Re: [PATCH v2 00/20] libnd: non-volatile memory device support

From: Williams, Dan J
Date: Fri May 08 2015 - 02:31:38 EST

On Tue, 2015-05-05 at 02:06 +0200, Rafael J. Wysocki wrote:
> On Tuesday, April 28, 2015 06:22:05 PM Dan Williams wrote:
> > On Tue, Apr 28, 2015 at 5:25 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > > On Tuesday, April 28, 2015 02:24:12 PM Dan Williams wrote:
> > >> Changes since v1 [1]: Incorporates feedback received prior to April 24.
> > >>
> [cut]
> > >
> > > I'm wondering what's wrong with CCing all of the series to linux-acpi?
> > >
> > > Is there anything in it that the people on that list should not see, by any
> > > chance?
> >
> > linux-acpi may not care about the dimm-metadata labeling patches that
> > are completely independent of ACPI, but might as well include
> > linux-acpi on the whole series at this point.
> I've gone through the ACPI-related patches in this series (other than [2/20]
> that I've commented directly) and while I haven't found anything horrible in
> them, I don't quite feel confident enough to ACK them.
> What I'm really missing in this series is a design document describing all that
> from a high-level perspective and making it clear where all of the pieces go
> and what their respective roles are. Also reordering the series to introduce
> the nd subsystem to start with and then its users might help here.

Here you go, and also see the "Supporting Documents" section if you need
more details, or just ask. This is the reworked document after pushing
NFIT specifics out of the core implementation. The core apis are
nd_bus_register(), nd_dimm_create(), nd_pmem_region_create(), and


LIBND: Non-volatile Devices
libnd - kernel / libndctl - userspace helper library

Supporting Documents
Git Trees
Why BLK?
BLK-REGIONs, PMEM-REGIONs, Atomic Sectors, and DAX
Example NVDIMM Platform
LIBND Kernel Device Model and LIBNDCTL Userspace API
libndctl: instantiate a new library context example
libnd: control class device in /sys/class
libnd: bus
libndctl: bus enumeration example
libnd: DIMM (NMEM)
libndctl: DIMM enumeration example
libnd: region
libndctl: region enumeration example
Why Not Encode the Region Type into the Region Name?
How Do I Determine the Major Type of a Region?
libnd: namespace
libndctl: namespace enumeration example
libndctl: namespace creation example
Why the Term "namespace"?
LIBND/LIBNDCTL: Block Translation Table "btt"
libnd: btt layout
libndctl: btt creation example
Summary LIBNDCTL Diagram


PMEM: A system physical address range where writes are persistent. A
block device composed of PMEM is capable of DAX. A PMEM address range
may span/interleave several DIMMs.

BLK: A set of one or more programmable memory mapped apertures provided
by a DIMM to access its media. This indirection precludes the
performance benefit of interleaving, but enables DIMM-bounded failure
modes .

DPA: DIMM Physical Address, is a DIMM-relative offset. With one DIMM in
the system there would be a 1:1 system-physical-address:DPA association.
Once more DIMMs are added an memory controller interleave must be
decoded to determine the DPA associated with a given
system-physical-address. BLK capacity always has a 1:1 relationship
with a single-dimm's DPA range.

DAX: File system extensions to bypass the page cache and block layer to
mmap persistent memory, from a PMEM block device, directly into a
process address space.

BTT: Block Translation Table: Persistent memory is byte addressable.
Existing software may have an expectation that the power-fail-atomicity
of writes is at least one sector, 512 bytes. The BTT is an indirection
table with atomic update semantics to front a PMEM/BLK block device
driver and present arbitrary atomic sector sizes.

LABEL: Metadata stored on a DIMM device that partitions and identifies
(persistently names) storage between PMEM and BLK. It also partitions
BLK storage to host BTTs with different parameters per BLK-partition.
Note that traditional partition tables, GPT/MBR, are layered on top of a
BLK or PMEM device.


