[PATCH 2/2] RISC-V: Extract base ISA from device tree

[Tsukasa OI]

This commit replaces print_isa function to extract base ISA from device
tree ("riscv,isa").  It uses a subset of Tsukasa's riscv_fill_hwcap ISA
parser.

Design choices (1):

It constructs base ISA string from the original string (does not cut
the original string like Atish's).  This is to strip version numbers
(too minor to represent as major ISA) but this behavior may be
debatable.

Design choices (2):

It skips when single-letter 'S' or 'U' is encountered.  It improves
familiarity with regular ISA string.

This commit is intended to be squashed into Atish's isa_framework_v4
PATCH 6/6.

Signed-off-by: Tsukasa OI <research_trasio at irq.a4lg.com>
---
 arch/riscv/kernel/cpu.c | 83 ++++++++++++++++++++++++++++++++++++-----
 1 file changed, 74 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index 6f9660c0a973..3f9607a66d95 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -101,17 +101,82 @@ static void print_isa_ext(struct seq_file *f)
 
 static void print_isa(struct seq_file *f, const char *isa)
 {
-	char *ext_start;
-	int isa_len = strlen(isa);
-	int base_isa_len = isa_len;
-
-	ext_start = strnchr(isa, isa_len, '_');
-	if (ext_start)
-		base_isa_len = isa_len - strlen(ext_start);
+	unsigned char parse_break = 0;
+	char base_isa[RISCV_ISA_EXT_BASE + 5];
+	char *base_isa_end = base_isa;
+	const char *isa_end = isa;
+#if IS_ENABLED(CONFIG_32BIT)
+	if (!strncmp(isa, "rv32", 4))
+		isa_end += 4;
+#elif IS_ENABLED(CONFIG_64BIT)
+	if (!strncmp(isa, "rv64", 4))
+		isa_end += 4;
+#endif
+	if (isa != isa_end) {
+		strncpy(base_isa, isa, isa_end - isa);
+		base_isa_end += isa_end - isa;
+		for (; *isa_end; ++isa_end) {
+			switch (*isa_end++) {
+			case 'x':
+			case 'z':
+				parse_break = 1;
+				break;
+			case 's':
+				if (*isa_end != 'u') {
+					parse_break = 1;
+					break;
+				}
+				/**
+				 * Workaround for invalid single-letter 's'
+				 * (QEMU).  Should break for valid ISA string.
+				 */
+				fallthrough;
+			default:
+				if (unlikely(!islower(isa_end[-1])
+				    || base_isa_end == base_isa + sizeof(base_isa))) {
+					parse_break = 2;
+					break;
+				}
+				switch (isa_end[-1]) {
+				case 's':
+				case 'u':
+					break;
+				default:
+					*base_isa_end++ = isa_end[-1];
+				}
+				/* Skip version number */
+				if (!isdigit(*isa_end))
+					break;
+				while (isdigit(*++isa_end))
+					;
+				if (*isa_end != 'p')
+					break;
+				if (!isdigit(*++isa_end)) {
+					--isa_end;
+					break;
+				}
+				while (isdigit(*++isa_end))
+					;
+				break;
+			}
+			if (*isa_end != '_')
+				--isa_end;
+			if (parse_break)
+				break;
+		}
+	}
 
-	/* Print only the base ISA as it is */
+	/**
+	 * Print only the base ISA
+	 * (original ISA string as is if an error is encountered)
+	 */
 	seq_puts(f, "isa\t\t: ");
-	seq_write(f, isa, base_isa_len);
+	if (parse_break < 2) {
+		seq_write(f, base_isa, base_isa_end - base_isa);
+	} else {
+		isa_end += strlen(isa_end);
+		seq_write(f, isa, isa_end - isa);
+	}
 	seq_puts(f, "\n");
 }
 
-- 
2.32.0