[Xilinx Zynq]: spurious UART receive on startup

[Norbert Braun]

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.pdf