4KB LEB's on QSPI and ubifs

[Alison Chaiken]

I'm helping to bring up a board that contains a Micron n25q512ax3 Quad
SPI-NOR.    As discussed previously at linux-mtd,

               http://lists.infradead.org/pipermail/linux-mtd/2013-July/047665.html

ubifs won't work with the 4 kB erase sectors like those used by this
chip, as it fails with

              UBIFS error (ubi0:0 pid 3289): init_constants_early: too
small LEBs (3968 bytes), min. is 15360 bytes

We are using a 4.1 PREEMPT_RT_FULL kernel, but as far as I can tell,
the relevant code in spi-nor.c and ubifs/super.c is unchanged in
recent 4.7.    Therefore, with SECT_4K  as a configuration for this
device, ubifs still will not work.

What is the best alternative?

0. Create a UBI volume, but mount a different kind of flash filesystem
on it.   Perhaps a different filesystem is a better choice for QSPI?
ubiattach and ubimkvol work fine with the device.  Our use case
involves a set of files that will likely only ever be written at
system installation time.

1. Remove SECT_4K from the entry in spi_nor_ids[] for the device and
use ubifs.   As I understand 548cd3a "mtd: spi-nor: Add quad I/O
support for Micron SPI NOR" that added SECT_4K, this removal will make
the device slower, but not otherwise cause problems.    The device's
datasheet reads,

                – Subsector erase 4KB uniform granularity blocks
                – Sector erase 64KB uniform granularity blocks

which suggests that removing SECT_4K from spi_nor_ids[] will result in
64KB sector erase blocks rather than 4KB subsector ones that ubiattach
detects now.

2. Something else entirely?

Thanks for your suggestions,
Alison Chaiken
Peloton Technology