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+ 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
+ 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).
+ 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.
+ 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.
+[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.