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

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Date:	Tue, 28 Apr 2015 15:30:19 -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 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.
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