Hi Gustavo, Em Wed, 24 May 2017 21:31:01 -0300 Gustavo Padovan <gustavo@xxxxxxxxxxx> escreveu: > Hi all, > > I've been working on the v2 of this series, but I think I hit a blocker > when trying to cover the case where the driver asks to requeue the > buffer. It is related to the out-fence side. > > In the current implementation we return on QBUF an out-fence fd that is not > tied to any buffer, because we don't know the queueing order until the > buffer is queued to the driver. Then when the buffer is queued we use > the BUF_QUEUED event to notify userspace of the index of the buffer, > so now userspace knows the buffer associated to the out-fence fd > received earlier. > > Userspace goes ahead and send a DRM Atomic Request to the kernel to > display that buffer on the screen once the fence signals. If it is > a nonblocking request the fence waiting is past the check phase, thus > it isn't allowed to fail anymore. > > But now, what happens if the V4L2 driver calls buffer_done() asking > to requeue the buffer. That means the operation failed and can't > signal the fence, starving the DRM side. > > We need to fix that. The only way I can see is to guarantee ordering of > buffers when out-fences are used. Ordering is something that HAL3 needs > to so maybe there is more than one reason to do it like this. I'm not > a V4L2 expert, so I don't know all the consequences of such a change. > > Any other ideas? > > The current patchset is at: > > https://git.kernel.org/pub/scm/linux/kernel/git/padovan/linux.git/log/?h=v4l2-fences Currently, nothing warrants that buffers will be returned in order, but that should be the case of most drivers. I guess the main exception would be mem2mem codec drivers. On those, the driver or the hardware may opt to reorder the buffers. If this is a mandatory requirement for explicit fences to work, then we'll need to be able to explicitly enable it, per driver, and clearly document that drivers using it *should* warrant that the dequeued buffer will follow the queued order. We may need to modify VB2 in order to enforce it or return an error if the order doesn't match. -- Thanks, Mauro
