linux-kernel - Re: [PATCH 1/6] pmem: remove indirection layer arch_has_pmem

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Message-ID: <CAPcyv4jun2dE9=24Wcf4Qh_JruNyf=5qoMckqdg3u0ROUgPZ8w@mail.gmail.com>
Date:	Fri, 7 Aug 2015 09:14:42 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 1/6] pmem: remove indirection layer arch_has_pmem_api()

On Thu, Aug 6, 2015 at 10:43 AM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> Prior to this change arch_has_wmb_pmem() was only called by
> arch_has_pmem_api().  Both arch_has_wmb_pmem() and arch_has_pmem_api()
> checked to make sure that CONFIG_ARCH_HAS_PMEM_API was enabled.
>
> Instead, remove one extra layer of indirection and the redundant
> CONFIG_ARCH_HAS_PMEM_API check, and just have arch_has_pmem_api()
> call __arch_has_wmb_pmem() directly.

So I think this patch takes us further away from where we want to go
in the near term which is a finer grained pmem api.  The class of
systems where (has_pmem_api() && !has_wmb_pmem()) is existing energy
backed nvdimm platforms.  I'm assuming those platforms will want to
assert persistence guarantees in the absence of a pcommit-like
instruction, and that we want to stop gating arch_has_pmem_api() on
the presence of wmb_pmem() capability.  In that case
arch_has_wmb_pmem() will be useful to have and that was the original
intent for including it, that intent did not seem to comprehended in
the changelog.
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