linux-kernel - Re: [PATCH v6 06/18] drm/virtio: remove ttm calls from in virtio_gpu_object

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Message-ID: <20190705085325.am2reucblvxc6qpg@sirius.home.kraxel.org>
Date:   Fri, 5 Jul 2019 10:53:25 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Chia-I Wu <olvaffe@...il.com>
Cc:     ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: 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 <kraxel@...hat.com> 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.

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