nvme: report capacity 0 for non supported ZNS SSDs

[hch at lst.de]

On Tue, Nov 03, 2020 at 03:10:19PM +0100, Javier Gonzalez wrote:
> One question here is that we are preparing a RFC for a io_uring passthru
> using the block device. Based on this discussion, it seems to me that
> you see this more suitable through the char device.
>
> Does it make sense that we post this RFC using the block device? It
> would be helpful to get early feedback before starting the char device.

If you wan to send the RFC with the block device that is ok.  But I
think the separate character device is pretty trivial, at least for
NVM command set derived command sets like ZNS (for others we'll need
to finish the Command Set Independent Identify Namespace TP first).

> I see that this does not make much sense for the other non-supported
> features in this patch (i.e., !po2 zone size and zoc). Since this is
> very much like PI today, is it OK we add these the same way (capacity 0)
> and then move to the char device when ready?

I'd rath avoid adding more of that capacity 0 mess if we can.  Especially
as the character device should be really easy to do.