[PATCH v2 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 wd_limit is constant 0, the ">=" comparisons are
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

Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
---
 arch/powerpc/Kconfig     |  1 +
 arch/powerpc/lib/Kconfig |  9 +++++++++
 arch/powerpc/lib/cache.c | 14 ++++++++++++--
 3 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 arch/powerpc/lib/Kconfig

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 6a2e88fed2..133447648c 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..115ae8e160 100644
--- a/arch/powerpc/lib/cache.c
+++ b/arch/powerpc/lib/cache.c
@@ -12,6 +12,8 @@
 void flush_cache(ulong start_addr, ulong size)
 {
 	ulong addr, start, end;
+	ulong wd_limit = CONFIG_CACHE_FLUSH_WATCHDOG_THRESHOLD;
+	ulong flushed = 0;
 
 	start = start_addr & ~(CONFIG_SYS_CACHELINE_SIZE - 1);
 	end = start_addr + size - 1;
@@ -19,7 +21,11 @@ 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 += CONFIG_SYS_CACHELINE_SIZE;
+		if (flushed >= wd_limit) {
+			WATCHDOG_RESET();
+			flushed = 0;
+		}
 	}
 	/* wait for all dcbst to complete on bus */
 	asm volatile("sync" : : : "memory");
@@ -27,7 +33,11 @@ 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 += CONFIG_SYS_CACHELINE_SIZE;
+		if (flushed >= wd_limit) {
+			WATCHDOG_RESET();
+			flushed = 0;
+		}
 	}
 	asm volatile("sync" : : : "memory");
 	/* flush prefetch queue */
-- 
2.29.2