SERIAL DEVICE NAMING
As of 2.6.10, serial devices on ia64 are named based on the
order of ACPI and PCI enumeration. The first device in the
ACPI namespace (if any) becomes /dev/ttyS0, the second becomes
/dev/ttyS1, etc., and PCI devices are named sequentially
starting after the ACPI devices.
Prior to 2.6.10, there were confusing exceptions to this:
- Firmware on some machines (mostly from HP) provides an HCDP
table that tells the kernel about devices that can be used
as a serial console. If the user specified "console=ttyS0"
or the EFI ConOut path contained only UART devices, the
kernel registered the device described by the HCDP as
- If there was no HCDP, we assumed there were UARTs at the
legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so
the kernel registered those as /dev/ttyS0 and /dev/ttyS1.
Any additional ACPI or PCI devices were registered sequentially
after /dev/ttyS0 as they were discovered.
With an HCDP, device names changed depending on EFI configuration
and "console=" arguments. Without an HCDP, device names didn't
change, but we registered devices that might not really exist.
For example, an HP rx1600 with a single built-in serial port
(described in the ACPI namespace) plus an MP (a PCI device) has
MMIO (EFI console (EFI console
address on builtin) on MP port) 2.6.10
========== ========== ========== ======
builtin 0xff5e0000 ttyS0 ttyS1 ttyS0
MP UPS 0xf8031000 ttyS1 ttyS2 ttyS1
MP Console 0xf8030000 ttyS2 ttyS0 ttyS2
MP 2 0xf8030010 ttyS3 ttyS3 ttyS3
MP 3 0xf8030038 ttyS4 ttyS4 ttyS4
EFI knows what your console devices are, but it doesn't tell the
kernel quite enough to actually locate them. The DIG64 HCDP
table does tell the kernel where potential serial console
devices are, but not all firmware supplies it. Also, EFI supports
multiple simultaneous consoles and doesn't tell the kernel which
should be the "primary" one.
So how do you tell Linux which console device to use?
- If your firmware supplies the HCDP, it is simplest to
configure EFI with a single device (either a UART or a VGA
card) as the console. Then you don't need to tell Linux
anything; the kernel will automatically use the EFI console.
(This works only in 2.6.6 or later; prior to that you had
to specify "console=ttyS0" to get a serial console.)
- Without an HCDP, Linux defaults to a VGA console unless you
specify a "console=" argument.
NOTE: Don't assume that a serial console device will be /dev/ttyS0.
It might be ttyS1, ttyS2, etc. Make sure you have the appropriate
entries in /etc/inittab (for getty) and /etc/securetty (to allow
EARLY SERIAL CONSOLE
The kernel can't start using a serial console until it knows where
the device lives. Normally this happens when the driver enumerates
all the serial devices, which can happen a minute or more after the
kernel starts booting.
2.6.10 and later kernels have an "early uart" driver that works
very early in the boot process. The kernel will automatically use
this if the user supplies an argument like "console=uart,io,0x3f8",
or if the EFI console path contains only a UART device and the
firmware supplies an HCDP.
TROUBLESHOOTING SERIAL CONSOLE PROBLEMS
No kernel output after elilo prints "Uncompressing Linux... done":
- You specified "console=ttyS0" but Linux changed the device
to which ttyS0 refers. Configure exactly one EFI console
device and remove the "console=" option.
- The EFI console path contains both a VGA device and a UART.
EFI and elilo use both, but Linux defaults to VGA. Remove
the VGA device from the EFI console path.
- Multiple UARTs selected as EFI console devices. EFI and
elilo use all selected devices, but Linux uses only one.
Make sure only one UART is selected in the EFI console
- You're connected to an HP MP port but have a non-MP UART
selected as EFI console device. EFI uses the MP as a
console device even when it isn't explicitly selected.
Either move the console cable to the non-MP UART, or change
the EFI console path to the MP UART.
Long pause (60+ seconds) between "Uncompressing Linux... done" and
start of kernel output:
- No early console because you used "console=ttyS<n>". Remove
the "console=" option if your firmware supplies an HCDP.
- If you don't have an HCDP, the kernel doesn't know where
your console lives until the driver discovers serial
devices. Use "console=uart, io,0x3f8" (or appropriate
address for your machine).
Kernel and init script output works fine, but no "login:" prompt:
- Add getty entry to /etc/inittab for console tty. Look for
the "Adding console on ttyS<n>" message that tells you which
device is the console.
"login:" prompt, but can't login as root:
- Add entry to /etc/securetty for console tty.
The table was originally defined as the "HCDP" for "Headless
Console/Debug Port." The current version is the "PCDP" for
"Primary Console and Debug Port Devices."
 The HP MP (management processor) is a PCI device that provides
several UARTs. One of the UARTs is often used as a console; the
EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
The external connection is usually a 25-pin connector, and a
special dongle converts that to three 9-pin connectors, one of
which is labelled "Console."
 EFI console devices are configured using the EFI Boot Manager
"Boot option maintenance" menu. You may have to interrupt the
boot sequence to use this menu, and you will have to reset the
box after changing console configuration.