[RFC PATCH 0/3] watchdog servicing during decompression

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

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

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2.20.1