[PATCH v3 3/3] RISC-V: Extract multi-letter extension names from "riscv, isa"

[Tsukasa OI]

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.

Signed-off-by: Tsukasa OI <research_trasio at irq.a4lg.com>
[Improved commit text and comments]
Signed-off-by: Atish Patra <atishp at rivosinc.com>
---
 arch/riscv/kernel/cpufeature.c | 38 ++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 9d5448542226..cd9eb34f8d11 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -119,9 +119,28 @@ 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;
-				/* ... but must be ignored. */
+				/* 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))) {
@@ -131,6 +150,7 @@ void __init riscv_fill_hwcap(void)
 				/* Find next extension */
 				if (!isdigit(*isa))
 					break;
+				/* Skip the minor version */
 				while (isdigit(*++isa))
 					;
 				if (*isa != 'p')
@@ -139,20 +159,20 @@ void __init riscv_fill_hwcap(void)
 					--isa;
 					break;
 				}
+				/* Skip the major version */
 				while (isdigit(*++isa))
 					;
 				break;
 			}
 			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'));
+			}
 		}
 
 		/*
-- 
2.32.0