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>

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 */

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

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

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