RISC-V: Minimal parser for "riscv, isa" strings

Current hart ISA ("riscv,isa") parser don't correctly parse:

1. Multi-letter extensions
2. Version numbers

All ISA extensions ratified recently has multi-letter extensions
(except 'H'). The current "riscv,isa" parser that is easily confused
by multi-letter extensions and "p" in version numbers can be a huge
problem for adding new extensions through the device tree.

Leaving it would create incompatible hacks and would make "riscv,isa"
value unreliable.

This commit implements minimal parser for "riscv,isa" strings.  With this,
we can safely ignore multi-letter extensions and version numbers.

[Improved commit text and fixed a bug around 's' in base extension]
Signed-off-by: Atish Patra <atishp@rivosinc.com>
[Fixed workaround for QEMU]
Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/cpufeature.c

@@ -7,6 +7,7 @@
  */
 
 #include <linux/bitmap.h>
+#include <linux/ctype.h>
 #include <linux/of.h>
 #include <asm/processor.h>
 #include <asm/hwcap.h>
@@ -66,7 +67,7 @@ void __init riscv_fill_hwcap(void)
 	struct device_node *node;
 	const char *isa;
 	char print_str[NUM_ALPHA_EXTS + 1];
-	size_t i, j, isa_len;
+	int i, j;
 	static unsigned long isa2hwcap[256] = {0};
 
 	isa2hwcap['i'] = isa2hwcap['I'] = COMPAT_HWCAP_ISA_I;
@@ -92,23 +93,72 @@ void __init riscv_fill_hwcap(void)
 			continue;
 		}
 
-		i = 0;
-		isa_len = strlen(isa);
 #if IS_ENABLED(CONFIG_32BIT)
 		if (!strncmp(isa, "rv32", 4))
-			i += 4;
+			isa += 4;
 #elif IS_ENABLED(CONFIG_64BIT)
 		if (!strncmp(isa, "rv64", 4))
-			i += 4;
+			isa += 4;
 #endif
-		for (; i < isa_len; ++i) {
-			this_hwcap |= isa2hwcap[(unsigned char)(isa[i])];
+		for (; *isa; ++isa) {
+			const char *ext = isa++;
+			const char *ext_end = isa;
+			bool ext_long = false, ext_err = false;
+
+			switch (*ext) {
+			case 's':
+				/**
+				 * Workaround for invalid single-letter 's' & 'u'(QEMU).
+				 * No need to set the bit in riscv_isa as 's' & 'u' are
+				 * not valid ISA extensions. It works until multi-letter
+				 * extension starting with "Su" appears.
+				 */
+				if (ext[-1] != '_' && ext[1] == 'u') {
+					++isa;
+					ext_err = true;
+					break;
+				}
+				fallthrough;
+			case 'x':
+			case 'z':
+				ext_long = true;
+				/* Multi-letter extension must be delimited */
+				for (; *isa && *isa != '_'; ++isa)
+					if (!islower(*isa) && !isdigit(*isa))
+						ext_err = true;
+				break;
+			default:
+				if (unlikely(!islower(*ext))) {
+					ext_err = true;
+					break;
+				}
+				/* Find next extension */
+				if (!isdigit(*isa))
+					break;
+				/* Skip the minor version */
+				while (isdigit(*++isa))
+					;
+				if (*isa != 'p')
+					break;
+				if (!isdigit(*++isa)) {
+					--isa;
+					break;
+				}
+				/* Skip the major version */
+				while (isdigit(*++isa))
+					;
+				break;
+			}
+			if (*isa != '_')
+				--isa;
 			/*
-			 * TODO: X, Y and Z extension parsing for Host ISA
-			 * bitmap will be added in-future.
+			 * TODO: Full version-aware handling including
+			 * multi-letter extensions will be added in-future.
 			 */
-			if ('a' <= isa[i] && isa[i] < 'x')
-				this_isa |= (1UL << (isa[i] - 'a'));
+			if (ext_err || ext_long)
+				continue;
+			this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
+			this_isa |= (1UL << (*ext - 'a'));
 		}
 
 		/*