linux-kernel - Re: [PATCH 02/15] libnvdimm: infrastructure for btt devices

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Date:	Tue, 23 Jun 2015 08:19:57 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
	Neil Brown <neilb@...e.de>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 02/15] libnvdimm: infrastructure for btt devices

On Tue, Jun 23, 2015 at 3:19 AM, Christoph Hellwig <hch@....de> wrote:
> On Mon, Jun 22, 2015 at 12:02:54PM -0700, Dan Williams wrote:
>> I don't see the need to re-invent partitioning which is the path this
>> requested rework is putting us on...
>>
>> However, when the need arises for smaller granularity BTT we can have
>> the partition fight then.  To be clear, I believe that need is already
>> here today, but I'm not in a position to push that agenda at this late
>> date.
>
>
> Instead of all this complaining and moaning let's figure out what
> architecture you'd actually want.  The one I had in mind is:
>
> +------------------------------+
> |  block layer (& partitions)  |
> +---------------+--------------+--------------------+
> |  pmem driver  |  btt driver  |  other consumers   |
> +---------------+--------------+--------------------+
> |        pmem API through libnvdimm                 |
> +---------------------------------------------------+
>

I've got this mostly coded up.  The nice property is that BTTs now
become another flavor of the same namespace.

> If you really want btt to stack on top of pmem it really
> needs to be moved out entirely of libnvdimm and be a
> generic block driver just using ->rw_bytes, e.g.:
>
>
> +------------------------------+
> |  btt driver                  |
> +------------------------------+
> |  block layer (& partitions)  |
> +------------------------------+--------------------+
> |  pmem driver                 | other consumers    |
> +------------------------------+--------------------+
> |        pmem API through libnvdimm                 |
> +---------------------------------------------------+
>
> Not the current mess where btt pretends to be a stacking block
> driver but still ties into libnvdimm.

That tie was only to enable autodetect so that we don't need to run a
BTT assembly step from an initramfs just to get an NVDIMM up and
running.  It was a convenience, not a requirement.

> Add blk mode access to all the schemes, but it's really just
> another next to the pmem driver each time.  In fact while
> looking over the code a bit more I start to wonder why
> we need the blk driver at all - just hook into the nfit
> do_io routines instead of the low-level API based on what
> libnvdimm provides, and don't offer DAX for it.  It mostly
> seems duplicate code.

Mostly, it does handle dis-contiguous dimm-physical-address ranges,
but you're right we might be able to unify it in the coming cycle.
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