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

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Message-ID: <CAPcyv4h1h=UFkJR_RzaLcJRZ4D5rNsoqxfzabYvw+dfBH1Fe3w@mail.gmail.com>
Date:	Fri, 1 May 2015 12:38:23 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@...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, 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.
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