[riscv] Add support for detecting T-Head vendor extensions

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/core/xthead.c

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2025 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** @file
+ *
+ * T-Head vendor extensions
+ *
+ */
+
+#include <ipxe/sbi.h>
+#include <ipxe/xthead.h>
+
+/** Colour for debug messages */
+#define colour THEAD_MVENDORID
+
+/**
+ * Check for a T-Head feature via SXSTATUS register
+ *
+ * @v feature		Feature bit
+ * @ret supported	Feature is supported
+ */
+int xthead_supported ( unsigned long feature ) {
+	struct sbi_return ret;
+	unsigned long sxstatus;
+
+	/* Check for a T-Head CPU */
+	ret = sbi_ecall_0 ( SBI_BASE, SBI_BASE_MVENDORID );
+	if ( ret.error )
+		return 0;
+	if ( ret.value != THEAD_MVENDORID ) {
+		DBGC ( colour, "THEAD vendor ID mismatch: %#08lx\n",
+		       ret.value );
+		return 0;
+	}
+
+	/* Read SXSTATUS CSR */
+	__asm__ ( "csrr %0, %1"
+		  : "=r" ( sxstatus ) : "i" ( THEAD_SXSTATUS ) );
+	DBGC ( colour, "THEAD sxstatus %#08lx\n", sxstatus );
+
+	/* Check feature bit */
+	return ( !! ( sxstatus & feature ) );
+}

src/arch/riscv/include/ipxe/sbi.h

@@ -193,6 +193,10 @@ sbi_legacy_ecall_1 ( int fid, unsigned long p0 ) {
 #define SBI_LEGACY_GETCHAR 0x02		/**< Console Get Character */
 #define SBI_LEGACY_SHUTDOWN 0x08	/**< System Shutdown */
 
+/** Base extension */
+#define SBI_BASE 0x10
+#define SBI_BASE_MVENDORID 0x04		/**< Get machine vendor ID */
+
 /** System reset extension */
 #define SBI_SRST SBI_EID ( 'S', 'R', 'S', 'T' )
 #define SBI_SRST_SYSTEM_RESET 0x00	/**< Reset system */

src/arch/riscv/include/ipxe/xthead.h

@@ -0,0 +1,21 @@
+#ifndef _IPXE_XTHEAD_H
+#define _IPXE_XTHEAD_H
+
+/** @file
+ *
+ * T-Head vendor extensions
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** T-Head machine vendor ID */
+#define THEAD_MVENDORID 0x5b7
+
+/** T-Head SXSTATUS CSR */
+#define THEAD_SXSTATUS 0x5c0
+#define THEAD_SXSTATUS_THEADISAEE 0x00400000	/**< General ISA extensions */
+
+extern int xthead_supported ( unsigned long feature );
+
+#endif /* _IPXE_XTHEAD_H */