| ============== |
| Serial Devices |
| ============== |
| |
| 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: |
| |
| ========== ========== ============ ============ ======= |
| Type MMIO pre-2.6.10 pre-2.6.10 2.6.10+ |
| address |
| (EFI console (EFI console |
| on builtin) on MP port) |
| ========== ========== ============ ============ ======= |
| 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. |
