[PATCH v3 2/2] powerpc: introduce CONFIG_CACHE_FLUSH_WATCHDOG_THRESHOLD

[Rasmus Villemoes]

When flush_cache() is called during boot on our ~7M kernel image, the
hundreds of thousands of WATCHDOG_RESET calls end up adding
significantly to boottime. Flushing a single cache line doesn't take
many microseconds, so doing these calls for every cache line is
complete overkill.

The generic watchdog_reset() provided by wdt-uclass.c actually
contains some rate-limiting logic that should in theory mitigate this,
but alas, that rate-limiting must be disabled on powerpc because of
its get_timer() implementation - get_timer() works just fine until
interrupts are disabled, but it just so happens that the "big"
flush_cache() call happens in the part of bootm where interrupts are
indeed disabled. [1] [2] [3]

I have checked with objdump that the generated code doesn't change
when this option is left at its default value of 0: gcc is smart
enough to see that the ">=" comparison is tautologically true, hence
all assignments to "flushed" are eliminated as dead stores.

On our board, setting the option to something like 65536 ends up
reducing total boottime by about 0.8 seconds.

[1] https://patchwork.ozlabs.org/project/uboot/patch/20200605111657.28773-1-rasmus.villemoes@prevas.dk/
[2] https://lists.denx.de/pipermail/u-boot/2021-April/446906.html
[3] https://lists.denx.de/pipermail/u-boot/2021-April/447280.html

Reviewed-by: Stefan Roese <sr at denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
---
 arch/powerpc/Kconfig     |  1 +
 arch/powerpc/lib/Kconfig |  9 +++++++++
 arch/powerpc/lib/cache.c | 15 +++++++++++++--
 3 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 arch/powerpc/lib/Kconfig

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c2c577f60c..1829de9e2c 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -49,5 +49,6 @@ source "arch/powerpc/cpu/mpc83xx/Kconfig"
 source "arch/powerpc/cpu/mpc85xx/Kconfig"
 source "arch/powerpc/cpu/mpc86xx/Kconfig"
 source "arch/powerpc/cpu/mpc8xx/Kconfig"
+source "arch/powerpc/lib/Kconfig"
 
 endmenu
diff --git a/arch/powerpc/lib/Kconfig b/arch/powerpc/lib/Kconfig
new file mode 100644
index 0000000000..b30b5edf7c
--- /dev/null
+++ b/arch/powerpc/lib/Kconfig
@@ -0,0 +1,9 @@
+config CACHE_FLUSH_WATCHDOG_THRESHOLD
+	int "Bytes to flush between WATCHDOG_RESET calls"
+	default 0
+	help
+	  The flush_cache() function periodically, and by default for
+	  every cache line, calls WATCHDOG_RESET(). When flushing a
+	  large area, that may add a significant amount of
+	  overhead. This option allows you to set a threshold for how
+	  many bytes to flush between each WATCHDOG_RESET call.
diff --git a/arch/powerpc/lib/cache.c b/arch/powerpc/lib/cache.c
index 3e487f50fe..19162511ce 100644
--- a/arch/powerpc/lib/cache.c
+++ b/arch/powerpc/lib/cache.c
@@ -9,9 +9,20 @@
 #include <asm/cache.h>
 #include <watchdog.h>
 
+static ulong maybe_watchdog_reset(ulong flushed)
+{
+	flushed += CONFIG_SYS_CACHELINE_SIZE;
+	if (flushed >= CONFIG_CACHE_FLUSH_WATCHDOG_THRESHOLD) {
+		WATCHDOG_RESET();
+		flushed = 0;
+	}
+	return flushed;
+}
+
 void flush_cache(ulong start_addr, ulong size)
 {
 	ulong addr, start, end;
+	ulong flushed = 0;
 
 	start = start_addr & ~(CONFIG_SYS_CACHELINE_SIZE - 1);
 	end = start_addr + size - 1;
@@ -19,7 +30,7 @@ void flush_cache(ulong start_addr, ulong size)
 	for (addr = start; (addr <= end) && (addr >= start);
 			addr += CONFIG_SYS_CACHELINE_SIZE) {
 		asm volatile("dcbst 0,%0" : : "r" (addr) : "memory");
-		WATCHDOG_RESET();
+		flushed = maybe_watchdog_reset(flushed);
 	}
 	/* wait for all dcbst to complete on bus */
 	asm volatile("sync" : : : "memory");
@@ -27,7 +38,7 @@ void flush_cache(ulong start_addr, ulong size)
 	for (addr = start; (addr <= end) && (addr >= start);
 			addr += CONFIG_SYS_CACHELINE_SIZE) {
 		asm volatile("icbi 0,%0" : : "r" (addr) : "memory");
-		WATCHDOG_RESET();
+		flushed = maybe_watchdog_reset(flushed);
 	}
 	asm volatile("sync" : : : "memory");
 	/* flush prefetch queue */
-- 
2.29.2