On 3/27/25 10:14, Kasireddy, Vivek wrote: >> Another question, why do we need this fencing for imported dmabuf? >> Fencing isn't done host/guest blobs in this code, while dmabuf is >> essentially a guest blob. Could you please clarify why this fence is >> needed? Maybe we shouldn't allocate fence in the first place for the dmabuf. > At-least for the non-virgl use-cases (where the rendering is done in the > Guest such > as in passthrough), this Guest/Host synchronization fence serves two purposes: > - It prevents the Guest from reusing/destroying the submitted buffer (Guest > compositor > FB) until the Host is done using it. Otherwise, the Guest compositor might > render > into this buffer at the same time while the Host is consuming it, leading > to issues > such as tearing/flickering. This problem is more noticeable in cases where > the > Guest compositor has only one backbuffer such as Xorg + dirtfyFb. > > - It also prevents the Guest compositor from rendering faster than the Host > refresh > rate. In other words, it just sets the upper bound in terms of the buffer > submission > rate as there is no point in going over the Host refresh rate, which would > likely > lead to wastage of GPU cycles and dropped frames. > > Therefore, this fence is really needed for Guest blobs including imported > dmabufs.
Thanks a lot -- Best regards, Dmitry
