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From:   ron minnich <rminnich@gmail.com>
Date:   Thu, 16 Apr 2020 07:55:01 -0700
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Subject: Re: [PATCH] mtd: parsers: Support '[]' for id in mtdparts
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     =?UTF-8?Q?Miqu=C3=A8l_Raynal?= <miquel.raynal@bootlin.com>,
        Ronald Minnich <rminnich@google.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh R <vigneshr@ti.com>, linux-mtd@lists.infradead.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Apr 16, 2020 at 2:51 AM Linus Walleij <linus.walleij@linaro.org> wrote:

> I suppose the use case is using PCI-based MTD devices for testing
> something android images on desktops? I'm surprised it didn't
> come up earlier.

Thanks. In this case it's for systems that companies are deploying
into their data centers, using linuxboot (linuxboot.org) and Intel
chipsets. On Intel  chipsets, there is a 64 MiB SPI part, but only 16
MiB is directly addressable.

Linux goes in the memory-addressable part of the SPI, and UEFI loads
it into RAM, since to UEFI the kernel is just another UEFI driver --
in fact in most cases we replace the UEFI shell with Linux.

But we need a file system, and with the huge amount of drivers that
come with UEFI there's not much room in the top 16M. (we're working to
fix that glitch, a process we call DXE-ectomy, but it takes time).

We wish to place a file system in the low 48 MiB -- lots of room there.

So what one can do is put a squashfs-formatted file system in that low
part of SPI, and, using this mtdparts capability, point the kernel at
it ("root=/dev/mtd1 mtdparts=[a:b.c]etc.etc"). It's a lifesaver for
those of us using u-root for our userland.

ron