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

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Date:	Fri, 01 May 2015 12:19:15 -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 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?

> > 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

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