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

[Chia-I Wu]

On Thu, Jul 4, 2019 at 4:10 AM Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>
>   Hi,
>
> > > -       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.

I can be wrong, but on the other hand, it seems fine for a BO to go
away before the vbuf using it is retired.  When that happens, the
driver emits a RESOURCE_UNREF vbuf which is *after* the original vbuf.


>
> cheers,
>   Gerd
>