using device-tree fragments/overlays

[Tim Harvey]

On Wed, Oct 13, 2021 at 1:38 AM Peter Robinson <pbrobinson at gmail.com> wrote:
>
> Hi Tim,
>
> > 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?
>
> A feature for overlays landed upstream in May [1], so the 2021,07
> release, so I'm sure there's most of the functionality you require
> already upstream. Not sure how a user would specify which of the
> variants to use or how it would load it from the FIT but I'm sure the
> patch series provides you a good start point.
>
> Peter
>
> [1] https://lists.denx.de/pipermail/u-boot/2021-May/448794.html


Peter,

Thanks - this is very helpful and is what I was looking for. My plan
is to use the hwconfig env var for users to select how they want
serial configured. I can use the methods used in the extension_apply
function and it seems appropriate for me to put dtbo's on the root
filesystem with the dtb's when using this.

Best regards,

Tim