[U-Boot] [PATCH 2/2] vexpress64: juno: add default NOR flash boot

[Linus Walleij]

On Thu, Feb 19, 2015 at 5:50 PM, Tom Rini <trini at ti.com> wrote:
> On Thu, Feb 19, 2015 at 05:19:54PM +0100, Linus Walleij wrote:
>
>> This modifies the vexpress64 Juno configuration so that
>> it will by default load and boot a kernel and a device tree
>> from the pre-defined location in the NOR flash. When we are
>> at it, also define the proper command line for the Juno and
>> indicate that the USB stick (/dev/sda1) is the default
>> root file system.
>>
>> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
>
> Is there a reason to not enable mtdparts and define + pass in the
> partition map on NOR?

I've looked closer at this, and the ARM reference designs actually
have a special "partition" format and does not rely on fixed
offsets that e.g the bootloader hardcodes and pass to the
kernel.

The format is implemented in the Linux kernel in
drivers/mtd/afs.c

I also made a stand-alone implementation at one point
to create these flash images:
http://www.df.lth.se/~triad/krad/integrator/arm-header.c

It involves scanning over the eraseblocks of the flash, reading
out the last 20 bytes of each, scanning for a magic
number indicating a footer. So this footer is added to
the last erase block for the "partition", not preceding
of the flash image, instead following it. Meaning XIP &c
will still work fine.

By scanning all erase blocks, all images in the flash can
be identified.

The proper implementation for U-Boot interaction with
the flash would be to copy over/implement this as AFS
partitions instead of MTD partitions, and select that
exclusively for the ARM Reference designs.

This image format has been used by ARM reference since
the first ARM Integrator in ~2001 and is still used in the
latest Juno design from last year, so it's likely not going
away.

Shall I just go ahead and implement this for U-boot?

It can be incremental on top of this patch.

I don't know if it even needs the MTD layer. Possibly
this "partition" type lives better on top of the raw flash
info, as a separate cmd file to read/write/load such
images.

Yours,
Linus Walleij