linux-kernel - Re: [Linux-nvdimm] [PATCH v2 11/20] libnd, nd_pmem: add libnd support to the pmem driver

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Message-ID: <CALCETrWcH76_P8pzK+cur50pk--VppkjG9z3ueXMAS_=x9jd1w@mail.gmail.com>
Date:	Tue, 28 Apr 2015 15:58:02 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Phil Pokorny <ppokorny@...guincomputing.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	linux-nvdimm <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [Linux-nvdimm] [PATCH v2 11/20] libnd, nd_pmem: add libnd support
 to the pmem driver

On Tue, Apr 28, 2015 at 3:21 PM, Phil Pokorny
<ppokorny@...guincomputing.com> wrote:
> On Tue, Apr 28, 2015 at 2:04 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Tue, Apr 28, 2015 at 11:25 AM, Dan Williams <dan.j.williams@...el.com> wrote:
>>> +config ND_E820
>>> +       tristate "E820: Support the E820-type-12 PMEM convention"
>>> +       depends on X86_PMEM_LEGACY
>>> +       default m if X86_PMEM_LEGACY
>>> +       select LIBND
>>> +       help
>>> +         Prior to ACPI 6 some platforms advertised peristent memory
>>> +         via type-12 e820 memory ranges.  Create a libnd bus and
>>> +         attach an instance of the pmem driver to these ranges.
>>> +
>>
>> How about something like:
>>
>> "This driver allows libnd to work with legacy, pre-ACPI 6 NVDIMMs.
>> This enables such devices to be exposed as block devices using PMEM.
>>
>> The legacy NVDIMM interface is problematic.  This driver will not work
>> if you boot using UEFI, and some NVDIMMs and motherboards that work
>> with this driver may require proprietary code in order to work
>> reliably."
>
> Perhaps not "problematic" but "requires a BIOS in Legacy mode"
>
> It might also mention that if you use the kernel command line
> memmap=nn!ss syntax it adds
> a type 12 region to the e820 map and so you would want this support.
>
> If you have a motherboard with UEFI support for NVDIMM's that would be
> the recommended
> configuration.

This is such a mess that I think this driver should maybe flat-out
refuse to load in this type of configuration without some scary module
option.  I have some NVDIMMs that report as type 12 but need two extra
out-of-tree drivers to work safely.  First, they need i2c_imc or the
equivalent (I'll try to resubmit that soon).  Second, they need secret
magic NDAed register poking.  The latter is very problematic.

At the very least, I think we should discourage people who don't
really know what they're doing from using this driver without care.

--Andy
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