Hi,
I am running into a problem with U-Boot v2020.07 on Xilinx Zynq (Zedboard). If I build U-Boot with the default config (xilinx_zynq_virt_defconfig, DEVICE_TREE=zynq-zed), autoboot will abort, even if no key is pressed. This happens regardless of whether the USB UART is even connected to a PC.
I am using boot.bin as generated by U-Boot as the SPL (plus u-boot.img for U-Boot proper).
While debugging this, I noticed two things:
1. The Zynq TRM [1] notes in section 19.2.3 ("Baud generator"): IMPORTANT: It is essential to disable the transmitter and receiver before writing to the Baud Rate Generator register (uart.Baud_rate_gen_reg0), or the baud rate divider register (uart.Baud_rate_divider_reg0). A soft reset must be issued to both the transmitter and receiver before they are re-enabled.
However, the code in _uart_zynq_serial_setbrg() (in drivers/serial/serial_zynq.c) does not seem to do that.
2. It seems that the Zynq BootROM actually touches the registers for UART1. I have no idea why it does that, but table 6-22 ("BootROM Modified Registers") in the TRM lists several UART1 registers that have been modified from their reset value. In particular, the control register at 0xE0001000 contains the value 0x00000114 after the BootROM ran.
As zynq_serial_probe() checks if TX is enabled (bit 4 in the control register), and only writes to the control register if it is not, the end result is that U-Boot never really initializes UART1 or resets its TX or RX path.
If the debug UART functionality (CONFIG_DEBUG_UART_ZYNQ) is enabled, _debug_uart_init() writes to the control register unconditionally and resets the TX/RX path. Indeed, enabling the debug UART functionality makes my problem disappear. The debug UART was enabled by default in zynq_zed_defconfig (in v2019.10), but this changed when switching to a single Zynq configuration (xilinx_zynq_virt_defconfig) for v2020.07.
Note that the above workaround fixes the problem for me, but I wanted to report it in case other people run into the same issue.
Best regards,
Norbert
[1]: https://www.xilinx.com/support/documentation/user_guides/ug585-Zynq-7000-TRM...