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

[Stefan Hajnoczi]

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.

Thanks for confirming!

Stefan

> 
> 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):
> 
> --
> diff --git a/nvme-print-json.c b/nvme-print-json.c
> index df5616e88119..489e9e15c3b5 100644
> --- a/nvme-print-json.c
> +++ b/nvme-print-json.c
> @@ -2507,6 +2507,7 @@ static void json_detail_list(nvme_root_t r)
>                                         uint64_t nuse =
> nvme_ns_get_lba_util(n) * lba;
> 
> 
> json_object_add_value_string(jns, "NameSpace", nvme_ns_get_name(n));
> + json_object_add_value_string(jns, "Generic", nvme_ns_get_generic_name(n));
>                                         json_object_add_value_int(jns,
> "NSID", nvme_ns_get_nsid(n));
> 
> json_object_add_value_uint64(jns, "UsedBytes", nuse);
> 
> json_object_add_value_uint64(jns, "MaximumLBA", nvme_ns_get_lba_count(n));
> @@ -2565,15 +2566,18 @@ static struct json_object *json_list_item(nvme_ns_t
> n)
>  {
>         struct json_object *jdevice = json_create_object();
>         char devname[128] = { 0 };
> +       char genname[128] = { 0 };
> 
>         int lba = nvme_ns_get_lba_size(n);
>         uint64_t nsze = nvme_ns_get_lba_count(n) * lba;
>         uint64_t nuse = nvme_ns_get_lba_util(n) * lba;
> 
>         nvme_dev_full_path(n, devname, sizeof(devname));
> +       nvme_generic_full_path(n, genname, sizeof(genname));
> 
>         json_object_add_value_int(jdevice, "NameSpace",
> nvme_ns_get_nsid(n));
>         json_object_add_value_string(jdevice, "DevicePath", devname);
> +       json_object_add_value_string(jdevice, "GenericPath", genname);
>         json_object_add_value_string(jdevice, "Firmware",
> nvme_ns_get_firmware(n));
>         json_object_add_value_string(jdevice, "ModelNumber",
> nvme_ns_get_model(n));
>         json_object_add_value_string(jdevice, "SerialNumber",
> nvme_ns_get_serial(n));
> diff --git a/nvme-print.c b/nvme-print.c
> index fee9aabfc176..ec98fe304c4b 100644
> --- a/nvme-print.c
> +++ b/nvme-print.c
> @@ -5108,7 +5108,7 @@ void nvme_dev_full_path(nvme_ns_t n, char *path,
> size_t len)
>         snprintf(path, len, "%s", nvme_ns_get_name(n));
>  }
> 
> -static void nvme_generic_full_path(nvme_ns_t n, char *path, size_t len)
> +void nvme_generic_full_path(nvme_ns_t n, char *path, size_t len)
>  {
>         int head_instance;
>         int instance;
> diff --git a/nvme-print.h b/nvme-print.h
> index ab11774daa28..7a4ceeea5131 100644
> --- a/nvme-print.h
> +++ b/nvme-print.h
> @@ -149,6 +149,7 @@ const char *nvme_feature_to_string(enum nvme_features_id
> feature);
>  const char *nvme_register_to_string(int reg);
> 
>  void nvme_dev_full_path(nvme_ns_t n, char *path, size_t len);
> +void nvme_generic_full_path(nvme_ns_t n, char *path, size_t len);
>  char *zone_type_to_string(__u8 cond);
>  char *zone_state_to_string(__u8 state);
>  const char *nvme_pel_event_to_string(int type);
> --
> 
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