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Subject: Re: [RFC PATCH 0/3] watchdog servicing during decompression
To:     Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc:     Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Gao Xiang <xiang@kernel.org>, kernel@pengutronix.de,
        Linus Walleij <linus.walleij@linaro.org>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
References: <20191017114906.30302-1-linux@rasmusvillemoes.dk>
 <20191017120310.GD25745@shell.armlinux.org.uk>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <c4b6805b-67fe-6bce-1777-2d81e96b4ac9@rasmusvillemoes.dk>
Date:   Thu, 17 Oct 2019 14:34:52 +0200
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On 17/10/2019 14.03, Russell King - ARM Linux admin wrote:
> We used to have this on ARM - it was called from the decompressor
> code via an arch_decomp_wdog() hook.
> 
> That code got removed because it is entirely unsuitable for a multi-
> platform kernel.  This looks like it takes an address for the watchdog
> from the Kconfig, and builds that into the decompressor, making the
> decompressor specific to that board or platform.
> 
> I'm not sure distros are going to like that given where we are with
> multiplatform kernels.

This is definitely not for multiplatform kernels or general distros,
it's for kernels that are built as part of a BSP for a specific board -
hence the "Say N unless you know you need this.".

I didn't know it used to exist. But I do know that something like this
is carried out-of-tree for lots of boards, so I thought I'd try to get
upstream support.

The first two patches, or something like them, would be nice on their
own, as that would minimize the conflicts when forward-porting the
board-specific patch. But such a half-implemented feature that requires
out-of-tree patches to actually do anything useful of course won't fly.

I'm not really a big fan of the third patch, even though it does work
for all the cases I've encountered so far - I'm sure there would be
boards where a much more complicated solution would be needed. Another
method I thought of was to just supply a __weak no-op
decompress_keepalive(), and then have a config option to point at an
extra object file to link into the compressed/vmlinux (similar to the
initramfs_source option that also points to some external resource).

But if the mainline kernel doesn't want anything like this
re-introduced, that's also fine, that just means a bit of job security.

Rasmus