[RFC PATCH 3/3] decompress/keepalive.h: add config option for toggling a set of bits

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

It's quite common to have an external watchdog which is serviced via
flipping a GPIO, with the value of that GPIO being settable directly
in a memory-mapped register. So add kconfig options defining the
physical address of such a register as well as a mask to have the
decompressor periodically xor into that register.

If and when other decompress_keepalive methods are added, this can be
made into a "choice DECOMPRESS_KEEPALIVE".

Since only LZ4 is wired up currently, this is "depends on KERNEL_LZ4"
for now. Also, prevent this option from being shown to the average
user.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 include/linux/decompress/keepalive.h |  8 +++++++
 init/Kconfig                         | 33 ++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/linux/decompress/keepalive.h b/include/linux/decompress/keepalive.h
index 39caa7693624..c62e49bee7cf 100644
--- a/include/linux/decompress/keepalive.h
+++ b/include/linux/decompress/keepalive.h
@@ -5,6 +5,14 @@
 
 #ifdef PREBOOT
 
+#ifdef CONFIG_DECOMPRESS_KEEPALIVE_TOGGLE
+#define decompress_keepalive() do {					\
+		long addr = CONFIG_DECOMPRESS_KEEPALIVE_TOGGLE_REG;	\
+		volatile unsigned *reg = (volatile unsigned *)addr;	\
+		*reg ^= CONFIG_DECOMPRESS_KEEPALIVE_TOGGLE_MASK;	\
+} while (0)
+#endif
+
 #endif
 
 #ifndef decompress_keepalive
diff --git a/init/Kconfig b/init/Kconfig
index b4daad2bac23..8a894d9fdd77 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -289,6 +289,39 @@ config KERNEL_UNCOMPRESSED
 
 endchoice
 
+config DECOMPRESS_KEEPALIVE_TOGGLE
+	depends on KERNEL_LZ4
+	depends on EXPERT
+	bool "Toggle some bits while decompressing"
+	help
+	  Some embedded boards have an always-running hardware
+	  watchdog with a timeout short enough that the board is reset
+	  during decompression, thus preventing the board from ever
+	  booting.
+
+	  Enable this to toggle certain bits in a memory register
+	  while decompressing the kernel. This can for example be used
+	  in the common case of an external watchdog serviced via a
+	  memory-mapped GPIO.
+
+	  Say N unless you know you need this.
+
+if DECOMPRESS_KEEPALIVE_TOGGLE
+
+config DECOMPRESS_KEEPALIVE_TOGGLE_REG
+	hex "Address of register to modify while decompressing"
+	help
+	  Set this to a physical address of a 32-bit memory word to
+	  modify while decompressing.
+
+config DECOMPRESS_KEEPALIVE_TOGGLE_MASK
+	hex "Bit mask to toggle while decompressing"
+	help
+	  The register selected above will periodically be xor'ed with
+	  this value during decompression.
+
+endif
+
 config DEFAULT_HOSTNAME
 	string "Default hostname"
 	default "(none)"
-- 
2.20.1