166 lines
6.6 KiB
ReStructuredText
166 lines
6.6 KiB
ReStructuredText
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==============
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Serial Devices
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==============
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Serial Device Naming
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====================
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As of 2.6.10, serial devices on ia64 are named based on the
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order of ACPI and PCI enumeration. The first device in the
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ACPI namespace (if any) becomes /dev/ttyS0, the second becomes
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/dev/ttyS1, etc., and PCI devices are named sequentially
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starting after the ACPI devices.
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Prior to 2.6.10, there were confusing exceptions to this:
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- Firmware on some machines (mostly from HP) provides an HCDP
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table[1] that tells the kernel about devices that can be used
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as a serial console. If the user specified "console=ttyS0"
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or the EFI ConOut path contained only UART devices, the
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kernel registered the device described by the HCDP as
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/dev/ttyS0.
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- If there was no HCDP, we assumed there were UARTs at the
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legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so
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the kernel registered those as /dev/ttyS0 and /dev/ttyS1.
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Any additional ACPI or PCI devices were registered sequentially
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after /dev/ttyS0 as they were discovered.
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With an HCDP, device names changed depending on EFI configuration
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and "console=" arguments. Without an HCDP, device names didn't
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change, but we registered devices that might not really exist.
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For example, an HP rx1600 with a single built-in serial port
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(described in the ACPI namespace) plus an MP[2] (a PCI device) has
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these ports:
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========== ========== ============ ============ =======
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Type MMIO pre-2.6.10 pre-2.6.10 2.6.10+
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address
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(EFI console (EFI console
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on builtin) on MP port)
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========== ========== ============ ============ =======
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builtin 0xff5e0000 ttyS0 ttyS1 ttyS0
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MP UPS 0xf8031000 ttyS1 ttyS2 ttyS1
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MP Console 0xf8030000 ttyS2 ttyS0 ttyS2
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MP 2 0xf8030010 ttyS3 ttyS3 ttyS3
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MP 3 0xf8030038 ttyS4 ttyS4 ttyS4
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========== ========== ============ ============ =======
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Console Selection
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=================
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EFI knows what your console devices are, but it doesn't tell the
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kernel quite enough to actually locate them. The DIG64 HCDP
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table[1] does tell the kernel where potential serial console
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devices are, but not all firmware supplies it. Also, EFI supports
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multiple simultaneous consoles and doesn't tell the kernel which
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should be the "primary" one.
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So how do you tell Linux which console device to use?
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- If your firmware supplies the HCDP, it is simplest to
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configure EFI with a single device (either a UART or a VGA
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card) as the console. Then you don't need to tell Linux
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anything; the kernel will automatically use the EFI console.
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(This works only in 2.6.6 or later; prior to that you had
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to specify "console=ttyS0" to get a serial console.)
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- Without an HCDP, Linux defaults to a VGA console unless you
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specify a "console=" argument.
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NOTE: Don't assume that a serial console device will be /dev/ttyS0.
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It might be ttyS1, ttyS2, etc. Make sure you have the appropriate
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entries in /etc/inittab (for getty) and /etc/securetty (to allow
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root login).
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Early Serial Console
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====================
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The kernel can't start using a serial console until it knows where
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the device lives. Normally this happens when the driver enumerates
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all the serial devices, which can happen a minute or more after the
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kernel starts booting.
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2.6.10 and later kernels have an "early uart" driver that works
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very early in the boot process. The kernel will automatically use
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this if the user supplies an argument like "console=uart,io,0x3f8",
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or if the EFI console path contains only a UART device and the
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firmware supplies an HCDP.
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Troubleshooting Serial Console Problems
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=======================================
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No kernel output after elilo prints "Uncompressing Linux... done":
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- You specified "console=ttyS0" but Linux changed the device
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to which ttyS0 refers. Configure exactly one EFI console
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device[3] and remove the "console=" option.
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- The EFI console path contains both a VGA device and a UART.
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EFI and elilo use both, but Linux defaults to VGA. Remove
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the VGA device from the EFI console path[3].
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- Multiple UARTs selected as EFI console devices. EFI and
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elilo use all selected devices, but Linux uses only one.
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Make sure only one UART is selected in the EFI console
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path[3].
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- You're connected to an HP MP port[2] but have a non-MP UART
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selected as EFI console device. EFI uses the MP as a
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console device even when it isn't explicitly selected.
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Either move the console cable to the non-MP UART, or change
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the EFI console path[3] to the MP UART.
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Long pause (60+ seconds) between "Uncompressing Linux... done" and
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start of kernel output:
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- No early console because you used "console=ttyS<n>". Remove
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the "console=" option if your firmware supplies an HCDP.
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- If you don't have an HCDP, the kernel doesn't know where
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your console lives until the driver discovers serial
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devices. Use "console=uart,io,0x3f8" (or appropriate
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address for your machine).
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Kernel and init script output works fine, but no "login:" prompt:
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- Add getty entry to /etc/inittab for console tty. Look for
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the "Adding console on ttyS<n>" message that tells you which
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device is the console.
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"login:" prompt, but can't login as root:
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- Add entry to /etc/securetty for console tty.
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No ACPI serial devices found in 2.6.17 or later:
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- Turn on CONFIG_PNP and CONFIG_PNPACPI. Prior to 2.6.17, ACPI
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serial devices were discovered by 8250_acpi. In 2.6.17,
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8250_acpi was replaced by the combination of 8250_pnp and
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CONFIG_PNPACPI.
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[1]
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http://www.dig64.org/specifications/agreement
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The table was originally defined as the "HCDP" for "Headless
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Console/Debug Port." The current version is the "PCDP" for
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"Primary Console and Debug Port Devices."
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[2]
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The HP MP (management processor) is a PCI device that provides
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several UARTs. One of the UARTs is often used as a console; the
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EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
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The external connection is usually a 25-pin connector, and a
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special dongle converts that to three 9-pin connectors, one of
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which is labelled "Console."
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[3]
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EFI console devices are configured using the EFI Boot Manager
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"Boot option maintenance" menu. You may have to interrupt the
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boot sequence to use this menu, and you will have to reset the
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box after changing console configuration.
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