On 8/13/26 11:29, Taimuraz Kaitmazov wrote:
Resending: my earlier reply does not appear on the lists, so I assume it
did not reach you either.
On 8/13/26 20:08, Lizhi Hou wrote:
The legacy NPU device is not cache coherent. And the next generation
(aie4) devices will be cache coherent.
Confirming that with numbers, since I had measured it before your reply
landed. On npu4, without a flush the CPU reads what the buffer held
before the NPU wrote it, and the NPU reads what DRAM held before the CPU
wrote it. Both reproduce on all 20 runs, and the stale read is most of
the buffer, not a stray line: 3931 of 4096 values on average.
Good to know aie4 is coherent -- that makes anything we do here a
legacy-only concern.
On 8/13/26 09:44, Christian König wrote:
An alternative would be to use DMA_BUF_IOCTL_SYNC from userspace
Tried it against amdgpu, imported into amdxdna: stale on all 20 runs, no
better than no sync at all. SYNC_BO on the same buffer is clean on all
20.
Which leaves me no legal way to import a buffer the CPU also reads. Is
there one I'm missing, or should a device like this just not import?
On 8/12/26 17:45, Lizhi Hou wrote:
we need to unconditionally return -EOPNOTSUPP for imported BO
is_import_bo() also covers ubuf and cbuf, so that stops maintaining our
own userptr and carve-out BOs too. They take that arm today: on a 64 MiB
userptr BO a 4 KiB sync and a full sync both cost 659 us, so the range
is already being ignored there.
I am working on removing the dma-buf part for ubuf BO because that is
also not a good usage of dma-buf. So the ubuf will be a object soon.
cbuf is mainly for debug and is disabled by default.
Lizhi
Keying on dma_buf->ops instead would confine it to foreign buffers.
Either is fine by me, tell me which you want.
Separately: XRT's buffer::sync() clflushes in userspace unless
Debug.force_driver_sync is set, so the stack does this to foreign
dma-bufs whatever the driver does. And when the ioctl is used, a
FROM_DEVICE sync returns -EINVAL after the flush has already run, out of
amdxdna_hwctx_sync_debug_bo() when the BO has no assigned hwctx. Happy
to send that as its own patch; the helper has one caller, so returning 0
there is the obvious shape unless you want it done elsewhere.
v3 is sent: patches 1, 3 and 4 only. Patch 5 is dropped, and 2 with it
since it only serves 5. I have not tested 5 on a matching tree and its
numbers came from the foreign import case.
Taimuraz
