PMEM, DAX, MTD, and the quest for the Theory of Everything

[Wilcox, Matthew R]

No, we haven't been considering the MTD code at all with DAX.  We leapt onto the S/390 filemap_xip code, and took it for a wild ride.  I did look at the MTD code a bit, but that was two or more years ago.  I vaguely remember thinking that it could be made to support DAX, but it really wasn't (and isn't) my focus.

I don't think I'd be terribly upset by the appearance of patches which made DAX sit on top of an MTD device instead of a block device, but I don't have a good feel for where embedded systems are these days.  Is it still worth configuring out the block subsystem?

In your last sentence you talk about 'EXT4 on ROM/NOR devices' -- have you divorced ext4 from the block layer?  Or are you talking about ext4 on mtdblock?

-----Original Message-----
From: Jared Hulbert [mailto:jaredeh at gmail.com] 
Sent: Monday, January 25, 2016 5:10 PM
To: linux-mtd; David Woodhouse; Wilcox, Matthew R; ross.zwisler at linux.intel.com; Chris Brandt
Subject: PMEM, DAX, MTD, and the quest for the Theory of Everything

I'm not seeing any traffic connecting MTD, PMEM, and DAX.  Any
discussions I should be aware of?

MTD devices like SPI-NOR and the inevitable spillover into the
embedded market of 3D Xpoint and similar technologies are excluded
from the DAX game.  Considering all the years we spent playing with
XIP in MTD...   well that's what happens when you disengage for
several years.

A few thoughts...

#1 - MTD provides various ways to partition memory semantic devices as
reprogramable ROMs for storage of bootloaders, kernel, rootfs images,
etc.  Does PMEM have anything to learn from this?  Or does the MTD
layer have an opportunity to gain something by using PMEM standards?

#2 - MTD has had an API that allowed direct access (see point() in
struct mtd_info) and block-ish semantic access of persistent memory
devices since, I dunno, before bitkeeper?  Can we hook this up with
DAX?  Read Only should be "trivial" especially if someone else does
it. :)

#3 - There has long been a desire to merge MTD with the block
subsystem with some members of the MTD community.  PMEM seems to
create a precedent for a block interface on a device that isn't
fundamentally block based.  Also the direct_access() in block has been
legitimized with the recent DAX stuff.   Is it time now?

My vision here is to enable Read-Only DAX images of EXT4, AXFS, PRAMFS
on ROM/NOR devices in a standard DAX way.

Thoughts?