[PATCH 0/3] hw/arm: xilinx_zynq: Fix upstream U-Boot boot failure

[Bin Meng]

As of today, when booting upstream U-Boot for Xilinx Zynq, the UART
does not receive anything. Debugging shows that the UART input clock
frequency is zero which prevents the UART from receiving anything as.
per the logic in uart_receive().

Note the U-Boot can still output data to the UART tx fifo, which should
not happen, as the design seems to prevent the data transmission when
clock is not enabled but somehow it only applies to the Rx side.

For anyone who is interested to give a try, here is the U-Boot defconfig:
$ make xilinx_zynq_virt_defconfig

and QEMU commands to test U-Boot:
$ qemu-system-arm -M xilinx-zynq-a9 -m 1G -display none -serial null -serial 
stdio \
    -device loader,file=u-boot-dtb.bin,addr=0x4000000,cpu-num=0

Note U-Boot used to boot properly in QEMU 4.2.0 which is the QEMU
version used in current U-Boot's CI testing. The UART clock changes
were introduced by the following 3 commits:

38867cb7ec90 ("hw/misc/zynq_slcr: add clock generation for uarts")
b636db306e06 ("hw/char/cadence_uart: add clock support")
5b49a34c6800 ("hw/arm/xilinx_zynq: connect uart clocks to slcr")


Bin Meng (3):
  hw/misc: zynq_slcr: Correctly compute output clocks in the reset exit
    phase
  hw/char: cadence_uart: Disable transmit when input clock is disabled
  hw/char: cadence_uart: Move clock/reset check to uart_can_receive()

 hw/char/cadence_uart.c | 16 +++++++++++-----
 hw/misc/zynq_slcr.c    | 31 ++++++++++++++++++-------------
 2 files changed, 29 insertions(+), 18 deletions(-)

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2.25.1