On Thu, Oct 6, 2016 at 10:56 AM, Trammell Hudson <hudson@trmm.net> wrote:

Is it possible to use the Skylake Servo debug UART in Linux or Xen?
It doesn't show up as a normal 16550 (setserial reports "uart type
unknown"), which is making debugging the payload kernel a little
frustrating.

I've added lots of "outb $0x80" calls to trace the Xen hypervisor and
have figured out that the Skylake doesn't work with the x2apic, the xsave
instruction or the MCE handler, but would prefer a higher bandwidth way
to report progress/errors.

The debug UART on Skylake is 32bit MMIO based, so it does not work with most existing serial drivers.  It should work in recent Linux kernels and come up as ttyS2, but you can also force it with earlyprintk using the 'mmio32' directive and the base address.

The device can be put into 16550/8-bit compatible mode, there is some code to do this on broadwell at soc/intel/skylake/broadwell/romstage/uart.c but on Skylake we left it in 32bit mode as putting it in compatible mode made it unable to suspend properly.

I *think* for skylake you can set the device PchSerialIoIndexUart2 to PchSerialIoLegacyUart (in devicetree.cb) and FSP will configure it for 16500 byte mode.  You will then need to update coreboot to use the uart in 8-bit mode instead of 32bit mode.

That still leaves the issue that it is mmio based and not IO based, so it will still have a hard time in things like seabios.  However I think Xen does support MMIO UART so it might work there...

-duncan