ARC: Dynamically determine BASE_BAUD from DeviceTree

8250 earlycon is broken on multi-platform ARC because the UART clk
value (BASE_BAUD) is fixed at build time.

Instead, determine the appropriate UART clk at runtime; parse the
devicetree early for platforms requiring alternate UART clk values
(currently only the TB10X platform).

Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/serial.h

@@ -10,26 +10,13 @@
 #define _ASM_ARC_SERIAL_H
 
 /*
- * early-8250 requires BASE_BAUD to be defined and includes this header.
+ * early 8250 (now earlycon) requires BASE_BAUD to be defined in this header.
- * We put in a typical value:
+ * However to still determine it dynamically (for multi-platform images)
- * 	(core clk / 16) - i.e. UART samples 16 times per sec.
+ * we do this in a helper by parsing the FDT early
- * Athough in multi-platform-image this might not work, specially if the
- * clk driving the UART is different.
- * We can't use DeviceTree as this is typically for early serial.
  */
 
-#include <asm/clk.h>
+extern unsigned int __init arc_early_base_baud(void);
 
-#define BASE_BAUD	(arc_get_core_freq() / 16)
+#define BASE_BAUD	arc_early_base_baud()
-
-/*
- * This is definitely going to break early 8250 consoles on multi-platform
- * images but hey, it won't add any code complexity for a debug feature of
- * one broken driver.
- */
-#ifdef CONFIG_ARC_PLAT_TB10X
-#undef BASE_BAUD
-#define BASE_BAUD	(arc_get_core_freq() / 16 / 3)
-#endif
 
 #endif /* _ASM_ARC_SERIAL_H */

arch/arc/kernel/devtree.c

@@ -17,6 +17,28 @@
 #include <asm/clk.h>
 #include <asm/mach_desc.h>
 
+#ifdef CONFIG_SERIAL_8250_CONSOLE
+
+static unsigned int __initdata arc_base_baud;
+
+unsigned int __init arc_early_base_baud(void)
+{
+	return arc_base_baud/16;
+}
+
+static void __init arc_set_early_base_baud(unsigned long dt_root)
+{
+	unsigned int core_clk = arc_get_core_freq();
+
+	if (of_flat_dt_is_compatible(dt_root, "abilis,arc-tb10x"))
+		arc_base_baud = core_clk/3;
+	else
+		arc_base_baud = core_clk;
+}
+#else
+#define arc_set_early_base_baud(dt_root)
+#endif
+
 static const void * __init arch_get_next_mach(const char *const **match)
 {
 	static const struct machine_desc *mdesc = __arch_info_begin;
@@ -56,5 +78,7 @@ const struct machine_desc * __init setup_machine_fdt(void *dt)
 	if (clk)
 		arc_set_core_freq(of_read_ulong(clk, len/4));
 
+	arc_set_early_base_baud(dt_root);
+
 	return mdesc;
 }