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From:   Daniel Kurtz <djkurtz@chromium.org>
Cc:     adurbin@chromium.org, Daniel Kurtz <djkurtz@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        linux-kernel@vger.kernel.org (open list:X86 ARCHITECTURE (32-BIT AND
        64-BIT))
Subject: [PATCH 1/2] x86/Kconfig: Add config for specifying BASE_BAUD
Date:   Wed,  7 Mar 2018 16:24:09 -0700
Message-Id: <20180307232410.248434-2-djkurtz@chromium.org>
In-Reply-To: <20180307232410.248434-1-djkurtz@chromium.org>
References: <20180307232410.248434-1-djkurtz@chromium.org>
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Sender: linux-kernel-owner@vger.kernel.org
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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 <djkurtz@chromium.org>
---
 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.
 
+config X86_SERIAL_BASE_BAUD
+	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.
  */
+#ifdef CONFIG_X86_SERIAL_BASE_BAUD
+#define BASE_BAUD CONFIG_X86_SERIAL_BASE_BAUD
+#else
 #define BASE_BAUD (1843200/16)
+#endif
 
 /* Standard COM flags (except for COM4, because of the 8514 problem) */
 #ifdef CONFIG_SERIAL_8250_DETECT_IRQ
-- 
2.16.2.395.g2e18187dfd-goog