Ira Weiny [off-list ref] writes:

On Wed, May 20, 2020 at 12:30:57AM +0530, Vaibhav Jain wrote:

quoted

Introduce support for Papr nvDimm Specific Methods (PDSM) in papr_scm
modules and add the command family to the white list of NVDIMM command
sets. Also advertise support for ND_CMD_CALL for the dimm
command mask and implement necessary scaffolding in the module to
handle ND_CMD_CALL ioctl and PDSM requests that we receive.

...

quoted

+ *
+ * Payload Version:
+ *
+ * A 'payload_version' field is present in PDSM header that indicates a specific
+ * version of the structure present in PDSM Payload for a given PDSM command.
+ * This provides backward compatibility in case the PDSM Payload structure
+ * evolves and different structures are supported by 'papr_scm' and 'libndctl'.
+ *
+ * When sending a PDSM Payload to 'papr_scm', 'libndctl' should send the version
+ * of the payload struct it supports via 'payload_version' field. The 'papr_scm'
+ * module when servicing the PDSM envelope checks the 'payload_version' and then
+ * uses 'payload struct version' == MIN('payload_version field',
+ * 'max payload-struct-version supported by papr_scm') to service the PDSM.
+ * After servicing the PDSM, 'papr_scm' put the negotiated version of payload
+ * struct in returned 'payload_version' field.

FWIW many people believe using a size rather than version is more sustainable.
It is expected that new payload structures are larger (more features) than the
previous payload structure.

I can't find references at the moment through.

I think clone_args is a good modern example:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/sched.h#n88

cheers