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 > <gregkh@xxxxxxxxxxxxxxxxxxx> 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. 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. thanks, greg k-h _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
