linux-kernel - Re: [PATCH v2 20/20] libnd, nd_acpi, nd_blk: driver for BLK-mode access persistent memory

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Date:	Wed, 29 Apr 2015 10:10:33 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-nvdimm <linux-nvdimm@...ts.01.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...com>, "H. Peter Anvin" <hpa@...or.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@....de>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2 20/20] libnd, nd_acpi, nd_blk: driver for BLK-mode
 access persistent memory

On Tue, Apr 28, 2015 at 4:06 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Apr 28, 2015 at 3:30 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> On Tue, Apr 28, 2015 at 2:10 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Tue, Apr 28, 2015 at 11:26 AM, Dan Williams <dan.j.williams@...el.com> wrote:
>>>> From: Ross Zwisler <ross.zwisler@...ux.intel.com>
>>>>
>>>> The libnd implementation handles allocating dimm address space (DPA)
>>>> between PMEM and BLK mode interfaces.  After DPA has been allocated from
>>>> a BLK-region to a BLK-namespace the nd_blk driver attaches to handle I/O
>>>> as a struct bio based block device. Unlike PMEM, BLK is required to
>>>> handle platform specific details like mmio register formats and memory
>>>> controller interleave.  For this reason the libnd generic nd_blk driver
>>>> calls back into the bus provider to carry out the I/O.
>>>>
>>>> This initial implementation handles the BLK interface defined by the
>>>> ACPI 6 NFIT [1] and the NVDIMM DSM Interface Example [2] composed from
>>>> DCR (dimm control region), BDW (block data window), IDT (interleave
>>>> descriptor) NFIT structures and the hardware register format.
>>>> [1]: http://www.uefi.org/sites/default/files/resources/ACPI_6.0.pdf
>>>> [2]: http://pmem.io/documents/NVDIMM_DSM_Interface_Example.pdf
>>>>
>>>> Cc: Andy Lutomirski <luto@...capital.net>
>>>> Cc: Boaz Harrosh <boaz@...xistor.com>
>>>> Cc: H. Peter Anvin <hpa@...or.com>
>>>> Cc: Jens Axboe <axboe@...com>
>>>> Cc: Ingo Molnar <mingo@...nel.org>
>>>> Cc: Christoph Hellwig <hch@....de>
>>>> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
>>>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>>>> ---
>>>>  drivers/block/nd/Kconfig          |   12 +
>>>>  drivers/block/nd/Makefile         |    3
>>>>  drivers/block/nd/acpi.c           |  422 +++++++++++++++++++++++++++++++++++--
>>>>  drivers/block/nd/acpi_nfit.h      |   47 ++++
>>>>  drivers/block/nd/blk.c            |  264 +++++++++++++++++++++++
>>>>  drivers/block/nd/libnd.h          |   11 +
>>>>  drivers/block/nd/namespace_devs.c |   47 ++++
>>>>  drivers/block/nd/nd-private.h     |    3
>>>>  drivers/block/nd/nd.h             |   16 +
>>>>  drivers/block/nd/region.c         |    8 +
>>>>  drivers/block/nd/region_devs.c    |   65 +++++-
>>>>  drivers/block/nd/test/nfit.c      |   29 +++
>>>>  drivers/block/nd/test/nfit_test.h |    2
>>>>  13 files changed, 891 insertions(+), 38 deletions(-)
>>>>  create mode 100644 drivers/block/nd/blk.c
>>>>
>>>> diff --git a/drivers/block/nd/Kconfig b/drivers/block/nd/Kconfig
>>>> index 612bf2b14283..bac4290129fc 100644
>>>> --- a/drivers/block/nd/Kconfig
>>>> +++ b/drivers/block/nd/Kconfig
>>>> @@ -95,6 +95,18 @@ config BLK_DEV_PMEM
>>>>
>>>>           Say Y if you want to use a NVDIMM described by ACPI, E820, etc...
>>>>
>>>> +config ND_BLK
>>>> +       tristate "BLK: Block data window (aperture) device support"
>>>> +       depends on LIBND
>>>> +       default ND_ACPI
>>>> +       help
>>>> +         This driver performs I/O using a set of mmio windows on a
>>>> +         dimm.  The set of apertures will all access the one DIMM.
>>>> +         Multiple windows allow multiple threads to have a different
>>>> +         portions of the dimm open at one time.
>>>> +
>>>> +         Say Y if you want to use a NVDIMM with BLK-mode capability
>>>> +
>>>
>>> This describes how it works, not what it is.  How about:
>>>
>>> This driver exposes NVDIMM BLK regions as block devices.  BLK regions
>>> are regions of NVDIMM storage that are sector-addressable, not
>>> byte-addressible, and do not support DAX.
>>
>> They *are* byte-addressable albeit through an indirection window.  The
>> indirection windows are too small for DAX to be a viable access mode.
>
> Right, I was assuming incorrectly that the sector-atomic thing was a
> necessary part of BLK, or at least of this implementation.
>
> Anyway, I think my point stands: let's describe what these drivers do
> from a user's perspective, not how they work.

Agreed.  How about:

"Support NVDIMMs, or other devices, that implement a BLK-mode access
capability.  BLK-mode access uses memory-mapped-i/o apertures to
access persistent media.

Say Y if your platform firmware emits an ACPI.NFIT table
(CONFIG_ND_ACPI), or otherwise exposes BLK-mode capabilities."
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