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

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Message-ID: <CAPaKu7S4Og7kda2ZjFtuRv=_W8gpFbD5y7s==0JB5Z34S4Hsjw@mail.gmail.com>
Date:   Thu, 4 Jul 2019 12:17:48 -0700
From:   Chia-I Wu <olvaffe@...il.com>
To:     Gerd Hoffmann <kraxel@...hat.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 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.

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
>