Kernel-2.6.32-573.12.1.el6_serial

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[1] 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
  /dev/ttyS0.

- 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[2] (a PCI device) has
these ports:

                              pre-2.6.10      pre-2.6.10
                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

CONSOLE SELECTION

EFI knows what your console devices are, but it doesn't tell the
kernel quite enough to actually locate them.  The DIG64 HCDP
table[1] 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
root login).

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[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.  Remove
  the VGA device from the EFI console path[3].

- 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
  path[3].

- You're connected to an HP MP port[2] 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[3] 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.

No ACPI serial devices found in 2.6.17 or later:

- Turn on CONFIG_PNP and CONFIG_PNPACPI.  Prior to 2.6.17, ACPI
  serial devices were discovered by 8250_acpi.  In 2.6.17,
  8250_acpi was replaced by the combination of 8250_pnp and
  CONFIG_PNPACPI.

[1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf
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.”

[2] 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.”

[3] 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.