Enable the debug UART and emit a single 'a' early in the init sequence to show that it is working.
Unfortunately the debug UART implementation needs a stack to work. I cannot seem to remove this limitation as the absolute 'jmp %eax' instruction goes off into the weeds.
So this means that the character output cannot be any earlier than car_init_ret, where memory is available for a stack.
Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Lukasz Majewski l.majewski@samsung.com ---
Changes in v2: - Put the debug UART setup code in comments since we don't always want it
arch/x86/cpu/start.S | 9 +++++++++ configs/minnowmax_defconfig | 4 ++++ 2 files changed, 13 insertions(+)
diff --git a/arch/x86/cpu/start.S b/arch/x86/cpu/start.S index fdef2bb..ac711ed 100644 --- a/arch/x86/cpu/start.S +++ b/arch/x86/cpu/start.S @@ -128,6 +128,15 @@ car_init_ret: andl $0xfffffff0, %esp post_code(POST_START_STACK)
+ /* + * Debug UART is available here although it may not be plumbed out + * to pins depending on the board. To use it: + * + * call debug_uart_init + * mov $'a', %eax + * call printch + */ + /* Zero the global data since it won't happen later */ xorl %eax, %eax movl $GENERATED_GBL_DATA_SIZE, %ecx diff --git a/configs/minnowmax_defconfig b/configs/minnowmax_defconfig index e98f5eb..a67597d 100644 --- a/configs/minnowmax_defconfig +++ b/configs/minnowmax_defconfig @@ -24,3 +24,7 @@ CONFIG_FRAMEBUFFER_VESA_MODE_11A=y CONFIG_DM_RTC=y CONFIG_USE_PRIVATE_LIBGCC=y CONFIG_SYS_VSNPRINTF=y +CONFIG_DEBUG_UART=y +CONFIG_DEBUG_UART_NS16550=y +CONFIG_DEBUG_UART_BASE=0x3f8 +CONFIG_DEBUG_UART_CLOCK=1843200