Re: [PATCH 3/4] drm/virtio: remove drm_dev_set_unique workaround

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Apr 06, 2018 at 01:56:15PM +0100, Emil Velikov wrote:
> Hi Laszlo,
> 
> On 6 April 2018 at 13:15, Laszlo Ersek <lersek@redhat.com> 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@redhat.com> wrote:
> >>>> On 03/28/18 16:35, Emil Velikov wrote:
> >>>>> On 28 March 2018 at 11:27, Laszlo Ersek <lersek@redhat.com> 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.

What would/could break if we do this? Why is this a hack?
-Daniel

> 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 ;-)

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