Hi Laszlo, On 6 April 2018 at 13:15, Laszlo Ersek <lersek@xxxxxxxxxx> wrote: > On 04/04/18 19:29, Laszlo Ersek wrote: >> Hi Emil, >> >> On 04/03/18 19:13, Emil Velikov wrote: >>> On 29 March 2018 at 12:17, Laszlo Ersek <lersek@xxxxxxxxxx> wrote: >>>> On 03/28/18 16:35, Emil Velikov wrote: >>>>> On 28 March 2018 at 11:27, Laszlo Ersek <lersek@xxxxxxxxxx> wrote: >>>>>> On 03/28/18 03:24, Emil Velikov wrote: >>>> >>>>>>> Gents, can someone double-check this please? Just in case. >>>>>> >>>>>> I guess I should test whether this series regresses the use case >>>>>> described in c2cbc38b97; is that correct? >>>>>> >>>>> Precisely. >>>> >>>>> [3] https://github.com/evelikov/linux/commits/ioctl-cleanups >>>> >>>> Unfortunately, this series seems to reintroduce the regression fixed >>>> / described earlier in commit c2cbc38b97. >>>> >>> Thank you very much for testing. >>> >>> Believe I've tracked it down to a broken commit from 2014 ;-) >>> Please try the following branch [1] - it's untested, but I'm 99% sure >>> it will work like a charm. >> >> Alas, it does not work. > > I've done some more digging. Let me quote the commit message on the > proposed patch again: > >> Ealier commit a325725633c26aa66ab940f762a6b0778edf76c0 did not attribute >> that virtio can be either PCI or a platform device and removed the >> .set_busid hook. Whereas only the "platform" instance should have been >> removed. >> >> Since then, two things have happened: >> - the driver manually calls drm_dev_set_unique, addressing the Xserver >> regression - see commit 9785b4321b0bd701f8d21d3d3c676a7739a5cf22 >> - core itself calls drm_pci_set_busid, on drm_set_busid IOCTL setting >> the busid, so we don't need to fallback to dev->unique - see commit >> 5c484cee7ef9c4fd29fa0ba09640d55960977145 >> >> With that in place we can remove the local workaround. > > This write-up of events is not precise enough. Instead, I think the > timeline is as follows: > > (1) Commit a325725633c2 ("drm: Lobotomize set_busid nonsense for !pci > drivers", 2016-06-21) introduced the regression. By removing > drm_virtio_set_busid(), commit a325725633c2 changed the behavior of > the following function: > >> static int drm_set_busid(struct drm_device *dev, struct drm_file *file_priv) >> { >> struct drm_master *master = file_priv->master; >> int ret; >> >> if (master->unique != NULL) >> drm_unset_busid(dev, master); >> >> if (dev->driver->set_busid) { >> ret = dev->driver->set_busid(dev, master); >> if (ret) { >> drm_unset_busid(dev, master); >> return ret; >> } >> } else { >> WARN_ON(!dev->unique); >> master->unique = kstrdup(dev->unique, GFP_KERNEL); >> if (master->unique) >> master->unique_len = strlen(dev->unique); >> } >> >> return 0; >> } > > When a325725633c2 removed drm_virtio_set_busid(), drm_set_busid() > started taking the second branch, which wasn't doing the right thing > for virtio-vga at the time. > > (2) There were two ways to fix the regression: either (a) return > drm_set_busid() to taking the first branch, or (b) tweak the > virtio-vga driver so that the second branch in drm_set_busid() start > to behave right. > > My commit c2cbc38b9715 ("drm: virtio: reinstate > drm_virtio_set_busid()", 2016-10-04) implemented (a), by reverting a > few chunks of a325725633c2. > > (3) Gerd thought that approach (b) was superior (and I totally defer to > him on this, now that I'm learning of his patches in the first place > :) ). Namely, in commit 9785b4321b0b ("drm/virtio: fix busid > regression", 2016-11-15), he implemented approach (b), and right > after, in commit 1775db074a32 ("Revert "drm: virtio: reinstate > drm_virtio_set_busid()"", 2016-11-15), he undid my commit > c2cbc38b9715. > > In other words, Gerd replaced approach (a) with approach (b). > > (4) Subsequently, commit 5c484cee7ef9 ("drm: Remove > drm_driver->set_busid hook", 2017-06-20), changed drm_set_busid() > to the following: > >> static int drm_set_busid(struct drm_device *dev, struct drm_file *file_priv) >> { >> struct drm_master *master = file_priv->master; >> int ret; >> >> if (master->unique != NULL) >> drm_unset_busid(dev, master); >> >> if (dev_is_pci(dev->dev)) { >> ret = drm_pci_set_busid(dev, master); >> if (ret) { >> drm_unset_busid(dev, master); >> return ret; >> } >> } else { >> WARN_ON(!dev->unique); >> master->unique = kstrdup(dev->unique, GFP_KERNEL); >> if (master->unique) >> master->unique_len = strlen(dev->unique); >> } >> >> return 0; >> } > > Perhaps surprisingly, this change did not affect (or "help") > virtio-vga at all, despite the fact that drm_virtio_set_busid() also > used to call drm_pci_set_busid(). > > The reason for commit 5c484cee7ef9 not affecting virtio-vga is that > the first branch would not be taken just the same, because > dev_is_pci() returns false for virtio-vga. (So, the difference with > drm_virtio_set_busid() is that drm_virtio_set_busid() would call > drm_pci_set_busid() without checking dev_is_pci() first.) > > Here's the definition of the dev_is_pci() macro, from > "include/linux/pci.h": > >> #define dev_is_pci(d) ((d)->bus == &pci_bus_type) > > However, the bus type for virtio-vga is "virtio_bus", not > "pci_bus_type". Namely, virtio_pci_probe() > [drivers/virtio/virtio_pci_common.c] calls register_virtio_device() > [drivers/virtio/virtio.c], and there we have > >> int register_virtio_device(struct virtio_device *dev) >> { >> int err; >> >> dev->dev.bus = &virtio_bus; > > This means that post-5c484cee7ef9, we remain reliant on the second > branch in drm_set_busid(), and therefore Gerd's commit 9785b4321b0b, > from point (3), should not be backed out. > > What I could see as justified is a loud comment in drm_virtio_init(), > just above the call to drm_dev_set_unique(), explaining why it is > necessary to set "unique" manually. The reason is that virtio-vga > technically has "virtio_bus", not "pci_bus_type", for bus type, and so > the generic PCI BusID-setting will not cover it. > I've reached to roughly the same conclusion yesterday. Or to put it in slightly differently: Unlike the other virtual GPU drivers (vmxgfx, qxl, bosh...) virtio abstracts the underlying bus type by using struct virtio_device. Hence the dev_is_pci() check will fail and the unique returned will be the virtio_device' "virtio0", while the "pci:..." one is required. Apart from a beefy comment I've also considered: - Extending the dev_is_pci() case [in drm_set_busid] to consider virtio. It seems like a bigger hack that what we have currently. - point drm_device::dev to the parent of the virtio_device The biggest hack imaginable, if even possible. Above said, consider patches 3+4 dropped and I'll follow-up with a patch adding inline documentation about this. Thank you very much for the help. Emil P.S. On the plus side, I am working on a X/modesetting series that removes all the mess which requires this workaround ;-) _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
