Am 17.06.24 um 12:29 schrieb Pierre Ossman:
On 17/06/2024 10:13, Christian König wrote:
Let me try to clarify a couple of things:
The DMA_BUF_IOCTL_SYNC function is to flush and invalidate caches so
that the GPU can see values written by the CPU and the CPU can see
values written by the GPU. But that IOCTL does *not* wait for any
async GPU operation to finish.
If you want to wait for async GPU operations you either need to call
the OpenGL functions to read pixels or do a select() (or poll, epoll
etc...) call on the DMA-buf file descriptor.
Thanks for the clarification!
Just to avoid any uncertainty, are both of these things done
implicitly by gbm_bo_map()/gbm_bo_unmap()?
gbm_bo_map() is *not* doing any synchronization whatsoever as far as I
know. It just does the steps necessary for the mmap().
I did test adding those steps just in case, but unfortunately did not
see an improvement. My order was:
1. gbm_bo_import(GBM_BO_USE_RENDERING)
2. gbm_bo_get_fd()
3. Wait for client to request displaying the buffer
4. gbm_bo_map(GBM_BO_TRANSFER_READ)
5. select(fd+1, &fds, NULL, NULL, NULL)
6. ioctl(DMA_BUF_IOCTL_SYNC, &{ .flags = DMA_BUF_SYNC_START |
DMA_BUF_SYNC_READ })
7. pixman_blt()
8. gbm_bo_unmap()
At least of hand that looks like it should work.
So if you want to do some rendering with OpenGL and then see the
result in a buffer memory mapping the correct sequence would be the
following:
1. Issue OpenGL rendering commands.
2. Call glFlush() to make sure the hw actually starts working on the
rendering.
3. Call select() on the DMA-buf file descriptor to wait for the
rendering to complete.
4. Use DMA_BUF_IOCTL_SYNC to make the rendering result CPU visible.
What I want to do is implement the X server side of DRI3 in just CPU.
It works for every application I've tested except gnome-shell.
I would assume that 1. and 2. are supposed to be done by the X client,
i.e. gnome-shell?
Yes, exactly that.
What I need to be able to do is access the result of that, once the X
client tries to draw using that GBM backed pixmap (e.g. using
PresentPixmap).
No idea why that doesn't work.
Regards,
Christian.
So far, we've only tested Intel GPUs, but we are setting up Nvidia and
AMD GPUs at the moment. It will be interesting to see if the issue
remains on those or not.
Regards
