linux-kernel - Re: [PATCH 0/2] Add SEAMA partition types

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Date:   Tue, 9 May 2023 20:30:33 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Rafał Miłecki <zajec5@...il.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        linux-mtd@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/2] Add SEAMA partition types

On Tue, May 9, 2023 at 9:31 AM Miquel Raynal <miquel.raynal@...tlin.com> wrote:
> linus.walleij@...aro.org wrote on Sat, 06 May 2023 17:29:43 +0200:
>
> > This type of firmware partition appear in some devices in
> > NAND flash, so we need to be able to tag the partitions
> > with the appropriate type.
> >
> > The origin of the "SEAttle iMAge" is unknown.
>
> I don't see any kernel changes, why do we need an additional binding?

The bindings are not strictly bound to Linux, it's more like all OS:es
uses the Linux DT binding repo because it is the biggest project.
Also we actually merge a bunch of bindings just to describe hardware
(or things like partitions), in the hope of making use of them in the
long run.

Anyways, for the record, the full story:

Currently this binding is used in out-of-tree OpenWrt code, where it
is used as magic for splitting partitions with mtdsplit.

I guess you might be familiar with mtdsplit. It is a software partition
splitter that makes it possible to split a big partition into smaller
partitions dynamically, using magic block identifiers.

The typical usecase is to put the kernel in the first flash blocks,
then pad up to the nearest even erase block, and then add a
JFFS2 or UBI filesystem immediately there.

This way it avoids using static partitioning, the tools rebuilding the
firmware can dynamically split off more flash as the kernel image
grows.

The mtdsplit code uses different magic numbers to identify where
the different partitions start.

One such type of partition is seama, so the code needs to know
that it should look for seama magic to determine the size and
split this partition in a kernel and rootfs part. This is the code:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=tree;f=target/linux/generic/files/drivers/mtd/mtdsplit;h=3e0df856713a84b1decf17190f171cb10ce7a757;hb=HEAD

It is a bit sad that no-one has the energy to propose mtdsplit
upstream, I think it is quite generic and generally useful. I started
to make an upstream patch but got exhausted with the task.

Yours,
Linus Walleij