mmap support for m25p80 device

[Brian Norris]

+ Philippe

On Tue, Nov 17, 2015 at 06:15:03PM -0800, Brian Norris wrote:
> + Vignesh
> 
> Vignesh is working on supporting mmap'd flash reads in the SPI core,
> particularly for the TI QSPI driver. I don't believe he's planning on
> exposing this to userspace, though, and I believe that might be pretty
> difficult to do now.
> 
> Perhaps Vignesh can comment.
> 
> On Tue, Oct 20, 2015 at 04:31:20PM +0200, Simon Falsig wrote:
> > Hi,
> > 
> > I'm currently working on a new custom board (with a TI AM3356 ARM-Cortex
> > A8
> > CPU), with a 32 kB Everspin MR25H256 MRAM chip, attached over SPI. It
> > works
> > fine using the m25p80 driver, but I was wondering how complex it would be
> > to
> > add the possibility of memory-mapping the device in userspace? - mainly to
> > make the interface consistent with the board that it is replacing, which
> > uses
> > a different, mmap-able, RAM chip.
> > 
> > I'm not very experienced in the deeper aspects of the kernel, but I've
> > been
> > poking around a bit in the mtd subsystem, and it seems as if the main
> > thing
> > that is missing, is a valid get_unmapped_area() function for the m25p80
> > driver, and then to change the mtdchar_mmap() function (in mtdchar.c) to
> > actually allow mmap'ing on MMU systems.
> 
> It's not quite so simple. Read the comments in mtdchar, and you'll see
> that there's some layering bugs that caused us to disable MMU mmap
> entirely. Apparently no one cared so far. Read the comments here:
> 
> commit f5cf8f07423b2677cebebcebc863af77223a4972
> Author: David Woodhouse <David.Woodhouse at intel.com>
> Date:   Tue Oct 9 15:08:10 2012 +0100
> 
>     mtd: Disable mtdchar mmap on MMU systems
> 
> But feel free to fix it.

I see there's an old patch that never got reviewed/tested, for doing
this:

http://patchwork.ozlabs.org/patch/327981/

You could check that out.

Brian

> > But - does it even make sense to create such a function for m25p80? - and
> > how would I start?
> 
> SPI drivers don't really expose a mmap-able interface, so this isn't
> possible at the moment. Even once it can be done, it would be restricted
> only to those controllers that can do memory map for you. AIUI, your SoC
> might (?).
> 
> But even if it supports it, I expect you'll have difficulty coordinating
> this properly, since the SPI bus technically can be shared with multiple
> devices, whereas mmap would kind of assume that user-space can access
> the flash at any time. So I guess m25p80 would have to grab exclusive
> access of the entire SPI bus? If your system design can handle that,
> then I guess it CAN be done...
> 
> ...but why do you want to do this, again?
> 
> Brian
> 
> > Any pointers and/or comments are appreciated!
> > Thanks and best regards,
> > Simon Falsig
> > simon at newtec.dk
> > 
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