arm64: dts: imx8mm-kontron: Use GPIO for RS485 transceiver control

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@kontron.de>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

diff --git a/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts b/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
index d16490d876874b7bfc9066efdd724bbb52f518b7..e756fe5db56b6a19c309fcbb94475629e5f2b2a0 100644 (file)
--- a/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
+++ b/arch/arm64/boot/dts/freescale/imx8mm-kontron-bl.dts
@@ -268,8 +268,16 @@
 &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 @@
                        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
                >;
        };