riscv: semihosting: replace inline assembly with assembly file

So far we used inline assembly to inject the actual instruction that
triggers the semihosting service. While this sounds elegant, as it's
really only about a few instructions, it has some serious downsides:
- We need some barriers in place to force the compiler to issue writes
  to a data structure before issuing the trap instruction.
- We need to convince the compiler to actually fill the structures that
  we use pointers to.
- We need a memory clobber to avoid the compiler caching the data in
  those structures, when semihosting writes data back.
- We need register arguments to make sure the function ID and the
  pointer land in the right registers.

This is all doable, but fragile and somewhat cumbersome. Since we now
have a separate function in an extra file anyway, we can do away with
all the magic and just write that in an actual assembler.
This is much more readable and robust.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>

arch/riscv/lib/semihosting.S

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2022 Ventana Micro Systems Inc.
+ */
+
+#include <asm/asm.h>
+#include <linux/linkage.h>
+
+.pushsection .text.smh_trap, "ax"
+ENTRY(smh_trap)
+	.align	2
+	.option	push
+	.option	norvc		/* semihosting sequence must be 32-bit wide */
+
+	slli zero, zero, 0x1f	/* Entry NOP to identify semihosting */
+	ebreak
+	srai zero, zero, 7	/* NOP encoding of semihosting call number */
+	.option pop
+
+	ret
+ENDPROC(smh_trap)
+.popsection

arch/riscv/lib/semihosting.c

@@ -1,24 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0+
-/*
- * Copyright (C) 2022 Ventana Micro Systems Inc.
- */
-
-#include <common.h>
-
-long smh_trap(int sysnum, void *addr)
-{
-	register int ret asm ("a0") = sysnum;
-	register void *param0 asm ("a1") = addr;
-
-	asm volatile (".align 4\n"
-		".option push\n"
-		".option norvc\n"
-
-		"slli zero, zero, 0x1f\n"
-		"ebreak\n"
-		"srai zero, zero, 7\n"
-		".option pop\n"
-		: "+r" (ret) : "r" (param0) : "memory");
-
-	return ret;
-}