On Tue, Nov 16, 2021 at 7:43 AM Daniel Vetter <dan...@ffwll.ch> 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=WwrXqDERFm8https://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": https://chromium.googlesource.com/chromiumos/platform2/+/main/vm_tools/sommelier/ https://chromium.googlesource.com/chromiumos/platform2/+/main/vm_tools/sommelier/virtualization/virtgpu_channel.cc Previously, we were using the out-of-tree virtio-wl device and there were many discussions on how we could get this upstream: https://lists.freedesktop.org/archives/dri-devel/2017-December/160309.html https://lists.oasis-open.org/archives/virtio-dev/202002/msg00005.html Extending virtgpu was deemed the least intrusive option: https://www.spinics.net/lists/kvm/msg159206.html 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 <gurchetansi...@chromium.org> > > > > > Acked-by: Nicholas Verne <nve...@chromium.org> > > > > > --- > > > > > 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 >
