[PATCH 2/4] arch: arm/xen: add putc() for debugging

[AKASHI Takahiro]

This new function, xen_debug_putc(), is intended to be used to
enable CONFIG_DEBUG_UART on xen guest.

Please note that the underlying functionality in Xen is available
only when Xen is configured with !NDEBUG but is much simpler than
a generic HYPERVISOR_console_io().

Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
---
 arch/arm/include/asm/xen/hypercall.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm/include/asm/xen/hypercall.h b/arch/arm/include/asm/xen/hypercall.h
index a4fd077079a9..121ccfcc608f 100644
--- a/arch/arm/include/asm/xen/hypercall.h
+++ b/arch/arm/include/asm/xen/hypercall.h
@@ -19,4 +19,10 @@ int HYPERVISOR_sched_op(int cmd, void *arg);
 int HYPERVISOR_event_channel_op(int cmd, void *arg);
 unsigned long HYPERVISOR_hvm_op(int op, void *arg);
 int HYPERVISOR_memory_op(unsigned int cmd, void *arg);
+
+static inline void xen_debug_putc(int c)
+{
+	register int v __asm__ ("x0") = c;
+	__asm__ __volatile__("hvc 0xfffe" : "=r" (v) : "0" (v));
+}
 #endif /* _ASM_ARM_XEN_HYPERCALL_H */
-- 
2.28.0