The libnd subsystem provides support for three types of NVDIMMs, PMEM,
BLK, and NVDIMM platforms that can simultaneously support PMEM and BLK
mode access capabilities on a given set of DIMMs. These three modes of
operation are described by the "NVDIMM Firmware Interface Table" (NFIT)
in ACPI 6. While the libnd implementation is generic and supports
pre-NFIT platforms, it was guided by the superset of capabilities need
to support this ACPI 6 definition for NVDIMM resources. The bulk of the
kernel implementation is in place to handle the case where DPA
accessible via PMEM is aliased with DPA accessible via BLK. When that
occurs a LABEL is needed to reserve DPA for exclusive access via one
mode a time.

Supporting Documents
NVDIMM Namespace:
DSM Interface Example:
Driver Writer's Guide:

Git Trees


Prior to the arrival of the NFIT, non-volatile memory was described to a
system in various ad-hoc ways. Usually only the bare minimum was
provided, namely, a single system-physical-address range where writes
are expected to be durable after a system power loss. Now, the NFIT
specification standardizes not only the description of PMEM, but also
BLK and platform message-passing entry points for control and

For each NVDIMM access method (PMEM, BLK), LIBND provides a block device driver:

1. PMEM (nd_pmem.ko): Drives a system-physical-address range. This
range is contiguous in system memory and may be interleaved (hardware
memory controller striped) across multiple DIMMs. When interleaved the
platform may optionally provide details of which DIMMs are participating
in the interleave.

Note, LIBND describes system-physical-address ranges that may alias with
BLK access ND_NAMESPACE_PMEM ranges and those without alias as
ND_NAMESPACE_IO ranges, to the nd_pmem driver there is no distinction.
The different device-types are an implementation detail that userspace
can exploit to implement policies like "only interface with address
ranges from certain DIMMs". It is worth noting that when aliasing is
present and a DIMM lacks a label, then no block device can be created by
default as userspace needs to do at least one allocation of DPA to the
PMEM range. In contrast ND_NAMESPACE_IO ranges, once registered, can be
immediately attached to nd_pmem.

2. BLK (nd_blk.ko): This driver performs I/O using a set of platform
defined apertures. A set of apertures will all access just one DIMM.
Multiple windows allow multiple concurrent accesses, much like
tagged-command-queuing, and would likely be used by different threads or
different CPUs.

The NFIT specification defines a standard format for a BLK-aperture, but
the spec also allows for vendor specific layouts, and non-NFIT BLK
implementations may other designs for BLK I/O. For this reason "nd_blk"
calls back into platform-specific code to perform the I/O. One such
implementation is defined in the "Driver Writer's Guide" an "DSM
Interface Example".

Why BLK?

While PMEM provides direct byte-addressable CPU-load/store access to
NVDIMM storage, it does not provide the best system RAS (recovery,
availability, and serviceability) model. An access to a corrupted
system-physical-address address causes a cpu exception while an access
to a corrupted address through an BLK-aperture causes that block window
to raise an error status in a register. The latter is more aligned with
the standard error model that host-bus-adapter attached disks present.
Also, if an administrator ever wants to replace a memory it is easier to
service a system at DIMM module boundaries. Compare this to PMEM where
data could be interleaved in an opaque hardware specific manner across
several DIMMs.

BLK-apertures solve this RAS problem, but their presence is also the
major contributing factor to the complexity of the ND subsystem. They
complicate the implementation because PMEM and BLK alias in DPA space.
Any given DIMM's DPA-range may contribute to one or more
system-physical-address sets of interleaved DIMMs, *and* may also be
accessed in its entirety through its BLK-aperture. Accessing a DPA
through a system-physical-address while simultaneously accessing the
same DPA through a BLK-aperture has undefined results. For this reason,
DIMM's with this dual interface configuration include a DSM function to
store/retrieve a LABEL. The LABEL effectively partitions the DPA-space
into exclusive system-physical-address and BLK-aperture accessible
regions. For simplicity a DIMM is allowed a PMEM "region" per each
interleave set in which it is a member. The remaining DPA space can be
carved into an arbitrary number of BLK devices with discontiguous

BLK-REGIONs, PMEM-REGIONs, Atomic Sectors, and DAX

One of the few
reasons to allow multiple BLK namespaces per REGION is so that each
BLK-namespace can be configured with a BTT with unique atomic sector
sizes. While a PMEM device can host a BTT the LABEL specification does
not provide for a sector size to be specified for a PMEM namespace.
This is due to the expectation that the primary usage model for PMEM is
via DAX, and the BTT is incompatible with DAX. However, for the cases
where an application or filesystem still needs atomic sector update
guarantees it can register a BTT on a PMEM device or partition. See
LIBND/NDCTL: Block Translation Table "btt"

Example NVDIMM Platform

For the remainder of this document the following diagram will be
referenced for any example sysfs layouts.

