2020

On Wed, Feb 19, 2020 at 7:19 PM Greg Kroah-Hartman
<gregkh at linuxfoundation.org> wrote:
>
> On Wed, Feb 19, 2020 at 07:36:52PM +0200, Laurent Pinchart wrote:
> > Hi Greg,
> >
> > On Wed, Feb 19, 2020 at 06:00:46PM +0100, Greg Kroah-Hartman wrote:
> > > On Wed, Feb 19, 2020 at 03:22:49PM +0100, Daniel Vetter wrote:
> > > > On Wed, Feb 19, 2020 at 2:33 PM Greg Kroah-Hartman wrote:
> > > > > On Wed, Feb 19, 2020 at 03:28:47PM +0200, Laurent Pinchart wrote:
> > > > > > 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.
> > >
> > > Fair enough, that makes sense.
> > >
> > > > - 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.
> > >
> > > I think moving over time to having this be a backend library is good.
> > > But no rush/issues here with this going in now, it solves a real need
> > > and we can refactor it later on to try to make it more "bus/class"
> > > generic as needed.
> >
> > >From a type checking point of view, it would then be nice to have a
> > structure that models a device node, other than just struct device that
> > is shared by all types of devices. As someone who was involve in the
> > creation of the device model we have today, and thus know the history,
> > what's your opinion on that ?
>
> My opinion is that 'struct device' was created just for that exact
> thing.  If "all you want" is a device node, it is trivial to use:
>         device_create();
> or device_create_varargs() or device_create_with_groups()
> and then use device_destroy() when you are done with it.

Yeah I think if we're going to share the backend code with devres.c
then probably the simplest way is to embed a struct device into
drm_device and give it a name like fake_dont_touch_for_drmm_only_dev
or so :-) And then use that internally in the wrappers, with a nice
properly typed interface exposed to drivers. C isn't C++ where you can
instantiate stuff with generics and all that.
-Daniel

> yes, it can do much more complex things, as needed, but the basics are
> there, so use it in a simple way if you want to, no objection from me.
>
> If there are things that are missing with it, please let me know.
>
> But creating a new structure/way for this, no, we do not want to go back
> to the 2.4 and older kernel methods where it was all totally disjointed
> and messy.


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