r600_dma_ring_test() failed - synchronization problem with write-combining memory

[Alexander Fyodorov]

09.10.2014, 21:42, "Christian König" <christian.koenig at amd.com>:
> Hi Alexander,
>
> in the ring test we write the value 0xDEADBEEF and 0xCAFEDEAD into
> registers, not VRAM.
>
> And the register bar shouldn't be accessed write combined, cause that
> could lead to a couple of ordering problems. Why do you think the access
> is done write combined?

Because there is this code in r600_dma_ring_test():
  void __iomem *ptr = (void *)rdev->vram_scratch.ptr;

And vram_scratch is allocated in r600_vram_scratch_init() with domain RADEON_GEM_DOMAIN_VRAM which implies write-combining. I assumed that this means it points to the video memory.

Also when I look at page table attributes I can see that it is indeed mapped as write-combining. In this test only "rdev->rmmio" area was mapped as UC (the one where radeon_ring_commit() writes to to start the execution).

> For VRAM it is true that we have a couple of different caches between
> the CPU and the actually memory, which need to be flushed explicitly if
> you want to see a value written by the GPU.

Then maybe such a flush is what I need. How do I put it in the instruction ring buffer?