discard cmd through ioctl

[Keith Busch]

On Fri, 6 Feb 2015, sheng qiu wrote:
> Hi Keith,
>
> thanks for your reply.
> for the struct "nvme_passthru_cmd", can you tell me what's the "flags"
> and "rsvd1" used for? which cmd filed does it associated to the NVMe
> Spec?

The IOCTL's structure tries to match an NVMe command except for parts
user space has no way of knowing about; those parts are command id and
bus addresses.

The 'flags' are CDW0's bits 8-15, just after the opcode. The only spec
defined use for them is fuse and SGL, neither of which are supported by
the driver, so should be cleared to 0 for now.

'rsvd1' is for CDW3. The spec has no defined use for it, but we have a
field in the IOCTL's arg so we have a placeholder for it in the future
without breaking ABI.