On Thu, Oct 6, 2016 at 11:08 AM, Duncan Laurie <[email protected]> wrote:
> On Thu, Oct 6, 2016 at 10:56 AM, Trammell Hudson <[email protected]> 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. > > I may be mis-remembering and this might come up as ttyS0 in linux for skylake. (it is ttyS2 on apollolake...) Or just use a custom command line like console=uart,mmio32,0xd1134000,115200n8 -duncan > 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 > >
-- coreboot mailing list: [email protected] https://www.coreboot.org/mailman/listinfo/coreboot
