powerpc: Add an in memory udbg console

This patch adds a new udbg early debug console which utilises
statically defined input and output buffers stored within the kernel
BSS. It is primarily designed to assist with bring up of new hardware
which may not have a working console but which has a method of
reading/writing kernel memory.

This version incorporates comments made by Ben H (thanks!).

Changes from v1:
	- Add memory barriers.
	- Ensure updating of read/write positions is atomic.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig.debug

@@ -262,8 +262,31 @@ config PPC_EARLY_DEBUG_OPAL_HVSI
 	  Select this to enable early debugging for the PowerNV platform
 	  using an "hvsi" console
 
+config PPC_EARLY_DEBUG_MEMCONS
+	bool "In memory console"
+	help
+	  Select this to enable early debugging using an in memory console.
+	  This console provides input and output buffers stored within the
+	  kernel BSS and should be safe to select on any system. A debugger
+	  can then be used to read kernel output or send input to the console.
 endchoice
 
+config PPC_MEMCONS_OUTPUT_SIZE
+	int "In memory console output buffer size"
+	depends on PPC_EARLY_DEBUG_MEMCONS
+	default 4096
+	help
+	  Selects the size of the output buffer (in bytes) of the in memory
+	  console.
+
+config PPC_MEMCONS_INPUT_SIZE
+	int "In memory console input buffer size"
+	depends on PPC_EARLY_DEBUG_MEMCONS
+	default 128
+	help
+	  Selects the size of the input buffer (in bytes) of the in memory
+	  console.
+
 config PPC_EARLY_DEBUG_OPAL
 	def_bool y
 	depends on PPC_EARLY_DEBUG_OPAL_RAW || PPC_EARLY_DEBUG_OPAL_HVSI

arch/powerpc/include/asm/udbg.h

@@ -52,6 +52,7 @@ extern void __init udbg_init_40x_realmode(void);
 extern void __init udbg_init_cpm(void);
 extern void __init udbg_init_usbgecko(void);
 extern void __init udbg_init_wsp(void);
+extern void __init udbg_init_memcons(void);
 extern void __init udbg_init_ehv_bc(void);
 extern void __init udbg_init_ps3gelic(void);
 extern void __init udbg_init_debug_opal_raw(void);

arch/powerpc/kernel/udbg.c

@@ -64,6 +64,9 @@ void __init udbg_early_init(void)
 	udbg_init_usbgecko();
 #elif defined(CONFIG_PPC_EARLY_DEBUG_WSP)
 	udbg_init_wsp();
+#elif defined(CONFIG_PPC_EARLY_DEBUG_MEMCONS)
+	/* In memory console */
+	udbg_init_memcons();
 #elif defined(CONFIG_PPC_EARLY_DEBUG_EHV_BC)
 	udbg_init_ehv_bc();
 #elif defined(CONFIG_PPC_EARLY_DEBUG_PS3GELIC)

arch/powerpc/sysdev/Makefile

@@ -64,6 +64,8 @@ endif
 
 obj-$(CONFIG_PPC_SCOM)		+= scom.o
 
+obj-$(CONFIG_PPC_EARLY_DEBUG_MEMCONS)	+= udbg_memcons.o
+
 subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
 
 obj-$(CONFIG_PPC_XICS)		+= xics/

arch/powerpc/sysdev/udbg_memcons.c

@@ -0,0 +1,105 @@
+/*
+ * A udbg backend which logs messages and reads input from in memory
+ * buffers.
+ *
+ * The console output can be read from memcons_output which is a
+ * circular buffer whose next write position is stored in memcons.output_pos.
+ *
+ * Input may be passed by writing into the memcons_input buffer when it is
+ * empty. The input buffer is empty when both input_pos == input_start and
+ * *input_start == '\0'.
+ *
+ * Copyright (C) 2003-2005 Anton Blanchard and Milton Miller, IBM Corp
+ * Copyright (C) 2013 Alistair Popple, IBM Corp
+ *
+ *      This program is free software; you can redistribute it and/or
+ *      modify it under the terms of the GNU General Public License
+ *      as published by the Free Software Foundation; either version
+ *      2 of the License, or (at your option) any later version.
+ */
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <asm/barrier.h>
+#include <asm/page.h>
+#include <asm/processor.h>
+#include <asm/udbg.h>
+
+struct memcons {
+	char *output_start;
+	char *output_pos;
+	char *output_end;
+	char *input_start;
+	char *input_pos;
+	char *input_end;
+};
+
+static char memcons_output[CONFIG_PPC_MEMCONS_OUTPUT_SIZE];
+static char memcons_input[CONFIG_PPC_MEMCONS_INPUT_SIZE];
+
+struct memcons memcons = {
+	.output_start = memcons_output,
+	.output_pos = memcons_output,
+	.output_end = &memcons_output[CONFIG_PPC_MEMCONS_OUTPUT_SIZE],
+	.input_start = memcons_input,
+	.input_pos = memcons_input,
+	.input_end = &memcons_input[CONFIG_PPC_MEMCONS_INPUT_SIZE],
+};
+
+void memcons_putc(char c)
+{
+	char *new_output_pos;
+
+	*memcons.output_pos = c;
+	wmb();
+	new_output_pos = memcons.output_pos + 1;
+	if (new_output_pos >= memcons.output_end)
+		new_output_pos = memcons.output_start;
+
+	memcons.output_pos = new_output_pos;
+}
+
+int memcons_getc_poll(void)
+{
+	char c;
+	char *new_input_pos;
+
+	if (*memcons.input_pos) {
+		c = *memcons.input_pos;
+
+		new_input_pos = memcons.input_pos + 1;
+		if (new_input_pos >= memcons.input_end)
+			new_input_pos = memcons.input_start;
+		else if (*new_input_pos == '\0')
+			new_input_pos = memcons.input_start;
+
+		*memcons.input_pos = '\0';
+		wmb();
+		memcons.input_pos = new_input_pos;
+		return c;
+	}
+
+	return -1;
+}
+
+int memcons_getc(void)
+{
+	int c;
+
+	while (1) {
+		c = memcons_getc_poll();
+		if (c == -1)
+			cpu_relax();
+		else
+			break;
+	}
+
+	return c;
+}
+
+void udbg_init_memcons(void)
+{
+	udbg_putc = memcons_putc;
+	udbg_getc = memcons_getc;
+	udbg_getc_poll = memcons_getc_poll;
+}