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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@denx.de> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
10 lines
402 B
Plaintext
10 lines
402 B
Plaintext
config CACHE_FLUSH_WATCHDOG_THRESHOLD
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int "Bytes to flush between WATCHDOG_RESET calls"
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default 0
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help
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The flush_cache() function periodically, and by default for
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every cache line, calls WATCHDOG_RESET(). When flushing a
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large area, that may add a significant amount of
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overhead. This option allows you to set a threshold for how
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many bytes to flush between each WATCHDOG_RESET call.
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