+------+ | pm0.0 | blk2.0 | pm1.0 | blk2.1 | 0 region2
| imc0 +--+- - - region0- - - +--------+ +--------+
+--+---+ | pm0.0 | blk3.0 | pm1.0 | blk3.1 | 1 region3
| +-------------------+--------v v--------+
+--+---+ | |
| cpu0 | region1
+--+---+ | |
| +----------------------------^ ^--------+
+--+---+ | blk4.0 | pm1.0 | blk4.0 | 2 region4
| imc1 +--+----------------------------| +--------+
+------+ | blk5.0 | pm1.0 | blk5.0 | 3 region5

In this platform we have four DIMMs and two memory controllers in one
socket. Each unique interface (BLK or PMEM) to DPA space is identified
by a region device with a dynamically assigned id (REGION0 - REGION5).

1. The first portion of DIMM0 and DIMM1 are interleaved as REGION0. A
single PMEM namespace is created in the REGION0-SPA-range that spans
DIMM0 and DIMM1 with a user-specified name of "pm0.0". Some of that
interleaved system-physical-address range is reclaimed as BLK-aperture
accessed space starting at DPA-offset (a) into each DIMM. In that
reclaimed space we create two BLK-aperture "namespaces" from REGION2 and
REGION3 where "blk2.0" and "blk3.0" are just human readable names that
could be set to any user-desired name in the LABEL.

2. In the last portion of DIMM0 and DIMM1 we have an interleaved
system-physical-address range, REGION1, that spans those two DIMMs as
well as DIMM2 and DIMM3. Some of REGION1 allocated to a PMEM namespace
named "pm1.0" the rest is reclaimed in 4 BLK-aperture namespaces (for
each DIMM in the interleave set), "blk2.1", "blk3.1", "blk4.0", and

3. The portion of DIMM2 and DIMM3 that do not participate in the REGION1
interleaved system-physical-address range (i.e. the DPA address below
offset (b) are also included in the "blk4.0" and "blk5.0" namespaces.
Note, that this example shows that BLK-aperture namespaces don't need to
be contiguous in DPA-space.

This bus is provided by the kernel under the device
/sys/devices/platform/nfit_test.0 when CONFIG_NFIT_TEST is enabled and
the nfit_test.ko module is loaded. This not only test LIBND but the
acpi_nfit.ko driver as well.

LIBND Kernel Device Model and LIBNDCTL Userspace API

What follows is a description of the LIBND sysfs layout and a
corresponding object hierarchy diagram as viewed through the LIBNDCTL
api. The example sysfs paths and diagrams are relative to the Example
NVDIMM Platform which is also the libnd bus used in the libndctl unit

Every api call in the LIBNDCTL library requires a context that holds the
logging parameters and other library instance state. The library is
based on the libabc template: libndctl:
instantiate a new library context example

struct ndctl_ctx *ctx;

if (ndctl_new(&ctx) == 0)
return ctx;
return NULL;


A bus has a 1:1 relationship with an NFIT. The current expectation for
ACPI based systems is that there is only ever one platform-global NFIT.
That said, it is trivial to register multiple NFITs, the specification
does not preclude it. The infrastructure supports multiple busses and
we we use this capability to test multiple NFIT configurations in the
unit test.

libnd: control class device in /sys/class

This character device accepts DSM messages to be passed to DIMM
identified by its NFIT handle.

