PXA255 QEM FYI

[Mark Tinguely]

This is just a FYI for those wanting to use QEMU with the PXA255.

1) The uart is not discovered causing a hang after the line:

warning: no time-of-day clock registered, system time will not be set accurately

I had this problem in qemu 0.9.1 and 0.10.0_1. This problem was keeping 
the NFS diskless and MD based kernels to even get to single user.

In FreeBSD, sys/arm/xscale/pxa/uart_bus_pxa.c in uart_pxa_probe(), a
check is made to make sure the port is enabled. QEM does not enable that
bit. It appears that Linux does not check for the bit.

A simple work around in FreeBSD would something like:

static int
uart_pxa_probe(device_t dev)
{
	bus_space_handle_t	base;
	struct			uart_softc *sc;

+#ifdef QEMU
+	base = (bus_space_handle_t)pxa_get_base(dev);
+	if (0x40100000 != (unsigned int) base)
+		return (ENXIO);
+#else
	/* Check to see if the enable bit's on. */
	if ((bus_space_read_4(obio_tag, base,
	    (REG_IER << 2)) & PXA_UART_UUE) == 0)
		return (ENXIO);
+#endif

	sc = device_get_softc(dev);
	sc->sc_class = &uart_ns8250_class;

	return(uart_bus_probe(dev, 2, PXA2X0_COM_FREQ, 0, 0));
}

2) the new QEMU (qemu-0.10.0_1) compile under FreeBSD-8.0-current, incorrect
brings a register value from the ARM enviroment to the QEMU SMC emulation
evironment and QEMU crashes with a message:

qemu: fatal: smc91c111_read: Bad reg 0:30e

R00=00000000 R01=c5e1f300 R02=0000000e R03=c0a778e0
R04=c0ba8300 R05=c0baab00 R06=00000003 R07=00000001
R08=00000000 R09=00000000 R10=c0bb4540 R11=c00fbc50
R12=c00fbc54 R13=c00fbc44 R14=c0403950 R15=c03e8bac
PSR=a0000113 N-C- A svc32

Notice register 2 has the correct value of 0x0e.

Compiled under FreeBSD 6.4, the emulator runs fine. I will mention this
to the QEMU people; it may be a difference in gcc3 vs gcc4. A temporary
work around in QEMU might be a mask (0xff) of the offset variable in
smc91c111_readb (and smc91c111_writeb ?) or removal of the ethernet card
from the kernel configuration.

A big THANK-YOU to Jacques Fourie for holding my hand to get these problems
resolved.

--Mark Tinguely.