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From:   Paul Burton <paul.burton@mips.com>
To:     George Hilliard <thirtythreeforty@gmail.com>
CC:     Aaro Koskinen <aaro.koskinen@iki.fi>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RESEND PATCH] mips: ralink: allow zboot
Thread-Topic: [RESEND PATCH] mips: ralink: allow zboot
Thread-Index: AQHU4AgLQNJaCLPr+E2sTRrwfQ+CsqYWo1mAgAATLgCAAAhLgA==
Date:   Thu, 21 Mar 2019 23:40:20 +0000
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Hi George,

On Thu, Mar 21, 2019 at 05:10:38PM -0600, George Hilliard wrote:
> My version of U-Boot complains if I compile out LZMA support:
> ---
> =3D> tftpboot 0x81000000 uImage; tftpboot 0x84000000 hawkeye.dtb; bootm
> 0x81000000 - 0x84000000
> (snip)
> Bytes transferred =3D 7349 (1cb5 hex)
> ## Booting kernel from Legacy Image at 81000000 ...
>    Image Name:   Linux-5.1.0-rc1
>    Image Type:   MIPS Linux Kernel Image (lzma compressed)
>    Data Size:    1465871 Bytes =3D 1.4 MiB
>    Load Address: 80000000
>    Entry Point:  80344338
>    Verifying Checksum ... OK
> ## Flattened Device Tree blob at 84000000
>    Booting using the fdt blob at 0x84000000
>    Uncompressing Kernel Image ... Unimplemented compression type 3
> exit not allowed from main input shell.
> =3D>
> ---

There you're using a uImage though, which is different to
CONFIG_SYS_SUPPORTS_ZBOOT. Even without CONFIG_SYS_SUPPORTS_ZBOOT
enabled, you can build either a compressed or uncompressed uImage.

Some platforms enable one of those targets by default but it doesn't
appear that ralink does, so I guess you're specifying a uImage.lzma
target when building the kernel? Try doing that when
CONFIG_SYS_SUPPORTS_ZBOOT isn't enabled - it'll still generate the same
type of LZMA-compressed uImage.

What CONFIG_SYS_SUPPORTS_ZBOOT does is produce a program (vmlinuz) which
contains a compressed version of vmlinux.bin along with some code to
decompress it. The decompression is performed by vmlinuz itself, not by
U-Boot.

A uImage in contrast is just the compressed (or uncompressed)
vmlinux.bin with a header prepended that U-Boot uses to understand what
it's loading. If the header says the data is compressed with LZMA then
U-Boot will need to support LZMA decompression.

What U-Boot is complaining about there is that you gave it a uImage
that's LZMA compressed & your build of U-Boot itself doesn't support
LZMA. That has nothing to do with CONFIG_SYS_SUPPORTS_ZBOOT or whether
the kernel binary itself supports any particular compression algorithm -
it's purely down to the configuration of your copy of U-Boot.

Options would be to rebuild U-Boot with LZMA support enabled, or to use
a different compression for your uImage that U-Boot already supports.
eg. you could specify that you want to use gzip:

  $ make ARCH=3Dmips uImage.gz

That would give you an image that will work so long as U-Boot supports
gzip decompression, or:

  $ make ARCH=3Dmips uImage.bin

That would give you an uncompressed uImage which U-Boot always supports.

Now it could still be valid to select CONFIG_SYS_SUPPORTS_ZBOOT if you
had a need for a better compression algorithm & couldn't update your
bootloader to support it, but that would be solving a different problem
than your commit message claims & it wouldn't matter at all whether the
bootloader supports a particular compression algorithm.

Thanks,
    Paul