|-- dev
|-- device -> ../../../ndbus0
|-- subsystem -> ../../../../../../../class/nd

libnd: bus

struct nd_bus *nd_bus_register(struct device *parent,
struct nd_bus_descriptor *nfit_desc);

|-- btt0
|-- btt_seed
|-- commands
|-- nd
|-- nfit
|-- nmem0
|-- nmem1
|-- nmem2
|-- nmem3
|-- power
|-- provider
|-- region0
|-- region1
|-- region2
|-- region3
|-- region4
|-- region5
|-- uevent
`-- wait_probe

libndctl: bus enumeration example
Find the bus handle that describes the bus from Example NVDIMM Platform

static struct ndctl_bus *get_bus_by_provider(struct ndctl_ctx *ctx,
const char *provider)
struct ndctl_bus *bus;

ndctl_bus_foreach(ctx, bus)
if (strcmp(provider, ndctl_bus_get_provider(bus)) == 0)
return bus;

return NULL;

bus = get_bus_by_provider(ctx, "nfit_test.0");


The DIMM device provides a character device for sending commands to
hardware, and it is a container for LABELs. If the DIMM is defined by
NFIT then an optional 'nfit' attribute sub-directory is available to add

Note that the kernel device name for "DIMMs" is "nmemX". The NFIT
describes these devices via "Memory Device to System Physical Address
Range Mapping Structure", and there is no requirement that they actually
be physical DIMMs, so we use a more generic name.

libnd: DIMM (NMEM)

struct nd_dimm *nd_dimm_create(struct nd_bus *nd_bus, void *provider_data,
const struct attribute_group **groups, unsigned long flags,
unsigned long *dsm_mask);

|-- nmem0
| |-- available_slots
| |-- commands
| |-- dev
| |-- devtype
| |-- driver -> ../../../../../bus/nd/drivers/nd_dimm
| |-- modalias
| |-- nfit
| | |-- device
| | |-- format
| | |-- handle
| | |-- phys_id
| | |-- rev_id
| | |-- serial
| | `-- vendor
| |-- state
| |-- subsystem -> ../../../../../bus/nd
| `-- uevent
|-- nmem1

libndctl: DIMM enumeration example

Note, in this example we are assuming NFIT-defined DIMMs which are
identified by an "nfit_handle" a 32-bit value where:
Bit 3:0 DIMM number within the memory channel
Bit 7:4 memory channel number
Bit 11:8 memory controller ID
Bit 15:12 socket ID (within scope of a Node controller if node controller is present)
Bit 27:16 Node Controller ID
Bit 31:28 Reserved

static struct ndctl_dimm *get_dimm_by_handle(struct ndctl_bus *bus,
unsigned int handle)
struct ndctl_dimm *dimm;

ndctl_dimm_foreach(bus, dimm)
if (ndctl_dimm_get_handle(dimm) == handle)
return dimm;

return NULL;

#define DIMM_HANDLE(n, s, i, c, d) \
(((n & 0xfff) << 16) | ((s & 0xf) << 12) | ((i & 0xf) << 8) \
| ((c & 0xf) << 4) | (d & 0xf))

dimm = get_dimm_by_handle(bus, DIMM_HANDLE(0, 0, 0, 0, 0));


A generic REGION device is registered for each PMEM range orBLK-aperture
set. Per the example there are 6 regions: 2 PMEM and 4 BLK-aperture
sets on the "nfit_test.0" bus. The primary role of regions are to be a
container of "mappings". A mapping is a tuple of <DIMM,
DPA-start-offset, length>.

LIBND provides a built-in driver for these REGION devices. This driver
is responsible for reconciling the aliased DPA mappings across all
regions, parsing the LABEL, if present, and then emitting NAMESPACE
devices with the resolved/exclusive DPA-boundaries for the nd_pmem or
nd_blk device driver to consume.

