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>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 5416e28a7538..863d877e0b5f 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/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