Deprecating NVME_IOCTL_SUBSYS_RESET

[Keith Busch]

On Thu, May 10, 2018 at 04:18:06PM +0000, Alex_Gagniuc at Dellteam.com wrote:
> 
> According to (my reading of) the NVMe spec, an NSSR and PCIe link reset 
> should be equivalent.

That can't possibly be the case. A PCIe link reset affects a single
device on that PCIe link, but a NSSR resets ALL controllers in the
subsystem, some of which may be on different links.

The NSSR does indeed lead to a PCIe link reset, but there's a lot more
to it that that.
 
> > That said, I have heard enough cases where this reset method is not
> > successful, so there's some work to do here. Most failures seem to be
> > around the handling of the rapid link down-up sequence, and success
> > seems very dependent on the platform and the device used.
> 
> There is also the controller reset, which seems less intrusive, though I 
> haven't looked into it. Can that be used instead of the subsystem reset 
> for those cases where a subsystem reset is allegedly needed?

According to the spec (section 5.11 Firmware Commit), a device may to
require a subsystem reset in order to complete a firmware upgrade. No
other type of reset will do the trick.