Re: [RESEND-PATCH v2] powerpc/papr_scm: Add support for reporting dirty-shutdown-count

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

On Thu, Jun 24, 2021 at 1:07 AM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
>
> Persistent memory devices like NVDIMMs can loose cached writes in case
> something prevents flush on power-fail. Such situations are termed as
> dirty shutdown and are exposed to applications as
> last-shutdown-state (LSS) flag and a dirty-shutdown-counter(DSC) as
> described at [1]. The latter being useful in conditions where multiple
> applications want to detect a dirty shutdown event without racing with
> one another.
>
> PAPR-NVDIMMs have so far only exposed LSS style flags to indicate a
> dirty-shutdown-state. This patch further adds support for DSC via the
> "ibm,persistence-failed-count" device tree property of an NVDIMM. This
> property is a monotonic increasing 64-bit counter thats an indication
> of number of times an NVDIMM has encountered a dirty-shutdown event
> causing persistence loss.
>
> Since this value is not expected to change after system-boot hence
> papr_scm reads & caches its value during NVDIMM probe and exposes it
> as a PAPR sysfs attributed named 'dirty_shutdown' to match the name of
> similarly named NFIT sysfs attribute. Also this value is available to
> libnvdimm via PAPR_PDSM_HEALTH payload. 'struct nd_papr_pdsm_health'
> has been extended to add a new member called 'dimm_dsc' presence of
> which is indicated by the newly introduced PDSM_DIMM_DSC_VALID flag.
>
> References:
> [1] https://pmem.io/documents/Dirty_Shutdown_Handling-V1.0.pdf
>
> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

Belated:

Acked-by: Dan Williams <dan.j.williams@intel.com>

It's looking like CXL will add one of these as well. Might be time to
add a unified location when that happens and deprecate these
bus-specific locations.