RISC-V: Extract multi-letter extension names from "riscv, isa"

Currently, there is no usage for version numbers in extensions as
any ratified non base ISA extension will always at v1.0.

Extract the extension names in place for future parsing.
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Reviewed-by: NAnup Patel <anup@brainfault.org>
Signed-off-by: NTsukasa OI <research_trasio@irq.a4lg.com>
[Improved commit text and comments]
Signed-off-by: NAtish Patra <atishp@rivosinc.com>
Signed-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/cpufeature.c

@@ -124,8 +124,28 @@ void __init riscv_fill_hwcap(void)
 				ext_long = true;
 				/* Multi-letter extension must be delimited */
 				for (; *isa && *isa != '_'; ++isa)
-					if (!islower(*isa) && !isdigit(*isa))
+					if (unlikely(!islower(*isa)
+						     && !isdigit(*isa)))
 						ext_err = true;
+				/* Parse backwards */
+				ext_end = isa;
+				if (unlikely(ext_err))
+					break;
+				if (!isdigit(ext_end[-1]))
+					break;
+				/* Skip the minor version */
+				while (isdigit(*--ext_end))
+					;
+				if (ext_end[0] != 'p'
+				    || !isdigit(ext_end[-1])) {
+					/* Advance it to offset the pre-decrement */
+					++ext_end;
+					break;
+				}
+				/* Skip the major version */
+				while (isdigit(*--ext_end))
+					;
+				++ext_end;
 				break;
 			default:
 				if (unlikely(!islower(*ext))) {
@@ -151,14 +171,13 @@ void __init riscv_fill_hwcap(void)
 			}
 			if (*isa != '_')
 				--isa;
-			/*
-			 * TODO: Full version-aware handling including
-			 * multi-letter extensions will be added in-future.
-			 */
-			if (ext_err || ext_long)
+
+			if (unlikely(ext_err))
 				continue;
-			this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
-			this_isa |= (1UL << (*ext - 'a'));
+			if (!ext_long) {
+				this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
+				this_isa |= (1UL << (*ext - 'a'));
+			}
 		}
 
 		/*