In addition to the generic attributes of "mapping"s, "interleave_ways"
and "size" the REGION device also exports some convenience attributes.
"nstype" indicates the integer type of namespace-device this region
emits, "devtype" duplicates the DEVTYPE variable stored by udev at the
'add' event, "modalias" duplicates the MODALIAS variable stored by udev
at the 'add' event, and finally, the optional "spa_index" is provided in
the case where the region is defined by a SPA.

libnd: region

struct nd_region *nd_pmem_region_create(struct nd_bus *nd_bus,
struct nd_region_desc *ndr_desc);
struct nd_region *nd_blk_region_create(struct nd_bus *nd_bus,
struct nd_region_desc *ndr_desc);

|-- region0
| |-- available_size
| |-- devtype
| |-- driver -> ../../../../../bus/nd/drivers/nd_region
| |-- init_namespaces
| |-- mapping0
| |-- mapping1
| |-- mappings
| |-- modalias
| |-- namespace0.0
| |-- namespace_seed
| |-- nfit
| | `-- spa_index
| |-- nstype
| |-- set_cookie
| |-- size
| |-- subsystem -> ../../../../../bus/nd
| `-- uevent
|-- region1

libndctl: region enumeration example

Sample region retrieval routines based on NFIT-unique data like
"spa_index" (interleave set id) for PMEM and "nfit_handle" (dimm id) for

static struct ndctl_region *get_pmem_region_by_spa_index(struct ndctl_bus *bus,
unsigned int spa_index)
struct ndctl_region *region;

ndctl_region_foreach(bus, region) {
if (ndctl_region_get_type(region) != ND_DEVICE_REGION_PMEM)
if (ndctl_region_get_spa_index(region) == spa_index)
return region;
return NULL;

static struct ndctl_region *get_blk_region_by_dimm_handle(struct ndctl_bus *bus,
unsigned int handle)
struct ndctl_region *region;

ndctl_region_foreach(bus, region) {
struct ndctl_mapping *map;

if (ndctl_region_get_type(region) != ND_DEVICE_REGION_BLOCK)
ndctl_mapping_foreach(region, map) {
struct ndctl_dimm *dimm = ndctl_mapping_get_dimm(map);

if (ndctl_dimm_get_handle(dimm) == handle)
return region;
return NULL;

Why Not Encode the Region Type into the Region Name?

At first glance it seems since NFIT defines just PMEM and BLK interface
types that we should simply name REGION devices with something derived
from those type names. However, the ND subsystem explicitly keeps the
REGION name generic and expects userspace to always consider the
region-attributes for 4 reasons:

1. There are already more than two REGION and "namespace" types. For
PMEM there are two subtypes. As mentioned previously we have PMEM where
the constituent DIMM devices are known and anonymous PMEM. For BLK
regions the NFIT specification already anticipates vendor specific
implementations. The exact distinction of what a region contains is in
the region-attributes not the region-name or the region-devtype.

2. A region with zero child-namespaces is a possible configuration. For
example, the NFIT allows for a DCR to be published without a
corresponding BLK-aperture. This equates to a DIMM that can only accept
control/configuration messages, but no i/o through a descendant block
device. Again, this "type" is advertised in the attributes ('mappings'
== 0) and the name does not tell you much.

3. What if a third major interface type arises in the future? Outside
of vendor specific implementations, it's not difficult to envision a
third class of interface type beyond BLK and PMEM. With a generic name
for the REGION level of the device-hierarchy old userspace
implementations can still make sense of new kernel advertised
region-types. Userspace can always rely on the generic region
attributes like "mappings", "size", etc and the expected child devices
named "namespace". This generic format of the device-model hierarchy
allows the LIBND and LIBNDCTL implementations to be more uniform and

4. There are more robust mechanisms for determining the major type of a
region than a device name. See the next section, How Do I Determine the
Major Type of a Region?

How Do I Determine the Major Type of a Region?

Outside of the blanket recommendation of "use libndctl", or simply
looking at the kernel header (/usr/include/linux/ndctl.h) to decode the
"nstype" integer attribute, here are some other options.

1. module alias lookup:

The whole point of region/namespace device type differentiation is to
decide which block-device driver will attach to a given LIBND namespace.
One can simply use the modalias to lookup the resulting module. It's
important to note that this method is robust in the presence of a
vendor-specific driver down the road. If a vendor-specific
implementation wants to supplant the standard nd_blk driver it can with
minimal impact to the rest of LIBND.

In fact, a vendor may also want to have a vendor-specific region-driver
(outside of nd_region). For example, if a vendor defined its own LABEL
format it would need its own region driver to parse that LABEL and emit
the resulting namespaces. The output from module resolution is more
accurate than a region-name or region-devtype.

