[PATCH v2] nvme: reserve a keep-alive admin tag for all transports

[Keith Busch]

On Fri, May 15, 2026 at 03:12:48AM -0400, Chao Shi wrote:
> Per NVMe 2.0a section 5.27.1.12 and the transport binding wording,
> PCIe MAY support KATO. Reserve one admin tag on all transports so
> the host is ready when a controller accepts the feature. Fabrics
> keeps two, the second being for the connect command.
> 
> A quirk-based approach was considered but no PCIe controller
> documented to declare KAS != 0 was found (two enterprise SSDs tested
> locally report KAS=0), so an allowlist has no entries today.

I totally get it's optional for PCIe, but that also means it's the
host's option on whether it wants to use it, and there's no requirement
we have to. We just need the driver react correctly when someone tries
to do it.

I am skeptical anyone would produce a PCIe device that supports it, but
let's say someone does: what is the use case motivating enabling this
optional feature in this driver? If it's just because the option is
there, then I think we can just reject the user command submitting the
feature for PCIe transports, like I earlier suggested. Requiring an
active command will just harm idle power states.