Hi Gerd, > > On Fri, Feb 12, 2021 at 08:15:12AM +0000, Kasireddy, Vivek wrote: > > Hi Gerd, > > [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. [Kasireddy, Vivek] Oh, what I meant is whether you were suggesting that we create a GBM device and create a GBM surface and BOs using this device or just create a raw/dumb GEM object and create a GBM BO by importing it. As we just discovered, the former means we have to initialize virgl which complicates things so we went with the latter. > > > [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. [Kasireddy, Vivek] Vdmabuf v4, that implements your suggestion -- to have Vdmabuf allocate pages -- is posted here: https://lists.freedesktop.org/archives/dri-devel/2021-February/297841.html and tested it with Weston Headless and Qemu: https://gitlab.freedesktop.org/Vivek/weston/-/blob/vdmabuf/libweston/backend-headless/headless.c#L522 https://lists.nongnu.org/archive/html/qemu-devel/2021-02/msg02976.html Having said that, after discussing with Daniel Vetter, we are now switching our focus to virtio-gpu to compare and contrast both solutions. > > 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. [Kasireddy, Vivek] Since we are testing with Weston, we are looking at pageflips (4 color buffers). And, this part so far seems to work where virtio-gpu is used for kms (max_outputs=1) and Iris/i915 is used for rendering. We are currently glueing virtio-gpu and i915 in Weston but eventually the plan is to glue them (virgl/virtio-gpu and Iris) in Mesa if possible using KMSRO (KMS render only) to avoid having to change Weston or X or other user-space components. > > 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. [Kasireddy, Vivek] IIUC, I think you are referring to creating the Pixman image in set_scanout. What additional features need to be implemented or what is your recommendation in terms of what needs to be done to turn the memcpy() into a dma-buf? Also, how should we ensure that access to the guest fb/dmabuf is synchronized to ensure that the Guest and the Host do not access the backing storage of the dmabuf at the same time? Thanks, Vivek > > take care, > Gerd
