linux-kernel - Re: [PATCH 7/8] drm/virtio: move virtio_gpu_object

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Message-ID: <CAPM=9tw1piSdEWAyOzcymoaFWp_kDP0JEaqX_Bxuvx2gK+RsZQ@mail.gmail.com>
Date:   Fri, 19 Oct 2018 10:33:15 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Dave Airlie <airlied@...ux.ie>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:VIRTIO CORE, NET..." 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 7/8] drm/virtio: move virtio_gpu_object_{attach, detach} calls.

On Thu, 18 Oct 2018 at 17:00, Gerd Hoffmann <kraxel@...hat.com> wrote:
>
> > > > This to me feels more like a bind/unbind operation rather than a
> > > > populate/unpopulate operation,
> > > >
> > > > bind is " Bind the backend pages into the aperture in the location"
> > > >
> > > > whereas populate is
> > > >
> > > > allocate pages for a ttm.
> > >
> > > I ran into that trap too ;)
> > >
> > > My first attempt was to map this to bind/unbind.  But this is not
> > > correct and therefore didn't work very well.
> > >
> > > virtio_gpu_object_attach() will send a scatter list of the pages
> > > allocated for the object to the host (so the host knows where to
> > > copy from/to when processing the transfer_from/to calls).  So IMO
> > > it should be done on population not when binding.
> >
> > Well bind on AGP is the same thing, we'd fill the AGP GART table on
> > bind, so that the AGP GPU could access the pages.
>
> > So I'm interested in why using bind/unbind failed if you have some more info?
>
> Need to try again to be sure, but IIRC I saw multiple bind/unbind calls
> for the same object.  ttm probably does it to not waste AGB GART address
> space for objects not in use.  But for virtio it is pointless overhead.
> But maybe it is just a matter of taking a reference and keeping it for
> the whole lifetime of the object to make the binding permanent ...

Hmm maybe for the bind/unbind, not sure what would cause unbind except
for the object being moved back out of the TT space and into system,
it might be worth confirming what happens here, as I really do feel
bind/unbind is the correct interface to use here.

Dave.