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

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Date:   Thu, 18 Oct 2018 08:10:59 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Dave Airlie <airlied@...il.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, Oct 18, 2018 at 11:41:52AM +1000, Dave Airlie wrote:
> On Mon, 1 Oct 2018 at 20:33, Gerd Hoffmann <kraxel@...hat.com> wrote:
> >
> > Remove the virtio_gpu_object_{attach,detach} calls from move_notify()
> > callback.  Add them to the ttm_tt_{populate,unpopulate} callbacks, which
> > is the correct place to handle this.
> >
> > The new ttm_tt_{populate,unpopulate} callbacks call the
> > ttm_pool_populate()/unpopulate() functions (which are the default
> > implementation in case the callbacks not present) for the actual ttm
> > work.  Additionally virtio_gpu_object_{attach,detach} is called to
> > update the state on the host.
> 
> 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.

cheers,
  Gerd