ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

Add support for debug communications channel based
hvc console for arm64 cpus.
Signed-off-by: NAbhimanyu Kapur <abhimany@codeaurora.org>
Signed-off-by: NTimur Tabi <timur@codeaurora.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/include/asm/dcc.h

+/* Copyright (c) 2014-2015 The Linux Foundation. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 and
+ * only version 2 as published by the Free Software Foundation.
+ *
+ * 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.
+ *
+ * A call to __dcc_getchar() or __dcc_putchar() is typically followed by
+ * a call to __dcc_getstatus().  We want to make sure that the CPU does
+ * not speculative read the DCC status before executing the read or write
+ * instruction.  That's what the ISBs are for.
+ *
+ * The 'volatile' ensures that the compiler does not cache the status bits,
+ * and instead reads the DCC register every time.
+ */
+#ifndef __ASM_DCC_H
+#define __ASM_DCC_H
+
+#include <asm/barrier.h>
+
+static inline u32 __dcc_getstatus(void)
+{
+	u32 ret;
+
+	asm volatile("mrs %0, mdccsr_el0" : "=r" (ret));
+
+	return ret;
+}
+
+static inline char __dcc_getchar(void)
+{
+	char c;
+
+	asm volatile("mrs %0, dbgdtrrx_el0" : "=r" (c));
+	isb();
+
+	return c;
+}
+
+static inline void __dcc_putchar(char c)
+{
+	/*
+	 * The typecast is to make absolutely certain that 'c' is
+	 * zero-extended.
+	 */
+	asm volatile("msr dbgdtrtx_el0, %0"
+			: : "r" ((unsigned long)(unsigned char)c));
+	isb();
+}
+
+#endif

drivers/tty/hvc/Kconfig

@@ -81,7 +81,7 @@ config HVC_UDBG
 
 config HVC_DCC
        bool "ARM JTAG DCC console"
-       depends on ARM
+       depends on ARM || ARM64
        select HVC_DRIVER
        help
          This console uses the JTAG DCC on ARM to create a console under the HVC