How to look up an ns cdev from block maj:min?

[Daniel Wagner]

On Wed, Feb 15, 2023 at 10:41:08AM +0200, Sagi Grimberg wrote:
> 
> > Hi,
> > Is there an official way for userspace to look up an ns cdev from a
> > block maj:min?
> > 
> > For example, how should a userspace program determine that /dev/ng0n1 is
> > the corresponding ns cdev when given just the block device node
> > /dev/nvme0n1?
> > 
> > Parsing the filename is an obvious option but it makes assumptions about
> > the naming convention of both the block device node and the character
> > device node.
> > 
> > Is there a better way via sysfs? (I couldn't find it.)
> 
> Not that I can see... It follows the same rules as the
> normal namespace, which means that it takes the subsystem or ctrl
> instance number (depending if multipathing is enabled) and it will have
> the same namespace instance number.
> 
> In other words, every namespace nvmeXnY will present an 'nvme generic'
> chardev in the form of ngXnY.
> 
> Looks like nvme-cli is missing the generic representation in the json
> output as well. Perhaps that is the best tool to use (once fixed with
> the json output with the below):

Looks good. I've created a PR for it.

https://github.com/linux-nvme/nvme-cli/pull/1820

Can I add your SoB?