nvme: report capacity 0 for non supported ZNS SSDs

[hch at lst.de]

On Mon, Nov 02, 2020 at 10:24:05PM +0100, Javier Gonzalez wrote:
> If I understand correctly, the model would be that a namespace will
> always have (i) a character device associated where I/O is always allowed
> through user formed commands, and if the namespace has full support in
> the kernel (ii) a block device where I/O is as it is today. In case of
> (ii) both interfaces can be used for I/O.

Yes.

> While we work on iterations for c), do you believe it is reasonable to
> merge a version of the current path that follows the PI convention for
> unsupported command sets and features? I would assume that we will have
> to convert PI to this model too when it is available.

I'm rather torn.  I think the model of the zero capacity block device
is a really, really bad one and we should haver never added it.  That
being said, for a ZNS namespace that does not support Zone Append I
can think of a model that actually makes sense:  expose it as a read-only
block device, as we can actually read from it perfectly fine, and that
would also allow ioctl access.