Re: [RFC PATCH 0/3] watchdog servicing during decompression

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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.

On Thu, Oct 17, 2019 at 01:49:03PM +0200, Rasmus Villemoes wrote:
> Many custom boards have an always-running external watchdog
> circuit. When the timeout of that watchdog is small, one cannot boot a
> compressed kernel since the board gets reset before it even starts
> booting the kernel proper.
> 
> One way around that is to do the decompression in a bootloader which
> knows how to service the watchdog. However, one reason to prefer using
> the kernel's own decompressor is to be able to take advantage of
> future compression enhancements (say, a faster implementation of the
> current method, or switching over when a new method such a zstd is
> invented) - often, the bootloader cannot be updated without physical
> access or is locked down for other reasons, so the decompressor has to
> be bundled with the kernel image for that to be possible.
> 
> This POC adds a linux/decompress/keepalive.h header which provides a
> decompress_keepalive() macro. Wiring up any given decompressor just
> amounts to including that header and adding decompress_keepalive() in
> the main loop - for simplicity, this series just does it for lz4.
> 
> The actual decompress_keepalive() implementation is of course very
> board-specific. The third patch adds a kconfig knob that handles a
> common case (and in fact suffices for all the various boards I've come
> across): An external watchdog serviced by toggling a gpio, with the
> value of that gpio being settable in a memory-mapped register.
> 
> Rasmus Villemoes (3):
>   decompress/keepalive.h: prepare for watchdog keepalive during kernel
>     decompression
>   lib: lz4: wire up watchdog keepalive during decompression
>   decompress/keepalive.h: add config option for toggling a set of bits
> 
>  include/linux/decompress/keepalive.h | 22 +++++++++++++++++++
>  init/Kconfig                         | 33 ++++++++++++++++++++++++++++
>  lib/lz4/lz4_decompress.c             |  2 ++
>  3 files changed, 57 insertions(+)
>  create mode 100644 include/linux/decompress/keepalive.h
> 
> -- 
> 2.20.1
> 
> 
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