Getting DIMM size in Redfish

[Lei YU]

This mail is to discuss how to get DIMM size in Redfish (bmcweb).

Currently, bmcweb does not report DIMM size because it's not
implemented, and we are trying to implement this.

The DIMM interface is defined in [phosphor-dbus-interface][1], which
does not provide any metadata.

For OpenPOWER systems, the size information is provided in `PrettyName`, e.g.

    "PrettyName": "DDR4-2666 32GiB 64-bit ECC RDIMM"

It is guaranteed to be a string with 5 parts, and we could parse this
string to get the size.
I do not know how other systems (e.g. x86 or ARM) get the DIMM size.

During the [review][2] , Ed suggested creating an appropriate
interface for the DIMM size.

It's a good suggestion, but it could be complicated to be implemented:
* The information is sent by host via inband IPMI as FRU;
* In ipmid, the code to handle FRU is generic and part of the code is
generated, so it could be hard to make a specific change for DIMM
size.

So the question to the community is, how other systems get the DIMM size?
Knowing this, we could try to design a generic method to handle the case.

Otherwise, we will have to write specific code, either in ipmid or in
bmcweb, to get such specific values.

Thanks!

[1]: https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Inventory/Item/Dimm.interface.yaml
[2]: https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/25754