linux-kernel - [PATCH] arm64: dts: arm: juno: Fix UART frequency

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Message-Id: <20191119120331.28243-1-andre.przywara@arm.com>
Date:   Tue, 19 Nov 2019 12:03:31 +0000
From:   Andre Przywara <andre.przywara@....com>
To:     Liviu Dudau <liviu.dudau@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] arm64: dts: arm: juno: Fix UART frequency

Older versions of the Juno *SoC* TRM [1] recommended that the UART clock
source should be 7.2738 MHz, whereas the *system* TRM [2] stated a more
correct value of 7.3728 MHz. Somehow the wrong value managed to end up in
our DT.
Doing a prime factorisation, a modulo divide by 115200 and trying
to buy a 7.2738 MHz crystal at your favourite electronics dealer suggest
that the old value was actually a typo. The actual UART clock is driven
by a PLL, configured via a parameter in some board.txt file in the
firmware, which reads 7.37 MHz (sic!).

Fix this to correct the baud rate divisor calculation on the Juno board.

Signed-off-by: Andre Przywara <andre.przywara@....com>

[1] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0515b.b/DDI0515B_b_juno_arm_development_platform_soc_trm.pdf
[2] http://infocenter.arm.com/help/topic/com.arm.doc.100113_0000_07_en/arm_versatile_express_juno_development_platform_(v2m_juno)_technical_reference_manual_100113_0000_07_en.pdf
---
 arch/arm64/boot/dts/arm/juno-clocks.dtsi | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/boot/dts/arm/juno-clocks.dtsi b/arch/arm64/boot/dts/arm/juno-clocks.dtsi
index e5e265dfa902..2870b5eeb198 100644
--- a/arch/arm64/boot/dts/arm/juno-clocks.dtsi
+++ b/arch/arm64/boot/dts/arm/juno-clocks.dtsi
@@ -8,10 +8,10 @@
  */
 / {
 	/* SoC fixed clocks */
-	soc_uartclk: refclk7273800hz {
+	soc_uartclk: refclk7372800hz {
 		compatible = "fixed-clock";
 		#clock-cells = <0>;
-		clock-frequency = <7273800>;
+		clock-frequency = <7372800>;
 		clock-output-names = "juno:uartclk";
 	};
 
-- 
2.17.1