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Subject[PATCH 1/2] x86/Kconfig: Add config for specifying BASE_BAUD
Not all x86 CPUs have UARTs that use a baud reference clock (BASE_BAUD) of
115200 = 1843200 / 16. For example, the UARTs on AMD Carrizo and later
use a fixed input clock of 48000000, and hence require BASE_BAUD=3000000.

The BASE_BAUD value is used by the generic earlycon driver to compute the
UART clock used early in boot before the real serial drivers have a
chance to load and read their input clock configuration from ACPI.

Add a Kconfig option to override this default hard coded value.

Signed-off-by: Daniel Kurtz <>
arch/x86/Kconfig | 9 +++++++++
arch/x86/include/asm/serial.h | 4 ++++
2 files changed, 13 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index eb7f43f23521..0a258bb30159 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -454,6 +454,15 @@ config INTEL_RDT

Say N if unsure.

+ int "UART input clock for platform 8250/16550 serial ports"
+ default "115200"
+ help
+ This specifies the default frequency for the reference clock of the
+ serial ports. For many systems this is 1843200 / 16, which is the
+ default, but may need to be changed to get earlycon to work on
+ some hardware, such as those that use AMD Carrizo or later SoCs.
if X86_32
config X86_BIGSMP
bool "Support for big SMP systems with more than 8 CPUs"
diff --git a/arch/x86/include/asm/serial.h b/arch/x86/include/asm/serial.h
index ece8299d2695..c28151090b51 100644
--- a/arch/x86/include/asm/serial.h
+++ b/arch/x86/include/asm/serial.h
@@ -9,7 +9,11 @@
* clock, since the 16550A is capable of handling a top speed of 1.5
* megabits/second; but this requires a faster clock.
#define BASE_BAUD (1843200/16)

/* Standard COM flags (except for COM4, because of the 8514 problem) */
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