linux-kernel - [PATCH AUTOSEL 4.4 173/174] arm64: dts: juno: Fix UART frequency

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Date:   Thu, 16 Jan 2020 12:42:50 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Andre Przywara <andre.przywara@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Sasha Levin <sashal@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org
Subject: [PATCH AUTOSEL 4.4 173/174] arm64: dts: juno: Fix UART frequency

From: Andre Przywara <andre.przywara@....com>

[ Upstream commit 39a1a8941b27c37f79508426e27a2ec29829d66c ]

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.

[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

Fixes: 71f867ec130e ("arm64: Add Juno board device tree.")
Signed-off-by: Andre Przywara <andre.przywara@....com>
Acked-by: Liviu Dudau <liviu.dudau@....com>
Signed-off-by: Sudeep Holla <sudeep.holla@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 25352ed943e6..00bcbf7688c7 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";
 	};
 
-- 
2.20.1