[PATCH 1/3] arm: smh: specify Thumb trap instruction

[Andre Przywara]

The ARM semihosting interface uses different trap instructions for
different architectures and instruction sets. So far we were using
AArch64 and ARMv7-M, and had an untested v7-A entry. The latter does
not work when building for Thumb, as can be verified by using
qemu_arm_defconfig, then enabling SEMIHOSTING and SYS_THUMB_BUILD:
==========
{standard input}:35: Error: invalid swi expression
{standard input}:35: Error: value of 1193046 too large for field of 2 bytes at 0
==========

Fix this by providing the recommended instruction[1] for Thumb, and
using the ARM instruction only when not building for Thumb. This also
removes some comment, as QEMU for ARM allows to now test this case.
Also use the opportunity to clean up the inline assembly, and just define
the actual trap instruction inside #ifdef's, to improve readability.

[1] https://developer.arm.com/documentation/dui0471/g/Semihosting/The-semihosting-interface?lang=en

Signed-off-by: Andre Przywara <andre.przywara at arm.com>
---
 arch/arm/lib/semihosting.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/arm/lib/semihosting.c b/arch/arm/lib/semihosting.c
index 01d652a6b83..acc6b1be4f3 100644
--- a/arch/arm/lib/semihosting.c
+++ b/arch/arm/lib/semihosting.c
@@ -4,11 +4,6 @@
  * Copyright 2014 Broadcom Corporation
  */
 
-/*
- * This code has been tested on arm64/aarch64 fastmodel only.  An untested
- * placeholder exists for armv7 architectures, but since they are commonly
- * available in silicon now, fastmodel usage makes less sense for them.
- */
 #include <common.h>
 #include <log.h>
 #include <semihosting.h>
@@ -25,20 +20,28 @@
 #define SYSFLEN		0x0C
 #define SYSERRNO	0x13
 
+#if defined(CONFIG_ARM64)
+	#define SMH_TRAP "hlt #0xf000"
+#elif defined(CONFIG_CPU_V7M)
+	#define SMH_TRAP "bkpt #0xAB"
+#elif defined(CONFIG_SYS_THUMB_BUILD)
+	#define SMH_TRAP "svc #0xab"
+#else
+	#define SMH_TRAP "svc #0x123456"
+#endif
+
 /*
  * Call the handler
  */
 static noinline long smh_trap(unsigned int sysnum, void *addr)
 {
 	register long result asm("r0");
-#if defined(CONFIG_ARM64)
-	asm volatile ("hlt #0xf000" : "=r" (result) : "0"(sysnum), "r"(addr) : "memory");
-#elif defined(CONFIG_CPU_V7M)
-	asm volatile ("bkpt #0xAB" : "=r" (result) : "0"(sysnum), "r"(addr) : "memory");
-#else
-	/* Note - untested placeholder */
-	asm volatile ("svc #0x123456" : "=r" (result) : "0"(sysnum), "r"(addr) : "memory");
-#endif
+
+	asm volatile (SMH_TRAP
+		      : "=r" (result)
+		      : "0"(sysnum), "r"(addr)
+		      : "memory");
+
 	return result;
 }
 
-- 
2.25.1