From 7816ebc1ddd16b5cc95febb75f778bf88411a365 Mon Sep 17 00:00:00 2001 From: Conor Dooley Date: Wed, 7 Jun 2023 21:28:29 +0100 Subject: [PATCH] RISC-V: remove decrement/increment dance in ISA string parser While expanding on the comments in the ISA string parsing code, I noticed that the conditional decrement of `isa` at the end of the loop was a bit odd. The parsing code expects that at the start of the for loop, `isa` will point to the first character of the next unparsed extension. However, depending on what the next extension is, this may not be true. Unless the next extension is a multi-letter extension preceded by an underscore, `isa` will either point to the string's null-terminator or to the first character of the next extension, once the switch statement has been evaluated. Obviously incrementing `isa` at the end of the loop could cause it to increment past the null terminator or miss a single letter extension, so `isa` is conditionally decremented, just so that the loop can increment it again. It's easier to understand the code if, instead of this decrement + increment dance, we instead use a while loop & rely on the handling of individual extension types to leave `isa` pointing to the first character of the next extension. As already mentioned, this won't be the case where the following extension is multi-letter & preceded by an underscore. To handle that, invert the check and increment rather than decrement. Hopefully this eliminates a "huh?!?" moment the next time somebody tries to understand this code. Reviewed-by: Andrew Jones Signed-off-by: Conor Dooley Reviewed-by: Sunil V L Link: https://lore.kernel.org/r/20230607-estate-left-f20faabefb89@spud Signed-off-by: Palmer Dabbelt --- arch/riscv/kernel/cpufeature.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 7dd4589..84dc44a 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -157,7 +157,7 @@ void __init riscv_fill_hwcap(void) isa += 4; bitmap_zero(this_isa, RISCV_ISA_EXT_MAX); - for (; *isa; ++isa) { + while (*isa) { const char *ext = isa++; const char *ext_end = isa; bool ext_long = false, ext_err = false; @@ -270,14 +270,12 @@ void __init riscv_fill_hwcap(void) /* * The parser expects that at the start of an iteration isa points to the - * character before the start of the next extension. This will not be the - * case if we have just parsed a single-letter extension and the next - * extension is not a multi-letter extension prefixed with an "_". It is - * also not the case at the end of the string, where it will point to the - * terminating null character. + * first character of the next extension. As we stop parsing an extension + * on meeting a non-alphanumeric character, an extra increment is needed + * where the succeeding extension is a multi-letter prefixed with an "_". */ - if (*isa != '_') - --isa; + if (*isa == '_') + ++isa; #define SET_ISA_EXT_MAP(name, bit) \ do { \ -- 2.7.4