| 1 | SERIAL DEVICE NAMING |
| 2 | |
| 3 | As of 2.6.10, serial devices on ia64 are named based on the |
| 4 | order of ACPI and PCI enumeration. The first device in the |
| 5 | ACPI namespace (if any) becomes /dev/ttyS0, the second becomes |
| 6 | /dev/ttyS1, etc., and PCI devices are named sequentially |
| 7 | starting after the ACPI devices. |
| 8 | |
| 9 | Prior to 2.6.10, there were confusing exceptions to this: |
| 10 | |
| 11 | - Firmware on some machines (mostly from HP) provides an HCDP |
| 12 | table[1] that tells the kernel about devices that can be used |
| 13 | as a serial console. If the user specified "console=ttyS0" |
| 14 | or the EFI ConOut path contained only UART devices, the |
| 15 | kernel registered the device described by the HCDP as |
| 16 | /dev/ttyS0. |
| 17 | |
| 18 | - If there was no HCDP, we assumed there were UARTs at the |
| 19 | legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so |
| 20 | the kernel registered those as /dev/ttyS0 and /dev/ttyS1. |
| 21 | |
| 22 | Any additional ACPI or PCI devices were registered sequentially |
| 23 | after /dev/ttyS0 as they were discovered. |
| 24 | |
| 25 | With an HCDP, device names changed depending on EFI configuration |
| 26 | and "console=" arguments. Without an HCDP, device names didn't |
| 27 | change, but we registered devices that might not really exist. |
| 28 | |
| 29 | For example, an HP rx1600 with a single built-in serial port |
| 30 | (described in the ACPI namespace) plus an MP[2] (a PCI device) has |
| 31 | these ports: |
| 32 | |
| 33 | pre-2.6.10 pre-2.6.10 |
| 34 | MMIO (EFI console (EFI console |
| 35 | address on builtin) on MP port) 2.6.10 |
| 36 | ========== ========== ========== ====== |
| 37 | builtin 0xff5e0000 ttyS0 ttyS1 ttyS0 |
| 38 | MP UPS 0xf8031000 ttyS1 ttyS2 ttyS1 |
| 39 | MP Console 0xf8030000 ttyS2 ttyS0 ttyS2 |
| 40 | MP 2 0xf8030010 ttyS3 ttyS3 ttyS3 |
| 41 | MP 3 0xf8030038 ttyS4 ttyS4 ttyS4 |
| 42 | |
| 43 | CONSOLE SELECTION |
| 44 | |
| 45 | EFI knows what your console devices are, but it doesn't tell the |
| 46 | kernel quite enough to actually locate them. The DIG64 HCDP |
| 47 | table[1] does tell the kernel where potential serial console |
| 48 | devices are, but not all firmware supplies it. Also, EFI supports |
| 49 | multiple simultaneous consoles and doesn't tell the kernel which |
| 50 | should be the "primary" one. |
| 51 | |
| 52 | So how do you tell Linux which console device to use? |
| 53 | |
| 54 | - If your firmware supplies the HCDP, it is simplest to |
| 55 | configure EFI with a single device (either a UART or a VGA |
| 56 | card) as the console. Then you don't need to tell Linux |
| 57 | anything; the kernel will automatically use the EFI console. |
| 58 | |
| 59 | (This works only in 2.6.6 or later; prior to that you had |
| 60 | to specify "console=ttyS0" to get a serial console.) |
| 61 | |
| 62 | - Without an HCDP, Linux defaults to a VGA console unless you |
| 63 | specify a "console=" argument. |
| 64 | |
| 65 | NOTE: Don't assume that a serial console device will be /dev/ttyS0. |
| 66 | It might be ttyS1, ttyS2, etc. Make sure you have the appropriate |
| 67 | entries in /etc/inittab (for getty) and /etc/securetty (to allow |
| 68 | root login). |
| 69 | |
| 70 | EARLY SERIAL CONSOLE |
| 71 | |
| 72 | The kernel can't start using a serial console until it knows where |
| 73 | the device lives. Normally this happens when the driver enumerates |
| 74 | all the serial devices, which can happen a minute or more after the |
| 75 | kernel starts booting. |
| 76 | |