2. udev:

The kernel "devtype" is registered in the udev database
# udevadm info --path=/devices/platform/nfit_test.0/ndbus0/region0
P: /devices/platform/nfit_test.0/ndbus0/region0
E: DEVPATH=/devices/platform/nfit_test.0/ndbus0/region0
E: DEVTYPE=nd_pmem

# udevadm info --path=/devices/platform/nfit_test.0/ndbus0/region4
P: /devices/platform/nfit_test.0/ndbus0/region4
E: DEVPATH=/devices/platform/nfit_test.0/ndbus0/region4
E: DEVTYPE=nd_blk

...and is available as a region attribute, but keep in mind that the
"devtype" does not indicate sub-type variations and scripts should
really be understanding the other attributes.

3. type specific attributes:

As it currently stands a BLK-aperture region will never have a
"nfit/spa_index" attribute, but neither will a non-NFIT PMEM region. A
BLK region with a "mappings" value of 0 is, as mentioned above, a DIMM
that does not allow I/O. A PMEM region with a "mappings" value of zero
is a simple system-physical-address range.


A REGION, after resolving DPA aliasing and LABEL specified boundaries,
surfaces one or more "namespace" devices. The arrival of a "namespace"
device currently triggers either the nd_blk or nd_pmem driver to load
and register a disk/block device.

libnd: namespace
Here is a sample layout from the three major types of NAMESPACE where
namespace0.0 represents DIMM-info-backed PMEM (note that it has a 'uuid'
attribute), namespace2.0 represents a BLK namespace (note it has a
'sector_size' attribute) that, and namespace6.0 represents an anonymous
PMEM namespace (note that has no 'uuid' attribute due to not support a

|-- alt_name
|-- devtype
|-- dpa_extents
|-- modalias
|-- resource
|-- size
|-- subsystem -> ../../../../../../bus/nd
|-- type
|-- uevent
`-- uuid
|-- alt_name
|-- devtype
|-- dpa_extents
|-- modalias
|-- sector_size
|-- size
|-- subsystem -> ../../../../../../bus/nd
|-- type
|-- uevent
`-- uuid
|-- block
| `-- pmem0
|-- devtype
|-- driver -> ../../../../../../bus/nd/drivers/pmem
|-- modalias
|-- resource
|-- size
|-- subsystem -> ../../../../../../bus/nd
|-- type
`-- uevent

libndctl: namespace enumeration example
Namespaces are indexed relative to their parent region, example below. These indexes are mostly static from boot to boot, but subsystem makes no guarantees in this regard. For a static namespace identifier use its 'uuid' attribute.

static struct ndctl_namespace *get_namespace_by_id(struct ndctl_region *region,
unsigned int id)
struct ndctl_namespace *ndns;

ndctl_namespace_foreach(region, ndns)
if (ndctl_namespace_get_id(ndns) == id)
return ndns;

return NULL;

libndctl: namespace creation example
Idle namespaces are automatically created by the kernel if a given region has enough available capacity to create a new namespace. Namespace instantiation involves finding an idle namespace and configuring it. For the most part the setting of namespace attributes can occur in any order, the only constraint is that 'uuid' must be set before 'size'. This enables the kernel to track DPA allocations internally with a static identifier.

static int configure_namespace(struct ndctl_region *region,
struct ndctl_namespace *ndns,
struct namespace_parameters *parameters)
char devname[50];

snprintf(devname, sizeof(devname), "namespace%d.%d",
ndctl_region_get_id(region), paramaters->id);

ndctl_namespace_set_alt_name(ndns, devname);
/* 'uuid' must be set prior to setting size! */
ndctl_namespace_set_uuid(ndns, paramaters->uuid);
ndctl_namespace_set_size(ndns, paramaters->size);
/* unlike pmem namespaces, blk namespaces have a sector size */
if (parameters->lbasize)
ndctl_namespace_set_sector_size(ndns, parameters->lbasize);

Why the Term "namespace"?

