Serial Devices

Serial Device Naming

As of 2.6.10, serial devices on ia64 are named based on theorder of ACPI and PCI enumeration. The first device in theACPI namespace (if any) becomes /dev/ttyS0, the second becomes/dev/ttyS1, etc., and PCI devices are named sequentiallystarting after the ACPI devices.

Prior to 2.6.10, there were confusing exceptions to this:

  • Firmware on some machines (mostly from HP) provides an HCDPtable[1] that tells the kernel about devices that can be usedas a serial console. If the user specified “console=ttyS0”or the EFI ConOut path contained only UART devices, thekernel registered the device described by the HCDP as/dev/ttyS0.
  • If there was no HCDP, we assumed there were UARTs at thelegacy COM port addresses (I/O ports 0x3f8 and 0x2f8), sothe kernel registered those as /dev/ttyS0 and /dev/ttyS1.

Any additional ACPI or PCI devices were registered sequentiallyafter /dev/ttyS0 as they were discovered.

With an HCDP, device names changed depending on EFI configurationand “console=” arguments. Without an HCDP, device names didn’tchange, 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[2] (a PCI device) hasthese ports:

TypeMMIOaddress

pre-2.6.10

(EFI consoleon builtin)

pre-2.6.10

(EFI consoleon MP port)

2.6.10+
builtin0xff5e0000ttyS0ttyS1ttyS0
MP UPS0xf8031000ttyS1ttyS2ttyS1
MP Console0xf8030000ttyS2ttyS0ttyS2
MP 20xf8030010ttyS3ttyS3ttyS3
MP 30xf8030038ttyS4ttyS4ttyS4

Console Selection

EFI knows what your console devices are, but it doesn’t tell thekernel quite enough to actually locate them. The DIG64 HCDPtable[1] does tell the kernel where potential serial consoledevices are, but not all firmware supplies it. Also, EFI supportsmultiple simultaneous consoles and doesn’t tell the kernel whichshould be the “primary” one.

So how do you tell Linux which console device to use?

  • If your firmware supplies the HCDP, it is simplest toconfigure EFI with a single device (either a UART or a VGAcard) as the console. Then you don’t need to tell Linuxanything; the kernel will automatically use the EFI console.

    (This works only in 2.6.6 or later; prior to that you hadto specify “console=ttyS0” to get a serial console.)

  • Without an HCDP, Linux defaults to a VGA console unless youspecify 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 appropriateentries in /etc/inittab (for getty) and /etc/securetty (to allowroot login).

Early Serial Console

The kernel can’t start using a serial console until it knows wherethe device lives. Normally this happens when the driver enumeratesall the serial devices, which can happen a minute or more after thekernel starts booting.

2.6.10 and later kernels have an “early uart” driver that worksvery early in the boot process. The kernel will automatically usethis if the user supplies an argument like “console=uart,io,0x3f8”,or if the EFI console path contains only a UART device and thefirmware supplies an HCDP.

Troubleshooting Serial Console Problems

No kernel output after elilo prints “Uncompressing Linux… done”:

  • You specified “console=ttyS0” but Linux changed the deviceto which ttyS0 refers. Configure exactly one EFI consoledevice[3] 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. Removethe VGA device from the EFI console path[3].
  • Multiple UARTs selected as EFI console devices. EFI andelilo use all selected devices, but Linux uses only one.Make sure only one UART is selected in the EFI consolepath[3].
  • You’re connected to an HP MP port[2] but have a non-MP UARTselected as EFI console device. EFI uses the MP as aconsole device even when it isn’t explicitly selected.Either move the console cable to the non-MP UART, or changethe EFI console path[3] to the MP UART.

Long pause (60+ seconds) between “Uncompressing Linux… done” andstart of kernel output:

  • No early console because you used “console=ttyS<n>”. Removethe “console=” option if your firmware supplies an HCDP.
  • If you don’t have an HCDP, the kernel doesn’t know whereyour console lives until the driver discovers serialdevices. Use “console=uart,io,0x3f8” (or appropriateaddress for your machine).

Kernel and init script output works fine, but no “login:” prompt:

  • Add getty entry to /etc/inittab for console tty. Look forthe “Adding console on ttyS<n>” message that tells you whichdevice is the console.

“login:” prompt, but can’t login as root:

  • Add entry to /etc/securetty for console tty.

No ACPI serial devices found in 2.6.17 or later:

  • Turn on CONFIG_PNP and CONFIG_PNPACPI. Prior to 2.6.17, ACPIserial devices were discovered by 8250_acpi. In 2.6.17,8250_acpi was replaced by the combination of 8250_pnp andCONFIG_PNPACPI.
[1]
http://www.dig64.org/specifications/agreementThe table was originally defined as the “HCDP” for “HeadlessConsole/Debug Port.” The current version is the “PCDP” for“Primary Console and Debug Port Devices.”
[2]
The HP MP (management processor) is a PCI device that providesseveral UARTs. One of the UARTs is often used as a console; theEFI Boot Manager identifies it as “Acpi(HWP0002,700)/Pci(…)/Uart”.The external connection is usually a 25-pin connector, and aspecial dongle converts that to three 9-pin connectors, one ofwhich is labelled “Console.”
[3]
EFI console devices are configured using the EFI Boot Manager“Boot option maintenance” menu. You may have to interrupt theboot sequence to use this menu, and you will have to reset thebox after changing console configuration.