ARM: 7338/1: add support for early console output via semihosting

This is a very simple method for code running in an emulator, or under
the supervision of a debugger, to use I/O facilities on the controlling
host.

Tested with OpenOCD, and ARM's Fast Models.

Details on semihosting can be found in chapter 8 of
DUI0203I_rvct_developer_guide.pdf from ARM Ltd.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 44 insertions, 5 deletions

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index e0d236d7ff73..755d9fbe3238 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -100,6 +100,22 @@ choice
                   Note that the system will appear to hang during boot if there
                   is nothing connected to read from the DCC.
 
+        config DEBUG_SEMIHOSTING
+                bool "Kernel low-level debug output via semihosting I"
+                help
+                  Semihosting enables code running on an ARM target to use
+                  the I/O facilities on a host debugger/emulator through a
+                  simple SVC calls. The host debugger or emulator must have
+                  semihosting enabled for the special svc call to be trapped
+                  otherwise the kernel will crash.
+
+                  This is known to work with OpenOCD, as wellas
+                  ARM's Fast Models, or any other controlling environment
+                  that implements semihosting.
+
+                  For more details about semihosting, please see
+                  chapter 8 of DUI0203I_rvct_developer_guide.pdf from ARM Ltd.
+
         config AT91_DEBUG_LL_DBGU0
                 bool "Kernel low-level debugging on rm9200, 9260/9g20, 9261/9g10 and 9rl"
                 depends on HAVE_AT91_DBGU0
diff --git a/arch/arm/kernel/debug.S b/arch/arm/kernel/debug.S
index 204e2160cfcc..e5a765c5f06a 100644
--- a/arch/arm/kernel/debug.S
+++ b/arch/arm/kernel/debug.S
@@ -100,7 +100,7 @@
 
 #endif  /* CONFIG_CPU_V6 */
 
-#else
+#elif !defined(CONFIG_DEBUG_SEMIHOSTING)
 #include <mach/debug-macro.S>
 #endif  /* CONFIG_DEBUG_ICEDCC */
 
@@ -155,6 +155,8 @@ hexbuf:		.space 16
 
                 .ltorg
 
+#ifndef CONFIG_DEBUG_SEMIHOSTING
+
 ENTRY(printascii)
                 addruart_current r3, r1, r2
                 b       2f
@@ -177,3 +179,24 @@ ENTRY(printch)
                 mov     r0, #0
                 b       1b
 ENDPROC(printch)
+
+#else
+
+ENTRY(printascii)
+                mov     r1, r0
+                mov     r0, #0x04               @ SYS_WRITE0
+        ARM(    svc     #0x123456       )
+        THUMB(  svc     #0xab           )
+                mov     pc, lr
+ENDPROC(printascii)
+
+ENTRY(printch)
+                adr     r1, hexbuf
+                strb    r0, [r1]
+                mov     r0, #0x03               @ SYS_WRITEC
+        ARM(    svc     #0x123456       )
+        THUMB(  svc     #0xab           )
+                mov     pc, lr
+ENDPROC(printch)
+
+#endif
diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index a2e9694a68ee..3bf0c7f8b043 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -265,7 +265,7 @@ __create_page_tables:
         str     r6, [r3]
 
 #ifdef CONFIG_DEBUG_LL
-#ifndef CONFIG_DEBUG_ICEDCC
+#if !defined(CONFIG_DEBUG_ICEDCC) && !defined(CONFIG_DEBUG_SEMIHOSTING)
         /*
          * Map in IO space for serial debugging.
          * This allows debug messages to be output
@@ -297,10 +297,10 @@ __create_page_tables:
         cmp     r0, r6
         blo     1b
 
-#else /* CONFIG_DEBUG_ICEDCC */
-        /* we don't need any serial debugging mappings for ICEDCC */
+#else /* CONFIG_DEBUG_ICEDCC || CONFIG_DEBUG_SEMIHOSTING */
+        /* we don't need any serial debugging mappings */
         ldr     r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags
-#endif /* !CONFIG_DEBUG_ICEDCC */
+#endif
 
 #if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)
         /*