Re: [Linux-nvdimm] [PATCH v2 05/20] libnd, nd_acpi: dimm/memory-devices

Toshi Kani
2015-05-01T18:38:16Z

On Fri, 2015-05-01 at 11:22 -0700, Dan Williams wrote:
> On Fri, May 1, 2015 at 10:48 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> > On Tue, 2015-04-28 at 14:24 -0400, Dan Williams wrote:
> >> Register the memory devices described in the nfit as libnd 'dimm'
> >> devices on an nd bus. The kernel assigned device id for dimms is
> >> dynamic. If userspace needs a more static identifier it should consult
> >> a provider-specific attribute. In the case where NFIT is the provider,
> >> the 'nmemX/nfit/handle' or 'nmemX/nfit/serial' attributes may be used
> >> for this purpose.
> > :
> >> +
> >> +static int nd_acpi_register_dimms(struct acpi_nfit_desc *acpi_desc)
> >> +{
> >> + struct nfit_mem *nfit_mem;
> >> +
> >> + list_for_each_entry(nfit_mem, &acpi_desc->dimms, list) {
> >> + struct nd_dimm *nd_dimm;
> >> + unsigned long flags = 0;
> >> + u32 nfit_handle;
> >> +
> >> + nfit_handle = __to_nfit_memdev(nfit_mem)->nfit_handle;
> >> + nd_dimm = nd_acpi_dimm_by_handle(acpi_desc, nfit_handle);
> >> + if (nd_dimm) {
> >> + /*
> >> + * If for some reason we find multiple DCRs the
> >> + * first one wins
> >> + */
> >> + dev_err(acpi_desc->dev, "duplicate DCR detected: %s\n",
> >> + nd_dimm_name(nd_dimm));
> >> + continue;
> >> + }
> >> +
> >> + if (nfit_mem->bdw && nfit_mem->memdev_pmem)
> >> + flags |= NDD_ALIASING;
> >
> > Does this check work for a NVDIMM card which has multiple pmem regions
> > with label info, but does not have any bdw region configured?
>
> If you have multiple pmem regions then you don't have aliasing and
> don't need a label. You'll get an nd_namespace_io per region.
>
> > The code assumes that namespace_pmem (NDD_ALIASING) and namespace_blk
> > have label info. There may be an NVDIMM card with a single blk region
> > without label info.
>
> I'd really like to suggest that labels are only for resolving aliasing
> and that if you have a BLK-only NVDIMM you'll get an automatic
> namespace created the same as a PMEM-only. Partitioning is always
> there to provide sub-divisions of a namespace. The only reason to
> support multiple BLK-namespaces per-region is to give each a different
> sector size. I may eventually need to relent on this position, but
> I'd really like to understand the use case for requiring labels when
> aliasing is not present as it seems like a waste to me.

By looking at the callers of is_namespace_pmem() and is_namespace_blk(),
such as nd_namespace_label_update(), I am concerned that the namespace
types are also used for indicating the presence a label. Is it OK for
nd_namespace_label_update() to do nothing when there is no aliasing?

> > Instead of using the namespace types to assume the label info, how about
> > adding a flag to indicate the presence of the label info? This avoids
> > the separation of namespace_io and namespace_pmem for the same pmem
> > driver.
>
> To what benefit?

Why do they need to be separated? Having alias or not should not make
the pmem namespace different.

Thanks,
-Toshi