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Date: Fri, 22 Sep 2017 22:04:55 +0200
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Chris Packham <Chris.Packham@alliedtelesis.co.nz>
Cc: Mathias Thore <Mathias.Thore@infinera.com>,
 "linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>
Subject: Re: Regression for NOR flash with multiple erase block regions
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On Fri, 22 Sep 2017 18:27:42 +0000
Chris Packham <Chris.Packham@alliedtelesis.co.nz> wrote:

> Hi Mathias,
> 
> On 23/09/17 01:12, Mathias Thore wrote:
> > Hello,
> > 
> > Commit 1eeef2d7483a7e3f8d2dd2a5b9939b3b814dc549 included in Linux 4.13 ( 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/mtd/mtdpart.c?h=v4.13&id=1eeef2d7483a7e3f8d2dd2a5b9939b3b814dc549 
> > ) introduces a regression for NOR flash with multiple erase block 
> > regions of different sizes.
> > 
> > Only the largest erase block size seems to be considered when 
> > determining if partitions are aligned. Partitions in smaller regions 
> > will be mounted as read-only. With Linux 4.12 and earlier, read/write 
> > access was available for these partitions.

I don't understand how this could work before this patch? I mean, we
were previously using mtd_mod_by_eb() to check part alignment and
this functions is just returning the remainder of the off / erasesize
division. So, assuming the erasesize of your NOR did not change
between 4.12 and 4.13, I don't see how this commit could cause the
regression you're describing here.

Maybe MTD_NO_ERASE is set on your NOR, and ->writesize is used in place
of ->erasesize to check the alignment, but ->writesize is normally set
to 1 on NOR devices, so again, no real reasons for this failure.

> 
> Sorry about that. I think a fix would be to re-calculate the 
> wr_alignment as we're looking at each erase block. Unfortunately I'm 
> about to get on a plane for 13 hours so I'm pretty much a write off for 
> the next couple of days.
> 
> I'll take a look when I get back on-line unless Boris beats me to it.