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

[Alex Deucher]

On Thu, Oct 9, 2014 at 3:10 PM, Alexander Fyodorov <halcy at yandex.ru> wrote:
> 09.10.2014, 22:32, "Christian König" <christian.koenig at amd.com>:
>> Am 09.10.2014 um 20:15 schrieb Alexander Fyodorov:
>>>  09.10.2014, 21:42, "Christian König" <christian.koenig at amd.com>:
>>>>  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?
>>
>> At least we need to flush the HDP, but what hardware generation is this
>> exactly? Some R6xx don't support hardware flushes in the ring buffer.
>
> I observed the problem on HD2400 and HD6670.
>
>> Try to call r600_mmio_hdp_flush(rdev) from the loop which checks the
>> value written.
>
> Yep, it helped. Here is the patch against 3.10.53, tested on HD2400.
>
>
> Flush VRAM cache before each read when polling.
>
> Signed-off-by: Alexander Fyodorov <halcy at yandex.ru>
>
>
> Index: drivers/gpu/drm/radeon/r600.c
> ===================================================================
> --- drivers/gpu/drm/radeon/r600.c       (revision 11647)
> +++ drivers/gpu/drm/radeon/r600.c       (working copy)
> @@ -2899,6 +2899,7 @@
>         radeon_ring_unlock_commit(rdev, ring);
>
>         for (i = 0; i < rdev->usec_timeout; i++) {
> +               r600_ioctl_wait_idle(rdev, rdev->vram_scratch.robj);
>                 tmp = readl(ptr);
>                 if (tmp == 0xDEADBEEF)
>                         break;

I think I'd prefer to just switch the test to use gart memory since
this code is shared by different asics thay may not all implement hdp
flush the same way.  We can just reserve a couple of slots in the wb
page.

Alex