Re: [RFC v3 2/3] virtio: Introduce Vdmabuf driver

[Gerd Hoffmann <kraxel@redhat.com>]

On Fri, Feb 12, 2021 at 08:15:12AM +0000, Kasireddy, Vivek wrote:
> Hi Gerd,
> 
> > > > You don't have to use the rendering pipeline.  You can let the i915
> > > > gpu render into a dma-buf shared with virtio-gpu, then use
> > > > virtio-gpu only for buffer sharing with the host.
> [Kasireddy, Vivek] Just to confirm my understanding of what you are suggesting, are
> you saying that we need to either have Weston allocate scanout buffers (GBM surface/BO)
> using virtio-gpu and render into them using i915; or have virtio-gpu allocate pages and 
> export a dma-buf and have Weston create a GBM BO by calling gbm_bo_import(fd) and
> render into the BO using i915?

Not sure what the difference between the former and the latter is.

> > Hmm, why a big mode switch?  You should be able to do that without modifying the
> > virtio-gpu guest driver.  On the host side qemu needs some work to support the most
> > recent virtio-gpu features like the buffer uuids (assuming you use qemu userspace), right
> > now those are only supported by crosvm.
> [Kasireddy, Vivek] We are only interested in Qemu UI at the moment but if we were to use
> virtio-gpu, we are going to need to add one more vq and support for managing buffers, 
> events, etc.

Should be easy and it should not need any virtio-gpu driver changes.

You can use virtio-gpu like a dumb scanout device.  Create a dumb
bo, create a framebuffer for the bo, map the framebuffer to the crtc.

Then export the bo, import into i915, use it as render target.  When
rendering is done flush (DRM_IOCTL_MODE_DIRTYFB).  Alternatively
allocate multiple bo's + framebuffers and pageflip.

Pretty standard workflow for cases where rendering and scanout are
handled by different devices.  As far I know not uncommon in the arm
world.

Right now this will involve a memcpy() for any display update because
qemu is a bit behind on supporting recent virtio-gpu features.

take care,
  Gerd

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