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

[Kirti Wankhede <kwankhede@nvidia.com>]

On 6/20/2017 8:30 PM, Alex Williamson wrote:
> On Tue, 20 Jun 2017 12:57:36 +0200
> Gerd Hoffmann <kraxel@redhat.com> wrote:
> 
>> On Tue, 2017-06-20 at 08:41 +0000, Zhang, Tina wrote:
>>> Hi,
>>>
>>> Thanks for all the comments. Here are the summaries:
>>>
>>> 1. Modify the structures to make it more general.
>>> 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;
>>> 	__u32 generation;
>>> };  
>>
>> Looks good to me.
>>
>>> 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; 
>>> };  
>>
>> I'm not convinced the flags are a great idea.  Whenever dmabufs or a
>> region is used is a static property of the device, not of each
>> individual plane.
>>
>>
>> I think we should have this for userspace to figure:
>>
>> enum vfio_device_gfx_type {
>>         VFIO_DEVICE_GFX_NONE,
>>         VFIO_DEVICE_GFX_DMABUF,
>>         VFIO_DEVICE_GFX_REGION,
>> };
>>
>> struct vfio_device_gfx_query_caps {
>>         __u32 argsz;
>>         __u32 flags;
>>         enum vfio_device_gfx_type;
>> };
> 
> We already have VFIO_DEVICE_GET_INFO which returns:
> 
> struct vfio_device_info {
>         __u32   argsz;
>         __u32   flags;
> #define VFIO_DEVICE_FLAGS_RESET (1 << 0)        /* Device supports reset */
> #define VFIO_DEVICE_FLAGS_PCI   (1 << 1)        /* vfio-pci device */
> #define VFIO_DEVICE_FLAGS_PLATFORM (1 << 2)     /* vfio-platform device */
> #define VFIO_DEVICE_FLAGS_AMBA  (1 << 3)        /* vfio-amba device */
> #define VFIO_DEVICE_FLAGS_CCW   (1 << 4)        /* vfio-ccw device */
>         __u32   num_regions;    /* Max region index + 1 */
>         __u32   num_irqs;       /* Max IRQ index + 1 */
> };
> 
> We could use two flag bits to indicate dmabuf or graphics region
> support.  vfio_device_gfx_query_caps seems to imply a new ioctl, which
> would be unnecessary.
>

Sounds good to me.

>> Then this to query the plane:
>>
>> struct vfio_device_gfx_query_plane {
>>         __u32 argsz;
>>         __u32 flags;
>>         struct vfio_device_gfx_plane_info plane_info;  /* out */
>>         __u32 plane_type;                              /* in  */
>> };
> 
> I'm not sure why we're using an enum for something that can currently
> be defined with 2 bits, seems like this would be another good use of
> flags.  We could even embed an enum into the flags if we want to
> leave some expansion room, 4 bits maybe?  Also, I was imagining that a
> device could support multiple graphics regions, that's where specifying
> the "id" as a region index seemed useful.  We lose that ability here
> unless we go back to defining a flag bit to specify how to interpret
> this last field.
> 

Right, as I mentioned in earlier reply, we need 2 seperate fields
- plane type : DRM_PLANE_TYPE_PRIMARY or DRM_PLANE_TYPE_CURSOR
- id : fd for dmabuf or region index for region type


>> 2. Remove dmabuf mgr fd and add these two ioctl commands to the vfio
>> device fd.
>>> VFIO_DEVICE_QUERY_GFX_PLANE : used to query
>>> vfio_device_gfx_plane_info.  
>>
>> Yes.
>>
>>> VFIO_DEVICE_GET_DMABUF_FD: used to create and return the dmabuf fd.  
> 
> I'm not convinced this adds value, but I'll list it as an option:
> 
> VFIO_DEVICE_QUERY(VFIO_DEVICE_GFX_PLANE)
> VFIO_DEVICE_GET_FD(VFIO_DEVICE_GFX_DMABUF_FD)
> 
> The benefit is that it might help to avoid a proliferation of ioctls on
> the device the pain is that we need to either define a field or section
> of flags which identify what is being queried or what type of device fd
> is being requested.
> 
>> Yes.  The plane might have changed between query-plane and get-dmabuf
>> ioctl calls though, we must make sure we handle that somehow.  Current
>> patches return plane_info on get-dmabuf ioctl too, so userspace can see
>> what it actually got.
>>
>> With the generation we can also do something different:  Pass in
>> plane_type and generation, and have VFIO_DEVICE_GET_DMABUF_FD return
>> an error in case the generation doesn't match.  In that case it doesn't
>> make much sense any more to have a separate plane_info struct, which
>> was added so we don't have to duplicate things in query-plane and get-
>> dmabuf ioctl structs.
> 
> I'm not sure I understand how this works for a region, the region is
> always the current generation, how can the user ever be sure the
> plane_info matches what is exposed in the region?  Thanks,
>

Userspace have to follow the sequence to query plane info
(VFIO_DEVICE_QUERY_GFX_PLANE) and then read primary surface from the region.
On kernel space side, from VFIO_DEVICE_QUERY_GFX_PLANE ioctl, driver
should update surface which is being exposed by the GFX region, fill
vfio_device_gfx_plane_info structure then return. GFX region surface
would only get updated from this ioctl.

Thanks,
Kirti