Re: using device-tree fragments/overlays

14 Oct 2021


      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";
    		};
    	};
    };
};

Re: using device-tree fragments/overlays

Alex G.