Re: [PATCH] mtd: parsers: Support '[]' for id in mtdparts

[ron minnich]

On Thu, Apr 16, 2020 at 2:51 AM Linus Walleij <linus.walleij@xxxxxxxxxx> 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