1. Why not "volume" for instance? "volume" ran the risk of confusing ND
as a volume manager like device-mapper.

2. The term originated to describe the sub-devices that can be created
within a NVME controller (see the nvme specification:, and NFIT namespaces are
meant to parallel the capabilities and configurability of

LIBND/LIBNDCTL: Block Translation Table "btt"

A BTT (design document: is a stacked
block device driver that fronts either the whole block device or a
partition of a block device emitted by either a PMEM or BLK NAMESPACE.

libnd: btt layout
Every bus will start out with at least one BTT device which is the seed
device. To activate it set the "backing_dev", "uuid", and "sector_size"
attributes and then bind the device to the nd_btt driver.

|-- backing_dev
|-- delete
|-- devtype
|-- modalias
|-- sector_size
|-- subsystem -> ../../../../../bus/nd
|-- uevent
`-- uuid

libndctl: btt creation example
Similar to namespaces an idle BTT device is automatically created per
bus. Each time this "seed" btt device is configured and enabled a new
seed is created. Creating a BTT configuration involves two steps of
finding and idle BTT and assigning it to front a PMEM or BLK namespace.

static struct ndctl_btt *get_idle_btt(struct ndctl_bus *bus)
struct ndctl_btt *btt;

ndctl_btt_foreach(bus, btt)
if (!ndctl_btt_is_enabled(btt) && !ndctl_btt_is_configured(btt))
return btt;

return NULL;

static int configure_btt(struct ndctl_bus *bus, struct btt_parameters *parameters)
btt = get_idle_btt(bus);

sprintf(bdevpath, "/dev/%s",
ndctl_btt_set_uuid(btt, parameters->uuid);
ndctl_btt_set_sector_size(btt, parameters->sector_size);
ndctl_btt_set_backing_dev(btt, parametes->bdevpath);

Once instantiated a "nd_btt" link will be created under the
"backing_dev" (pmem0) block device:

|-- alignment_offset
|-- bdi -> ../../../../../../../virtual/bdi/259:0
|-- capability
|-- dev
|-- device -> ../../../namespace0.0
|-- discard_alignment
|-- ext_range
|-- holders
|-- inflight
|-- nd_btt -> ../../../../btt0

...and a new inactive seed device will appear on the bus.

Once a "backing_dev" is disabled its associated BTT will be
automatically deleted. This deletion is only at the device model level.
In order to destroy a BTT the "info block" needs to be destroyed.

Summary LIBNDCTL Diagram

For the given example above, here is the view of the objects as seen by the LIBNDCTL api:
|CTX| +---------+ +--------------+ +---------------+
+-+-+ +-> REGION0 +---> NAMESPACE0.0 +--> PMEM8 "pm0.0" |
| | +---------+ +--------------+ +---------------+
+-------+ | | +---------+ +--------------+ +---------------+
| DIMM0 <-+ | +-> REGION1 +---> NAMESPACE1.0 +--> PMEM6 "pm1.0" |
+-------+ | | | +---------+ +--------------+ +---------------+
| DIMM1 <-+ +-v--+ | +---------+ +--------------+ +---------------+
+-------+ +-+BUS0+---> REGION2 +-+-> NAMESPACE2.0 +--> ND6 "blk2.0" |
| DIMM2 <-+ +----+ | +---------+ | +--------------+ +----------------------+
+-------+ | | +-> NAMESPACE2.1 +--> ND5 "blk2.1" | BTT2 |
| DIMM3 <-+ | +--------------+ +----------------------+
+-------+ | +---------+ +--------------+ +---------------+
+-> REGION3 +-+-> NAMESPACE3.0 +--> ND4 "blk3.0" |
| +---------+ | +--------------+ +----------------------+
| +-> NAMESPACE3.1 +--> ND3 "blk3.1" | BTT1 |
| +--------------+ +----------------------+
| +---------+ +--------------+ +---------------+
+-> REGION4 +---> NAMESPACE4.0 +--> ND2 "blk4.0" |
| +---------+ +--------------+ +---------------+
| +---------+ +--------------+ +----------------------+
+-> REGION5 +---> NAMESPACE5.0 +--> ND1 "blk5.0" | BTT0 |
+---------+ +--------------+ +---------------+------+