[PATCH v2 2/2] risc-v: implement DBCN based debug console

[Heinrich Schuchardt]

Use the DBCN SBI extension to implement a debug console.
Make it the default for S-mode RISC-V.

Signed-off-by: Heinrich Schuchardt <heinrich.schucha...@canonical.com>
---
v2:
        check that we are in S-mode
---
 drivers/serial/Kconfig      |  5 ++++-
 drivers/serial/serial_sbi.c | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 7ca42df6a7..fb108f0189 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -218,6 +218,7 @@ config DEBUG_UART
 choice
        prompt "Select which UART will provide the debug UART"
        depends on DEBUG_UART
+       default DEBUG_SBI_CONSOLE if RISCV_SMODE
        default DEBUG_UART_NS16550
 
 config DEBUG_UART_ALTERA_JTAGUART
@@ -289,11 +290,13 @@ config DEBUG_EFI_CONSOLE
 
 config DEBUG_SBI_CONSOLE
        bool "SBI"
-       depends on SBI_V01
+       depends on RISCV_SMODE
        help
          Select this to enable a debug console which calls back to SBI to
          output to the console. This can be useful for early debugging of
          U-Boot when running on top of SBI (Supervisor Binary Interface).
+         This implementation of the debug UART is not available while in
+         M-mode (e.g. during SPL).
 
 config DEBUG_UART_S5P
        bool "Samsung S5P"
diff --git a/drivers/serial/serial_sbi.c b/drivers/serial/serial_sbi.c
index b9f35ed36e..a51a96c1ef 100644
--- a/drivers/serial/serial_sbi.c
+++ b/drivers/serial/serial_sbi.c
@@ -3,6 +3,8 @@
 #include <debug_uart.h>
 #include <asm/sbi.h>
 
+#ifdef CONFIG_SBI_V01
+
 static inline void _debug_uart_init(void)
 {
 }
@@ -13,4 +15,22 @@ static inline void _debug_uart_putc(int c)
                sbi_console_putchar(c);
 }
 
+#else
+
+static int sbi_dbcn_available;
+
+static inline void _debug_uart_init(void)
+{
+       if (CONFIG_IS_ENABLED(RISCV_SMODE))
+               sbi_dbcn_available = sbi_probe_extension(SBI_EXT_DBCN);
+}
+
+static inline void _debug_uart_putc(int ch)
+{
+       if (CONFIG_IS_ENABLED(RISCV_SMODE) && sbi_dbcn_available)
+               sbi_dbcn_write_byte(ch);
+}
+
+#endif
+
 DEBUG_UART_FUNCS
-- 
2.40.1