linux-kernel - Re: [Linux-nvdimm] [PATCH v2 05/20] libnd, nd

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Date:	Fri, 01 May 2015 14:08:08 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Neil Brown <neilb@...e.de>,
	Greg KH <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Robert Moore <robert.moore@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [Linux-nvdimm] [PATCH v2 05/20] libnd, nd_acpi:
 dimm/memory-devices

On Fri, 2015-05-01 at 12:38 -0700, Dan Williams wrote:
> On Fri, May 1, 2015 at 12:15 PM, Toshi Kani <toshi.kani@...com> wrote:
> > On Fri, 2015-05-01 at 11:43 -0700, Dan Williams wrote:
> >> On Fri, May 1, 2015 at 11:19 AM, Toshi Kani <toshi.kani@...com> wrote:
> >> > 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@...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?
> >
> > Did you forget to answer this question?  I am not asking to have a
> > label.  I am asking if the namespace types can handle it correctly.
> > Restating the nd_namespace_label_update() example:
> >  - namespace_io case: Skip, but a label may still exist. Correct?
> >  - namespace_blk case: Proceed, but blk does not require a label.
> 
> Ah, ok.  This is handled by nd_namespace_attr_visible() only labelled
> namespaces have writable sysfs attributes.  This would need to be
> extended for a label-less BLK namespace type.

I prefer not to duplicate each namespace type with and without a label,
but I am OK as long as the presence of labels is handled properly.

Thanks,
-Toshi

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