On Tue, Nov 16, 2021 at 7:43 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
On Mon, Nov 15, 2021 at 07:26:14PM +0000, Kasireddy, Vivek wrote:
> Hi Daniel, Greg,
>
> If it is the same or a similar crash reported here:
> https://lists.freedesktop.org/archives/dri-devel/2021-November/330018.html
> and here: https://lists.freedesktop.org/archives/dri-devel/2021-November/330212.html
> then the fix is already merged:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d89c0c8322ecdc9a2ec84b959b6f766be082da76
Yeah but that still leaves the problem of why exaxtly virtgpu is
reinventing drm_poll here?
Can you please replace it with drm_poll like all other drivers, including
the ones that have private events?
Hi Daniel,
Allow me to explain the use case a bit. It's for when virtgpu KMS is not used, but a special Wayland compositor does wayland passthrough instead:
https://www.youtube.com/watch?v=WwrXqDERFm8 https://www.youtube.com/watch?v=EkNBsBx501Q
This technique has gained much popularity in the virtualized laptop space, where it offers better performance/user experience than virtgpu KMS. The relevant paravirtualized userspace is "Sommelier":Previously, we were using the out-of-tree virtio-wl device and there were many discussions on how we could get this upstream:Extending virtgpu was deemed the least intrusive option:We ultimately settled on the context type abstraction and used virtio_gpu_poll to tell the guest "hey, we have a Wayland event". The host response is actually in a buffer of type BLOB_MEM_GUEST.It is possible to use drm_poll(..), but that would have to be accompanied by a drm_read(..). You'll need to define a dummy VIRTGPU_EVENT_FENCE_SIGNALED in the uapi too.That's originally how I did it, but some pointed out that's unnecessary since the host response is in the BLOB_MEM_GUEST buffer and virtgpu event is a dummy event. So we decided just to modify virtio_gpu_poll(..) to have the desired semantics in that case.For the regular virtio-gpu KMS path, things remain unchanged.There are of course other ways to do it (perhaps polling a dma_fence), but that was the cleanest way we could find.It's not rare for virtio to "special things" (see virtio_dma_buf_ops, virtio_dma_ops), since they are in fake devices.We're open to other ideas, but hopefully that answers some of your questions.
Thanks, Daniel
>
> Thanks,
> Vivek
>
> > On Sat, Nov 13, 2021 at 03:51:48PM +0100, Greg KH wrote:
> > > On Tue, Sep 21, 2021 at 04:20:23PM -0700, Gurchetan Singh wrote:
> > > > Similar to DRM_VMW_EVENT_FENCE_SIGNALED. Sends a pollable event
> > > > to the DRM file descriptor when a fence on a specific ring is
> > > > signaled.
> > > >
> > > > One difference is the event is not exposed via the UAPI -- this is
> > > > because host responses are on a shared memory buffer of type
> > > > BLOB_MEM_GUEST [this is the common way to receive responses with
> > > > virtgpu]. As such, there is no context specific read(..)
> > > > implementation either -- just a poll(..) implementation.
> > > >
> > > > Signed-off-by: Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx>
> > > > Acked-by: Nicholas Verne <nverne@xxxxxxxxxxxx>
> > > > ---
> > > > drivers/gpu/drm/virtio/virtgpu_drv.c | 43 +++++++++++++++++++++++++-
> > > > drivers/gpu/drm/virtio/virtgpu_drv.h | 7 +++++
> > > > drivers/gpu/drm/virtio/virtgpu_fence.c | 10 ++++++
> > > > drivers/gpu/drm/virtio/virtgpu_ioctl.c | 34 ++++++++++++++++++++
> > > > 4 files changed, 93 insertions(+), 1 deletion(-)
> > >
> > > This commit seems to cause a crash in a virtual drm gpu driver for
> > > Android. I have reverted this, and the next commit in the series from
> > > Linus's tree and all is good again.
> > >
> > > Any ideas?
> >
> > Well no, but also this patch looks very questionable of hand-rolling
> > drm_poll. Yes you can do driver private events like
> > DRM_VMW_EVENT_FENCE_SIGNALED, that's fine. But you really should not need
> > to hand-roll the poll callback. vmwgfx (which generally is a very old
> > driver which has lots of custom stuff, so not a great example) doesn't do
> > that either.
> >
> > So that part should go no matter what I think.
> > -Daniel
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
