UART5 is typically used as the default debug UART on the AST2600, but
UART1 is also designed to be a debug UART. All the AST2600 UART's have
semi-configurable clock rates through registers in the System Control
Unit (SCU), but only UART5 works out of the box with zero-initialized
values. The rest of the UART's expect a few of the registers to be
initialized to non-zero values, or else the clock rate calculation will
yield zero or undefined (due to a divide-by-zero).

For reference, the U-Boot clock rate driver here shows the calculation:

    https://github.com/facebook/openbmc-uboot/blob/15f7e0dc01d8/drivers/clk/aspeed/clk_ast2600.c#L357



To summarize, UART5 allows selection from 4 rates: 24 MHz, 192 MHz, 24 /
13 MHz, and 192 / 13 MHz. The other UART's allow selecting either the
"low" rate (UARTCLK) or the "high" rate (HUARTCLK). UARTCLK and HUARTCLK
are configurable themselves:

    UARTCLK = UXCLK * R / (N * 2)
    HUARTCLK = HUXCLK * HR / (HN * 2)

UXCLK and HUXCLK are also configurable, and depend on the APLL and/or
HPLL clock rates, which also derive from complicated calculations. Long
story short, there's lots of multiplication and division from
configurable registers, and most of these registers are zero-initialized
in QEMU, which at best is unexpected and at worst causes this clock rate
driver to hang from divide-by-zero's. This can also be difficult to
diagnose, because it may cause U-Boot to hang before serial console
initialization completes, requiring intervention from gdb.

This change just initializes all of these registers with default values
from the datasheet.

To test this, I used Facebook's AST2600 OpenBMC image for "fuji", with
the following diff applied (because fuji uses UART1 for console output,
not UART5).

  @@ -323,8 +323,8 @@ static void aspeed_soc_ast2600_realize(DeviceState *dev, Error **errp)
       }

      /* UART - attach an 8250 to the IO space as our UART5 */
  -    serial_mm_init(get_system_memory(), sc->memmap[ASPEED_DEV_UART5], 2,
  -                   aspeed_soc_get_irq(s, ASPEED_DEV_UART5),
  +    serial_mm_init(get_system_memory(), sc->memmap[ASPEED_DEV_UART1], 2,
  +                   aspeed_soc_get_irq(s, ASPEED_DEV_UART1),
                    38400, serial_hd(0), DEVICE_LITTLE_ENDIAN);

       /* I2C */

Without these clock rate registers being initialized, U-Boot hangs in
the clock rate driver from a divide-by-zero, because the UART1 clock
rate register reads return zero, and there's no console output. After
initializing them with default values, fuji boots successfully.

Signed-off-by: Peter Delevoryas <pdel@fb.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
[ clg: Removed _PARAM suffix ]
Message-Id: <20210906134023.3711031-2-pdel@fb.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>