Re: [PATCH v6 06/18] drm/virtio: remove ttm calls from in virtio_gpu_object_{reserve,unreserve}

On Thu, Jul 04, 2019 at 12:17:48PM -0700, Chia-I Wu wrote:

On Thu, Jul 4, 2019 at 4:10 AM Gerd Hoffmann [off-list ref] wrote:

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  Hi,

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-       r = ttm_bo_reserve(&bo->tbo, true, false, NULL);
+       r = reservation_object_lock_interruptible(bo->gem_base.resv, NULL);

Can you elaborate a bit about how TTM keeps the BOs alive in, for
example, virtio_gpu_transfer_from_host_ioctl?  In that function, only
three TTM functions are called: ttm_bo_reserve, ttm_bo_validate, and
ttm_bo_unreserve.  I am curious how they keep the BO alive.

It can't go away between reserve and unreserve, and I think it also
can't be evicted then.  Havn't checked how ttm implements that.

Hm, but the vbuf using the BO outlives the reserve/unreserve section.
The NO_EVICT flag applies only when the BO is still alive.  Someone
needs to hold a reference to the BO to keep it alive, otherwise the BO
can go away before the vbuf is retired.

Note that patches 14+15 rework virtio_gpu_transfer_*_ioctl to keep
gem reference until the command is finished and patch 17 drops
virtio_gpu_object_{reserve,unreserve} altogether.

Maybe I should try to reorder the series, then squash 6+17 to reduce
confusion.  I suspect that'll cause quite a few conflicts though ...

cheers,
  Gerd