linux-kernel - Re: [PATCH 01/13] driver core, libnvdimm: disable manual unbind of dimms while region active

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Date:	Mon, 6 Jun 2016 13:36:25 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Linda Knippers <linda.knippers@....com>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	david <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 01/13] driver core, libnvdimm: disable manual unbind of
 dimms while region active

On Mon, Jun 6, 2016 at 1:20 PM, Linda Knippers <linda.knippers@....com> wrote:
[..]
>> A solution to the posted-write-queue flushing needs to be available
>> and a platform can choose to use flush hints or ADR.  If the NFIT
>> defines an NVDIMM device without hints we assume the platform must
>> have ADR.  If the platform NFIT neglects to define an NVDIMM to
>> physical address range mapping, we warn about a potentially broken
>> BIOS.  Hopefully we can make this clearer in future versions of the
>> spec.
>
> You lost me on those last 2 sentences.  An NVDIMM doesn't have to have
> an SPA range, but that seems to be unrelated to pcommit or flushes.

Ok, now you've lost me...

We need a SPA range to be able to do I/O whether that SPA range is
direct access to media or a block-window aperture.  If an NFIT
inculdes a "System Physical Address (SPA) Range Structure", but
neglects to include a corresponding "NVDIMM Region Mapping Structure"
then the kernel has no idea what actual dimm device(s) back that
memory region.  Without a memory device mapping it is undefined
whether I/O to a SPA range requires flushing or not.

This patch set silences the warning about "not being able to guarantee
persistence" when the BIOS provides a "NVDIMM Region Mapping
Structure".  When that structure is present the kernel uses flush
hints when provided, but ADR otherwise.  See the implementation of
nvdimm_flush() in patch 4.