arm64: early_printk: add support for FastModel console output

Enable early_printk to use the FastModel semihosting to output
the early kernel messages. Works both for host and guest kernels.

To use this feature, pass "early_printk=smh" to the kernel.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/early_printk.c b/arch/arm64/kernel/early_printk.c
index 7e320a2..0bb7436 100644 (file)
--- a/arch/arm64/kernel/early_printk.c
+++ b/arch/arm64/kernel/early_printk.c
@@ -40,6 +40,17 @@ static void pl011_printch(char ch)
                ;
 }
 
+/*
+ * Semihosting-based debug console
+ */
+static void smh_printch(char ch)
+{
+       asm volatile("mov  x1, %0\n"
+                    "mov  x0, #3\n"
+                    "hlt  0xf000\n"
+                    : : "r" (&ch) : "x0", "x1", "memory");
+}
+
 struct earlycon_match {
        const char *name;
        void (*printch)(char ch);
@@ -47,6 +58,7 @@ struct earlycon_match {
 
 static const struct earlycon_match earlycon_match[] __initconst = {
        { .name = "pl011", .printch = pl011_printch, },
+       { .name = "smh", .printch = smh_printch, },
        {}
 };