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

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Date:   Mon, 19 Jun 2017 08:54:09 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Gerd Hoffmann <kraxel@...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

On Mon, 19 Jun 2017 08:34:13 +0200
Gerd Hoffmann <kraxel@...hat.com> wrote:

>   Hi,
> 
> > So perhaps this becomes:
> > 
> > struct vfio_device_gfx_plane_info {
> > 	__u64 start;
> > 	__u64 drm_format_mod;
> > 	__u32 drm_format;
> > 	__u32 width;
> > 	__u32 height;
> > 	__u32 stride;
> > 	__u32 size;
> > 	__u32 x_pos;
> > 	__u32 y_pos;
> > };  
> 
> Looks good.
> 
> > struct vfio_device_query_gfx_plane {
> > 	__u32 argsz;
> > 	__u32 flags;
> > #define VFIO_GFX_PLANE_FLAGS_REGION_ID		(1 << 0)
> > #define VFIO_GFX_PLANE_FLAGS_PLANE_ID		(1 << 1)
> > 	struct vfio_device_gfx_plane_info plane_info;
> > 	__u32 id; 
> > };  
> 
> 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.

> What are the nvidia plane for cursor support btw?
> 
> > The flag defines the data in the id field as either referring to a
> > region (perhaps there could be multiple regions with only one active)  
> 
> Well, we have a "start" field in vfio_device_gfx_plane_info (maybe we
> should rename that to "offset"), which can be used to place multiple
> planes into a single, fixed region.

That seems reasonable.
 
> 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.  VFIO_DEVICE_QUERY_GFX_PLANE would indicate the
plane as a region index.

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,

Alex