Re: [RESEND PATCH v9 4/5] ndctl/papr_scm,uapi: Add support for PAPR nvdimm specific methods

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Jun 5, 2020 at 8:22 AM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
[..]
> > Oh, why not define a maximal health payload with all the attributes
> > you know about today, leave some room for future expansion, and then
> > report a validity flag for each attribute? This is how the "intel"
> > smart-health payload works. If they ever needed to extend the payload
> > they would increase the size and add more validity flags. Old
> > userspace never groks the new fields, new userspace knows to ask for
> > and parse the larger payload.
> >
> > See the flags field in 'struct nd_intel_smart' (in ndctl) and the
> > translation of those flags to ndctl generic attribute flags
> > intel_cmd_smart_get_flags().
> >
> > In general I'd like ndctl to understand the superset of all health
> > attributes across all vendors. For the truly vendor specific ones it
> > would mean that the health flags with a specific "papr_scm" back-end
> > just would never be set on an "intel" device. I.e. look at the "hpe"
> > and "msft" health backends. They only set a subset of the valid flags
> > that could be reported.
>
> Thanks, this sounds good. Infact papr_scm implementation in ndctl does
> advertises support for only a subset of ND_SMART_* flags right now.
>
> Using 'flags' instead of 'version' was indeed discussed during
> v7..v9. However re-looking at the 'msft' and 'hpe' implementations the
> approach of maximal health payload tagged with a flags field looks more
> intuitive and I would prefer implementing this scheme in this patch-set.
>
> The current set health data exchanged with between libndctl and
> papr_scm via 'struct nd_papr_pdsm_health' (e.g various health status
> bits , nvdimm arming status etc) are guaranteed to be always available
> hence associating their availability with a flag wont be much useful as
> the flag will be always set.
>
> However as you suggested, extending the 'struct nd_papr_pdsm_health' in
> future to accommodate new attributes like 'life-remaining' can be done
> via adding them to the end of the struct and setting a flag field to
> indicate its presence.
>
> So I have the following proposal:
> * Add a new '__u32 extension_flags' field at beginning of 'struct
>   nd_papr_pdsm_health'
> * Set the size of the struct to 184-bytes which is the maximum possible
>   size for a pdsm payload.
> * 'papr_scm' kernel driver will currently set 'extension_flag' to 0
>   indicating no extension fields.
>
> * Future patch that adds support for 'life-remaining' add the new-field
>   at the end of known fields in 'struct nd_papr_pdsm_health'.
> * When provided to  papr_scm kernel module, if 'life-remaining' data is
>   available its populated and corresponding flag set in
>   'extension_flags' field indicating its presence.
> * When received by libndctl papr_scm implementation its tests if the
>   extension_flags have associated 'life-remaining' flag set and if yes
>   then return ND_SMART_USED_VALID flag back from
>   ndctl_cmd_smart_get_flags().
>
> Implementing first 3 items above in the current patchset should be
> fairly trivial.
>
> Does that sounds reasonable ?

This sounds good to me.