Unverified Commit 7816ebc1 authored by Conor Dooley's avatar Conor Dooley Committed by Palmer Dabbelt
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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: default avatarAndrew Jones <ajones@ventanamicro.com>
Signed-off-by: default avatarConor Dooley <conor.dooley@microchip.com>
Reviewed-by: default avatarSunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20230607-estate-left-f20faabefb89@spud


Signed-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
parent 6b913e3d

arch/riscv/kernel/cpufeature.c

+6 −8
Original line number Diff line number Diff line
@@ -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 {								\