| 77 | 2.6.10 and later kernels have an "early uart" driver that works |
| 78 | very early in the boot process. The kernel will automatically use |
| 79 | this if the user supplies an argument like "console=uart,io,0x3f8", |
| 80 | or if the EFI console path contains only a UART device and the |
| 81 | firmware supplies an HCDP. |
| 82 | |
| 83 | TROUBLESHOOTING SERIAL CONSOLE PROBLEMS |
| 84 | |
| 85 | No kernel output after elilo prints "Uncompressing Linux... done": |
| 86 | |
| 87 | - You specified "console=ttyS0" but Linux changed the device |
| 88 | to which ttyS0 refers. Configure exactly one EFI console |
| 89 | device[3] and remove the "console=" option. |
| 90 | |
| 91 | - The EFI console path contains both a VGA device and a UART. |
| 92 | EFI and elilo use both, but Linux defaults to VGA. Remove |
| 93 | the VGA device from the EFI console path[3]. |
| 94 | |
| 95 | - Multiple UARTs selected as EFI console devices. EFI and |
| 96 | elilo use all selected devices, but Linux uses only one. |
| 97 | Make sure only one UART is selected in the EFI console |
| 98 | path[3]. |
| 99 | |
| 100 | - You're connected to an HP MP port[2] but have a non-MP UART |
| 101 | selected as EFI console device. EFI uses the MP as a |
| 102 | console device even when it isn't explicitly selected. |
| 103 | Either move the console cable to the non-MP UART, or change |
| 104 | the EFI console path[3] to the MP UART. |
| 105 | |
| 106 | Long pause (60+ seconds) between "Uncompressing Linux... done" and |
| 107 | start of kernel output: |
| 108 | |
| 109 | - No early console because you used "console=ttyS<n>". Remove |
| 110 | the "console=" option if your firmware supplies an HCDP. |
| 111 | |
| 112 | - If you don't have an HCDP, the kernel doesn't know where |
| 113 | your console lives until the driver discovers serial |
| 114 | devices. Use "console=uart, io,0x3f8" (or appropriate |
| 115 | address for your machine). |
| 116 | |
| 117 | Kernel and init script output works fine, but no "login:" prompt: |
| 118 | |
| 119 | - Add getty entry to /etc/inittab for console tty. Look for |
| 120 | the "Adding console on ttyS<n>" message that tells you which |
| 121 | device is the console. |
| 122 | |
| 123 | "login:" prompt, but can't login as root: |
| 124 | |
| 125 | - Add entry to /etc/securetty for console tty. |
| 126 | |
| 127 | No ACPI serial devices found in 2.6.17 or later: |
| 128 | |
| 129 | - Turn on CONFIG_PNP and CONFIG_PNPACPI. Prior to 2.6.17, ACPI |
| 130 | serial devices were discovered by 8250_acpi. In 2.6.17, |
| 131 | 8250_acpi was replaced by the combination of 8250_pnp and |
| 132 | CONFIG_PNPACPI. |
| 133 | |
| 134 | |
| 135 | |
| 136 | [1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf |
| 137 | The table was originally defined as the "HCDP" for "Headless |
| 138 | Console/Debug Port." The current version is the "PCDP" for |
| 139 | "Primary Console and Debug Port Devices." |
| 140 | |
| 141 | [2] The HP MP (management processor) is a PCI device that provides |
| 142 | several UARTs. One of the UARTs is often used as a console; the |
| 143 | EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart". |
| 144 | The external connection is usually a 25-pin connector, and a |
| 145 | special dongle converts that to three 9-pin connectors, one of |
| 146 | which is labelled "Console." |
| 147 | |
| 148 | [3] EFI console devices are configured using the EFI Boot Manager |
| 149 | "Boot option maintenance" menu. You may have to interrupt the |
| 150 | boot sequence to use this menu, and you will have to reset the |
| 151 | box after changing console configuration. |