Using secondary UART for board initialization

[Alexander Koch]

> All `serial_putc()`-like functions that I could find are either static to the
> actual driver and thus not accessible from `board_init()` or do not accept a
> `struct udevice *` as a parameter and seem to work on the main U-Boot serial
> console device only.

Reading other board code I think I have found a supposed-to-be-working sequence
of calls:

(simplified code)

    struct udevice *dev = uclass_get_device_by_name(UCLASS_SERIAL,
                              "serial at 30860000", &dev);
    struct dm_serial_ops *ops = serial_get_ops(dev);
    ops->setbrg(dev, 115200);
    ops->putc(dev, 'f');

Unfortunately this yields a CPU reset:

    "Error" handler, esr 0xbf000002
    elr: 00000000402605f0 lr : 00000000402610f8 (reloc)
    elr: 00000000bff865f0 lr : 00000000bff870f8
    x0 : 000000000000006d x1 : 000000000000081a
    x2 : 00000000bffabf00 x3 : 00000000bdf1c630
    x4 : 00000000bdf1cf20 x5 : 00000000bdf21804
    x6 : 000000000000000f x7 : 00000000bdf1cf50
    x8 : 00000000bdf1ce80 x9 : 00000000ffffffd0
    x10: 0000000000000002 x11: 00000000ffffffff
    x12: 00000000bdf1d080 x13: 0000000000004884
    x14: 0000000000000000 x15: 00000000bdf1c6a8
    x16: 0000000040234dc4 x17: 000000000000aa80
    x18: 00000000bdf25d80 x19: 00000000bdf1cf20
    x20: 00000000bdf1c670 x21: 00000000ffffffc8
    x22: 00000000bdf1ce8a x23: 00000000bdf1cf20
    x24: 00000000bdf1c670 x25: 000000000000081a
    x26: 00000000bff9c000 x27: 00000000bffabf00
    x28: 0000000000000000 x29: 00000000bdf1c560
    
    Code: d00000ba 91263340 f90037e0 39400360 (350001a0)
    Resetting CPU ...

I traced this down to a writel() call to the UART TXD register in the serial
driver (`mxc_serial_putc()` in 'serial_mxc.c').

The documentation for `uclass_get_device_by_name()` says the device is 'probed
to activate it ready for use' so I think this is not an initialization issue.

Any ideas what I might be missing there?


Best regards,

Alex