Does gbm_bo_map() implicitly synchronise?

[Michel Dänzer]

On 2024-06-24 21:08, James Jones wrote:
> FWIW, the NVIDIA binary driver's implementation of gbm_bo_map/unmap()
> 
> 1) Don't do any synchronization against in-flight work. The assumption is that if the content is going to be read, the API writing the data has established that coherence. Likewise, if it's going to be written, the API reading it afterwards does any invalidates or whatever are needed for coherence.
> 
> 2) We don't blit anything or format convert, because our GBM implementation has no DMA engine access, and I'd like to keep it that way. Setting up a DMA-capable driver instance is much more expensive as far as runtime resources than setting up a simple allocator+mmap driver, at least in our driver architecture. Our GBM map just does an mmap(), and if it's not linear, you're not going to be able to interpret the data unless you've read up on our tiling formats. I'm aware this is different from Mesa, and no one has complained thus far.

I've seen at least one webkitgtk issue report about gbm_bo_map not working as intended with nvidia.

gbm_bo_map definitely has to handle tiling, that's one of its main purposes.

It also really has to handle implicit synchronization, since there's no GBM API for explicit synchronization.


Just doing a direct mmap for gbm_bo_map can be bad for other reasons as well. E.g. if the BO storage is in VRAM and the application does CPU reads, it'll fall down a performance cliff.


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer