MIPS, ttyFDC: Add early FDC console support

Add support for early console of MIPS Fast Debug Channel (FDC) on
channel 1 with a call very early from the MIPS setup_arch().

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9145/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

4 files changed, 46 insertions, 0 deletions

diff --git a/arch/mips/include/asm/cdmm.h b/arch/mips/include/asm/cdmm.h
index b7d520f28d30..16e22ce9719f 100644
--- a/arch/mips/include/asm/cdmm.h
+++ b/arch/mips/include/asm/cdmm.h
@@ -84,4 +84,15 @@ void mips_cdmm_driver_unregister(struct mips_cdmm_driver *);
         module_driver(__mips_cdmm_driver, mips_cdmm_driver_register, \
                         mips_cdmm_driver_unregister)
 
+/* drivers/tty/mips_ejtag_fdc.c */
+
+#ifdef CONFIG_MIPS_EJTAG_FDC_EARLYCON
+int setup_early_fdc_console(void);
+#else
+static inline int setup_early_fdc_console(void)
+{
+        return -ENODEV;
+}
+#endif
+
 #endif /* __ASM_CDMM_H */
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index 058929041368..be73c491182b 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -31,6 +31,7 @@
 #include <asm/bootinfo.h>
 #include <asm/bugs.h>
 #include <asm/cache.h>
+#include <asm/cdmm.h>
 #include <asm/cpu.h>
 #include <asm/sections.h>
 #include <asm/setup.h>
@@ -763,6 +764,7 @@ void __init setup_arch(char **cmdline_p)
         cpu_probe();
         prom_init();
 
+        setup_early_fdc_console();
 #ifdef CONFIG_EARLY_PRINTK
         setup_early_printk();
 #endif
diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 39469ca4231c..e0c18e5b7057 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -437,4 +437,17 @@ config MIPS_EJTAG_FDC_TTY
 
           If unsure, say N.
 
+config MIPS_EJTAG_FDC_EARLYCON
+        bool "Early FDC console"
+        depends on MIPS_EJTAG_FDC_TTY
+        help
+          This registers a console on FDC channel 1 very early during boot (from
+          MIPS arch code). This is useful for bring-up and debugging early boot
+          issues.
+
+          Do not enable unless there is a debug probe attached to drain the FDC
+          TX FIFO.
+
+          If unsure, say N.
+
 endif # TTY
diff --git a/drivers/tty/mips_ejtag_fdc.c b/drivers/tty/mips_ejtag_fdc.c
index 51672cfe7e45..8d9bf6f90110 100644
--- a/drivers/tty/mips_ejtag_fdc.c
+++ b/drivers/tty/mips_ejtag_fdc.c
@@ -69,6 +69,9 @@
 #define REG_FDSTAT_TXE                  BIT(1)  /* Tx Empty */
 #define REG_FDSTAT_TXF                  BIT(0)  /* Tx Full */
 
+/* Default channel for the early console */
+#define CONSOLE_CHANNEL      1
+
 #define NUM_TTY_CHANNELS     16
 
 #define RX_BUF_SIZE 1024
@@ -1124,3 +1127,20 @@ static int __init mips_ejtag_fdc_init_console(void)
         return mips_ejtag_fdc_console_init(&mips_ejtag_fdc_con);
 }
 console_initcall(mips_ejtag_fdc_init_console);
+
+#ifdef CONFIG_MIPS_EJTAG_FDC_EARLYCON
+static struct mips_ejtag_fdc_console mips_ejtag_fdc_earlycon = {
+        .cons   = {
+                .name   = "early_fdc",
+                .write  = mips_ejtag_fdc_console_write,
+                .flags  = CON_PRINTBUFFER | CON_BOOT,
+                .index  = CONSOLE_CHANNEL,
+        },
+        .lock   = __RAW_SPIN_LOCK_UNLOCKED(mips_ejtag_fdc_earlycon.lock),
+};
+
+int __init setup_early_fdc_console(void)
+{
+        return mips_ejtag_fdc_console_init(&mips_ejtag_fdc_earlycon);
+}
+#endif