linux-kernel - Re: [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

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Date:   Tue, 20 Jun 2017 10:35:03 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Kirti Wankhede <kwankhede@...dia.com>,
        Xiaoguang Chen <xiaoguang.chen@...el.com>,
        chris@...is-wilson.co.uk, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, zhenyuw@...ux.intel.com,
        zhiyuan.lv@...el.com, intel-gvt-dev@...ts.freedesktop.org,
        zhi.a.wang@...el.com, kevin.tian@...el.com
Subject: Re: [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

  Hi,

> > Hmm, plane isn't really an ID, it is a type, with type being either
> > DRM_PLANE_TYPE_PRIMARY or DRM_PLANE_TYPE_CURSOR, so I don't think
> > the
> > flage above make sense.
> 
> The intention was that ..._REGION_ID and ...PLANE_ID are describing
> what the vfio_device_query_gfx_plane.id field represents, either a
> region index or a plane identifier.  The type of plane would be
> represented within the vfio_device_gfx_plane_info struct.

The planes don't really have an id, we should rename that to
plane_type, or maybe drm_plane_type (simliar to the drm_format_*
fields), to avoid that confusion.

plane_type is set by userspace to specify what kind of plane it asks
for.

> > Also I think it would be useful to have some way to figure the
> > device
> > capabilities as the userspace workflow will look quite different
> > for
> > the two cases.
> 
> In the region case, VFIO_DEVICE_GET_REGION_INFO would include a
> device
> specific region with a hopefully common identifier to identify it as
> a
> graphics framebuffer.

Ok, that should work to figure whenever the mdev supports a plane
region or not.

> In the dmabuf case,VFIO_DEVICE_QUERY_GFX_PLANE would indicate the
> plane as a "plane ID" and some sort of
> VFIO_DEVICE_GET_GFX_PLANE(VFIO_GFX_TYPE_DMABUF) ioctl would be
> necessary to get a file descriptor to that plane.
> 
> What else are you thinking we need?  Thanks,

I need to know whenever the mdev supports dmabufs or not, at device
initialization time (because dmabufs require opengl support), when
VFIO_DEVICE_QUERY_GFX_PLANE doesn't work due to the guest not having
the device initialized yet.

Maybe we should have a error field in the ioctl struct, or we need to
clearly define error codes so the kernel doesn't just throw EINVAL in
all cases.

Or just a VFIO_DEVICE_GFX_CAPS ioctl which returns NONE, REGION or
DMABUF.

cheers,
  Gerd