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-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/agreement
- 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.