// SPDX-License-Identifier: GPL-2.0

THEORY OF OPERATION
-------------------
The capacity of an NVDIMM REGION (contiguous span of persistent memory)
is accessed via one or more NAMESPACE devices. REGION is the Linux term
for what ACPI and UEFI call a DIMM-interleave-set, or a
system-physical-address-range that is striped (by the memory controller)
across one or more memory modules.

The UEFI specification defines the 'NVDIMM Label Protocol' as the
combination of label area access methods and a data format for
provisioning one or more NAMESPACE objects from a REGION. Note that
label support is optional and if Linux does not detect the label
capability it will automatically instantiate a "label-less" namespace
per region. Examples of label-less namespaces are the ones created by
the kernel's 'memmap=ss!nn' command line option (see the nvdimm wiki on
kernel.org), or NVDIMMs without a valid 'namespace index' in their label
area.

NOTE: Label-less namespaces lack many of the features of their label-rich
cousins. For example, their size cannot be modified, or they cannot be
fully 'destroyed' (i.e. the space reclaimed). A destroy operation will
zero any mode-specific metadata. Finally, for create-namespace operations
on label-less namespaces, ndctl bypasses the region capacity availability
checks, and always satisfies the request using the full region capacity.
The only reconfiguration operation supported on a label-less namespace
is changing its 'mode'.

A namespace can be provisioned to operate in one of 4 modes, 'fsdax',
'devdax', 'sector', and 'raw'. Here are the expected usage models for
these modes:

	- fsdax: Filesystem-DAX mode is the default mode of a namespace
	  when specifying 'ndctl create-namespace' with no options. It creates
	  a block device (/dev/pmemX[.Y]) that supports the DAX capabilities
	  of Linux filesystems (xfs and ext4 to date). DAX removes the page
	  cache from the I/O path and allows mmap(2) to establish direct
	  mappings to persistent memory media. The DAX capability enables
	  workloads / working-sets that would exceed the capacity of the page
	  cache to scale up to the capacity of persistent memory. Workloads
	  that fit in page cache or perform bulk data transfers may not see
	  benefit from DAX. When in doubt, pick this mode.

	- devdax: Device-DAX mode enables similar mmap(2) DAX mapping
	  capabilities as Filesystem-DAX. However, instead of a block-device
	  that can support a DAX-enabled filesystem, this mode emits a single
	  character device file (/dev/daxX.Y). Use this mode to assign
	  persistent memory to a virtual-machine, register persistent memory
	  for RDMA, or when gigantic mappings are needed.

	- sector: Use this mode to host legacy filesystems that do
	  not checksum metadata or applications that are not prepared for torn
	  sectors after a crash. Expected usage for this mode is for small
	  boot volumes. This mode is compatible with other operating systems.

	- raw: Raw mode is effectively just a memory disk that does
	  not support DAX. Typically this indicates a namespace that was
	  created by tooling or another operating system that did not know how
	  to create a Linux 'fsdax' or 'devdax' mode namespace. This mode is
	  compatible with other operating systems, but again, does not support
	  DAX operation.
