using device-tree fragments/overlays

[Alex G.]

Hi Tim,

On 10/7/21 4:07 PM, Tim Harvey wrote:
> Greetings,
> 
> I'm working on some features for the imx8mm-venice boards I support
> which depend on making small changes to the dt before booting Linux.
> 
> The purpose is not to have any of this apply to the U-Boot controlling
> dt but instead to the Linux kernel dt applied within ft_board_setup. I
> could apply these changes with code but it would be way more readable
> if there was a way to store these as fragments in the FIT image and
> apply them.
> 
> The fragments would be small and have to do with two UART's routed to
> a multi-protocol RS232/RS485 transceiver that can be used as either of
> the following:
>   - two tx/rx UART's (the default configuration for my dt's)
>   - one rs232 uart with CTS/RTS flow control
>   - one rs485 uart
> 
> Here would be an example dt overlay to apply over the default
> arch/arm/dts/imx8mm-venice-gw73xx-0x.dts to change it from 2x RS232
> UARTS with TX/RX (uart2/uart4) to RS485 half duplex:
> 
> /* For RS485 HD:
>   *  - UART4_TX is DE for RS485 transmitter
>   *  - RS485_EN needs to be pulled high
>   *  - RS485_HALF needs to be pulled high
>   *  - RS485_TERM enables on-chip termination
>   */
> &uart2 {
>          rts-gpios = <&gpio5 29 GPIO_ACTIVE_HIGH>;
>          rs485-term-gpios = <&gpio1 0 GPIO_ACTIVE_HIGH>;
> };
> 
> &uart4 {
>          status = "disabled";
> };
> 
> &iomuxc {
>          pinctrl-names = "default";
>          pinctrl-0 = <&pinctrl_hog>;
> 
>          pinctrl_hog: hoggrp {
>                  fsl,pins = <
>                          MX8MM_IOMUXC_GPIO1_IO00_GPIO1_IO0
> 0x40000104 /* RS485_TERM */
>                          MX8MM_IOMUXC_SAI1_RXFS_GPIO4_IO0
> 0x40000144 /* RS485_EN */
>                          MX8MM_IOMUXC_SAI1_RXD0_GPIO4_IO2
> 0x40000144 /* RS485_HALF */
>                  >;
>          };
> 
>          pinctrl_uart2: uart2grp {
>                  fsl,pins = <
>                          MX8MM_IOMUXC_UART2_RXD_UART2_DCE_RX     0x140
>                          MX8MM_IOMUXC_UART2_TXD_UART2_DCE_TX     0x140
>                          MX8MM_IOMUXC_UART4_TXD_GPIO5_IO29       0x140
>                  >;
>          };
> };
> 
> Anyone do anything like this before or work with dt fragments/overlays
> in U-Boot or have any other suggestions?
> 

I had the exact same use case. A couple of prerequisites:

   * Compile kernel DTB with __symbols__ ('-@' argument to dtc)
   * Enable FIT devicetree overlays in u-boot
     - Slightly different CONFIG_s for loading FITs from u-boot or SPL
   * Assemble a FIT with overlays in it

I think there's also a way to do apply overlays from the u-boot 
commandline, but I never found that to be intuitive.

You might have to specify a "loadaddr=" for the overlays if you're 
loading the FIT from u-boot instead of SPL. AFAIR the u-boot FIT loader 
is defective in this regard.

Alex


APPENDIX A: Example FIT ITS

/dts-v1/
;/ {
	description = "Flu-boot image with overlays";
	#address-cells = <1>;
	images {
		kernel-1 {
			description = "Linux kernel";
			data = /incbin/("kernel/zImage");
			type = "kernel";
			arch = "arm";
			os = "linux";
			compression = "none";
			load = <0xc2000040>;
			entry = <0xc2000040>;
			hash-1 {
				algo = "sha256";
			};
		};

		fdt-stm32mp157c-dk2.dtb {
			description = "Flattened Device Tree blob";
			data = /incbin/("kernel/stm32mp157c-dk2.dtb");
			type = "flat_dt";
			arch = "arm";
			compression = "none";
			hash-1 {
				algo = "sha256";
			};
		};
		fdt-dk2-can1-enable.dto {
			description = "Flattened Device Tree blob";
			data = /incbin/("overlays/dk2-can1-enable.dto");
			type = "flat_dt";
			arch = "arm";
			compression = "none";
			hash-1 {
				algo = "sha256";
			};
		};
	};
	configurations {
		secure-stm32mp157c-dk2.dtb {
			description = "Linux for stm32mp157c-dk2.dtb";
			kernel = "kernel-1";
			fdt = "fdt-stm32mp157c-dk2.dtb", "fdt-dk2-can1-enable.dto";
			hash-1 {
				algo = "sha256";
			};
		};
	};
};