Help Needed: No UART Output in Mainline U-Boot on MT7620A (ZBT WE826-T2)

[Richard Thanki]

Dear U-Boot Community,

I am working on porting mainline U-Boot to a standard MT7620A wireless router, specifically the ZBT WE826-T2 (https://openwrt.org/toh/zbtlink/we-826). After building and flashing the mainline U-Boot, I'm encountering an issue where there is no output over UART.

So far, I've tried the default configurations for both MT7620 RFB and MT7620-MT7530 RFB targets, with the latter being more of an experimental test as my board lacks a gigabit switch. I’m flashing the standard `u-boot.bin` image. I’ve tried disabling the 'skip low-level configuration' in 'make menuconfig,’ but the issue persists. Despite these efforts, the router's behaviour remains unchanged, showing the same LED behaviour (one of the led’s infrequently but regularly flashing) whether the new U-Boot is flashed or the flash chip is entirely erased.

In comparing the Device Trees (DTs) for OpenWrt:
- https://github.com/openwrt/openwrt/blob/master/target/linux/ramips/dts/mt7620a_zbtlink_zbt-we826.dtsi
- https://github.com/openwrt/openwrt/blob/master/target/linux/ramips/dts/mt7620a.dtsi

with those in U-Boot 
- https://github.com/u-boot/u-boot/blob/master/arch/mips/dts/mediatek%2Cmt7620-rfb.dts
- https://github.com/u-boot/u-boot/blob/master/arch/mips/dts/mt7620.dtsi

both look similar to all important characteristics. I noticed both use the uartlite for serial.

I’m using Ubuntu 20.04 to compile using OpenWrt’s pre-built toolchain.

Any insights or guidance on this matter would be greatly appreciated, especially if someone has experience with UART issues on MT7620A-based hardware in mainline U-Boot. Resolving this could pave the way for easier ports to similar devices.

Thank you in advance for your support.

Rich