powerpc: Add support for early debugging via Serial 16550 console

Currently powerpc early debugging contains lot of platform specific
options, but does not support standard UART / serial 16550 console.

Later legacy_serial.c code supports registering UART as early debug console
from device tree but it is not early during booting, but rather later after
machine description code finishes.

So for real early debugging via UART is current code unsuitable.

Add support for new early debugging option CONFIG_PPC_EARLY_DEBUG_16550
which enable Serial 16550 console on address defined by new option
CONFIG_PPC_EARLY_DEBUG_16550_PHYSADDR and by stride by option
CONFIG_PPC_EARLY_DEBUG_16550_STRIDE.

With this change it is possible to debug powerpc machine descriptor code.
For example this early debugging code can print on serial console also
"No suitable machine description found" error which is done before
legacy_serial.c code.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220822231501.16827-1-pali@kernel.org

arch/powerpc/Kconfig.debug

@@ -283,6 +283,12 @@ config PPC_EARLY_DEBUG_MEMCONS
 	  This console provides input and output buffers stored within the
 	  kernel BSS and should be safe to select on any system. A debugger
 	  can then be used to read kernel output or send input to the console.
+
+config PPC_EARLY_DEBUG_16550
+	bool "Serial 16550"
+	depends on PPC_UDBG_16550
+	help
+	  Select this to enable early debugging via Serial 16550 console
 endchoice
 
 config PPC_MEMCONS_OUTPUT_SIZE
@@ -354,6 +360,15 @@ config PPC_EARLY_DEBUG_CPM_ADDR
 	  platform probing is done, all platforms selected must
 	  share the same address.
 
+config PPC_EARLY_DEBUG_16550_PHYSADDR
+	hex "Early debug Serial 16550 physical address"
+	depends on PPC_EARLY_DEBUG_16550
+
+config PPC_EARLY_DEBUG_16550_STRIDE
+	int "Early debug Serial 16550 stride"
+	depends on PPC_EARLY_DEBUG_16550
+	default 1
+
 config FAIL_IOMMU
 	bool "Fault-injection capability for IOMMU"
 	depends on FAULT_INJECTION

arch/powerpc/include/asm/udbg.h

@@ -52,6 +52,7 @@ extern void __init udbg_init_ehv_bc(void);
 extern void __init udbg_init_ps3gelic(void);
 extern void __init udbg_init_debug_opal_raw(void);
 extern void __init udbg_init_debug_opal_hvsi(void);
+extern void __init udbg_init_debug_16550(void);
 
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_UDBG_H */

arch/powerpc/kernel/udbg.c

@@ -67,6 +67,8 @@ void __init udbg_early_init(void)
 	udbg_init_debug_opal_raw();
 #elif defined(CONFIG_PPC_EARLY_DEBUG_OPAL_HVSI)
 	udbg_init_debug_opal_hvsi();
+#elif defined(CONFIG_PPC_EARLY_DEBUG_16550)
+	udbg_init_debug_16550();
 #endif
 
 #ifdef CONFIG_PPC_EARLY_DEBUG

arch/powerpc/kernel/udbg_16550.c

@@ -8,6 +8,7 @@
 #include <asm/udbg.h>
 #include <asm/io.h>
 #include <asm/reg_a2.h>
+#include <asm/early_ioremap.h>
 
 extern u8 real_readb(volatile u8 __iomem  *addr);
 extern void real_writeb(u8 data, volatile u8 __iomem *addr);
@@ -296,3 +297,35 @@ void __init udbg_init_40x_realmode(void)
 }
 
 #endif /* CONFIG_PPC_EARLY_DEBUG_40x */
+
+#ifdef CONFIG_PPC_EARLY_DEBUG_16550
+
+static void __iomem *udbg_uart_early_addr;
+
+void __init udbg_init_debug_16550(void)
+{
+	udbg_uart_early_addr = early_ioremap(CONFIG_PPC_EARLY_DEBUG_16550_PHYSADDR, 0x1000);
+	udbg_uart_init_mmio(udbg_uart_early_addr, CONFIG_PPC_EARLY_DEBUG_16550_STRIDE);
+}
+
+static int __init udbg_init_debug_16550_ioremap(void)
+{
+	void __iomem *addr;
+
+	if (!udbg_uart_early_addr)
+		return 0;
+
+	addr = ioremap(CONFIG_PPC_EARLY_DEBUG_16550_PHYSADDR, 0x1000);
+	if (WARN_ON(!addr))
+		return -ENOMEM;
+
+	udbg_uart_init_mmio(addr, CONFIG_PPC_EARLY_DEBUG_16550_STRIDE);
+	early_iounmap(udbg_uart_early_addr, 0x1000);
+	udbg_uart_early_addr = NULL;
+
+	return 0;
+}
+
+early_initcall(udbg_init_debug_16550_ioremap);
+
+#endif /* CONFIG_PPC_EARLY_DEBUG_16550 */