linux-kernel - Re: [RFC PATCH 5/7] libnvdimm, e820: make CONFIG_X86_PMEM

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Date:	Mon, 17 Aug 2015 08:32:07 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	Rik van Riel <riel@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	david <david@...morbit.com>, Ingo Molnar <mingo@...nel.org>,
	Linux MM <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 5/7] libnvdimm, e820: make CONFIG_X86_PMEM_LEGACY a
 tristate option

On Mon, Aug 17, 2015 at 8:01 AM, Christoph Hellwig <hch@....de> wrote:
> On Sat, Aug 15, 2015 at 09:04:02AM -0700, Dan Williams wrote:
>> What other layer? /sys/devices/platform/e820_pmem is that exact same
>> device we had before this patch.  We just have a proper driver for it
>> now.
>
> We're adding another layer of indirection between the old e820 file
> and the new module.

Ok, yes, I was confused by "another layer of platform_devices".

That said here are the non-unit-test related reasons for this change
that I would include in a new changelog:

---

We currently register a platform device for e820 type-12 memory and
register a nvdimm bus beneath it.  Registering the platform device
triggers the device-core machinery to probe for a driver, but that
search currently comes up empty.  Building the nvdimm-bus registration
into the e820_pmem platform device registration in this way forces
libnvdimm to be built-in.  Instead, convert the built-in portion of
CONFIG_X86_PMEM_LEGACY to simply register a platform device and move
the rest of the logic to the driver for e820_pmem, for the following
reasons:

1/ Letting libnvdimm be a module allows building and testing
libnvdimm.ko changes without rebooting

2/ All the normal policy around modules can be applied to e820_pmem
(unbind to disable and/or blacklisting the module from loading by
default)

3/ Moving the driver to a generic location and converting it to scan
"iomem_resource" rather than "e820.map" means any other architecture
can take advantage of this simple nvdimm resource discovery mechanism
by registering a resource named "Persistent Memory (legacy)"
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