[PATCH] MIPS: loongson2ef: don't build serial.o unconditionally

[Randy Dunlap]

LOONGSON_UART_BASE depends on EARLY_PRINTK || SERIAL_8250, but when
neither of these Kconfig symbols is set, the kernel build has errors:

../arch/mips/loongson2ef/common/serial.c: In function 'serial_init':
../arch/mips/loongson2ef/common/serial.c:66:25: error: 'loongson_uart_base' undeclared (first use in this function)
   66 |                         loongson_uart_base;
../arch/mips/loongson2ef/common/serial.c:66:25: note: each undeclared identifier is reported only once for each function it appears in
../arch/mips/loongson2ef/common/serial.c:68:41: error: '_loongson_uart_base' undeclared (first use in this function)
   68 |                         (void __iomem *)_loongson_uart_base;

Fix this by building serial.o only when one (or both) of these
Kconfig symbols is enabled.

Tested with:
	(a) EARLY_PRINTK=y, SERIAL_8250 not set;
	(b) EARLY_PRINTK=y, SERIAL_8250=y;
	(c) EARLY_PRINTK=y, SERIAL_8250=m.
	(d) EARLY_PRINTK not set, SERIAL_8250=y;
	(e) EARLY_PRINTK not set, SERIAL_8250=m;
	(f) EARLY_PRINTK not set, SERIAL_8250 not set.

Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Cc: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
---
 arch/mips/loongson2ef/common/Makefile |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- linext-20210803.orig/arch/mips/loongson2ef/common/Makefile
+++ linext-20210803/arch/mips/loongson2ef/common/Makefile
@@ -4,12 +4,14 @@
 #
 
 obj-y += setup.o init.o env.o time.o reset.o irq.o \
-    bonito-irq.o mem.o machtype.o platform.o serial.o
+    bonito-irq.o mem.o machtype.o platform.o
 obj-$(CONFIG_PCI) += pci.o
 
 #
 # Serial port support
 #
+obj-$(CONFIG_LOONGSON_UART_BASE) += serial.o
+obj-$(CONFIG_EARLY_PRINTK) += serial.o
 obj-$(CONFIG_LOONGSON_UART_BASE) += uart_base.o
 obj-$(CONFIG_LOONGSON_MC146818) += rtc.o