linux-kernel - [PATCH v2 03/12] arm64: dts: imx8mm-kontron: Use GPIO for RS485 transceiver control

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Message-ID: <20250721100701.115548-4-frieder@fris.de>
Date: Mon, 21 Jul 2025 12:05:37 +0200
From: Frieder Schrempf <frieder@...s.de>
To: linux-arm-kernel@...ts.infradead.org,
	Conor Dooley <conor+dt@...nel.org>,
	devicetree@...r.kernel.org,
	imx@...ts.linux.dev,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	linux-kernel@...r.kernel.org,
	Rob Herring <robh@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Shawn Guo <shawnguo@...nel.org>
Cc: Eberhard Stoll <eberhard.stoll@...tron.de>,
	Frieder Schrempf <frieder.schrempf@...tron.de>,
	Fabio Estevam <festevam@...il.com>,
	Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: [PATCH v2 03/12] arm64: dts: imx8mm-kontron: Use GPIO for RS485 transceiver control

From: Eberhard Stoll <eberhard.stoll@...tron.de>

For this IP the correct control of the CTS signal for transceiver direction
switching is difficult and - maybe also buggy - in the driver. Especially
the bootup requires special handling for most hardware implementations.

Therefore we simply use a GPIO now, which is fully under software control
and which is not problematic on bootup.

Signed-off-by: Eberhard Stoll <eberhard.stoll@...tron.de>
Signed-off-by: Frieder Schrempf <frieder.schrempf@...tron.de>
---
 arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts b/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
index d16490d876874..e756fe5db56b6 100644
--- a/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
+++ b/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
@@ -268,8 +268,16 @@ &uart1 {
 &uart2 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_uart2>;
+	/*
+	 * During bootup the CTS needs to stay LOW, which is only possible if this
+	 * pin is controlled by a GPIO. The UART IP always sets CTS to HIGH if not
+	 * running. So using 'uart-has-rtscts' is not a good choice here! There are
+	 * workarounds for this, but they introduce unnecessary complexity and are
+	 * therefore avoided here. For more information about this see:
+	 * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=79d0224f6bf296d04cd843cfc49921b19c97bb09
+	 */
+	rts-gpios = <&gpio4 29 GPIO_ACTIVE_HIGH>;
 	linux,rs485-enabled-at-boot-time;
-	uart-has-rtscts;
 	status = "okay";
 };
 
@@ -439,7 +447,7 @@ pinctrl_uart2: uart2grp {
 			MX8MM_IOMUXC_SAI3_TXFS_UART2_DCE_RX		0x0
 			MX8MM_IOMUXC_SAI3_TXC_UART2_DCE_TX		0x0
 			MX8MM_IOMUXC_SAI3_RXD_UART2_DCE_RTS_B		0x0
-			MX8MM_IOMUXC_SAI3_RXC_UART2_DCE_CTS_B		0x0
+			MX8MM_IOMUXC_SAI3_RXC_GPIO4_IO29		0x19
 		>;
 	};
 
-- 
2.50.1