Does gbm_bo_map() implicitly synchronise?

[Christian König]

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