1============== 2Serial Devices 3============== 4 5Serial Device Naming 6==================== 7 8 As of 2.6.10, serial devices on ia64 are named based on the 9 order of ACPI and PCI enumeration. The first device in the 10 ACPI namespace (if any) becomes /dev/ttyS0, the second becomes 11 /dev/ttyS1, etc., and PCI devices are named sequentially 12 starting after the ACPI devices. 13 14 Prior to 2.6.10, there were confusing exceptions to this: 15 16 - Firmware on some machines (mostly from HP) provides an HCDP 17 table[1] that tells the kernel about devices that can be used 18 as a serial console. If the user specified "console=ttyS0" 19 or the EFI ConOut path contained only UART devices, the 20 kernel registered the device described by the HCDP as 21 /dev/ttyS0. 22 23 - If there was no HCDP, we assumed there were UARTs at the 24 legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so 25 the kernel registered those as /dev/ttyS0 and /dev/ttyS1. 26 27 Any additional ACPI or PCI devices were registered sequentially 28 after /dev/ttyS0 as they were discovered. 29 30 With an HCDP, device names changed depending on EFI configuration 31 and "console=" arguments. Without an HCDP, device names didn't 32 change, but we registered devices that might not really exist. 33 34 For example, an HP rx1600 with a single built-in serial port 35 (described in the ACPI namespace) plus an MP[2] (a PCI device) has 36 these ports: 37 38 ========== ========== ============ ============ ======= 39 Type MMIO pre-2.6.10 pre-2.6.10 2.6.10+ 40 address 41 (EFI console (EFI console 42 on builtin) on MP port) 43 ========== ========== ============ ============ ======= 44 builtin 0xff5e0000 ttyS0 ttyS1 ttyS0 45 MP UPS 0xf8031000 ttyS1 ttyS2 ttyS1 46 MP Console 0xf8030000 ttyS2 ttyS0 ttyS2 47 MP 2 0xf8030010 ttyS3 ttyS3 ttyS3 48 MP 3 0xf8030038 ttyS4 ttyS4 ttyS4 49 ========== ========== ============ ============ ======= 50 51Console Selection 52================= 53 54 EFI knows what your console devices are, but it doesn't tell the 55 kernel quite enough to actually locate them. The DIG64 HCDP 56 table[1] does tell the kernel where potential serial console 57 devices are, but not all firmware supplies it. Also, EFI supports 58 multiple simultaneous consoles and doesn't tell the kernel which 59 should be the "primary" one. 60 61 So how do you tell Linux which console device to use? 62 63 - If your firmware supplies the HCDP, it is simplest to 64 configure EFI with a single device (either a UART or a VGA 65 card) as the console. Then you don't need to tell Linux 66 anything; the kernel will automatically use the EFI console. 67 68 (This works only in 2.6.6 or later; prior to that you had 69 to specify "console=ttyS0" to get a serial console.) 70 71 - Without an HCDP, Linux defaults to a VGA console unless you 72 specify a "console=" argument. 73 74 NOTE: Don't assume that a serial console device will be /dev/ttyS0. 75 It might be ttyS1, ttyS2, etc. Make sure you have the appropriate 76 entries in /etc/inittab (for getty) and /etc/securetty (to allow 77 root login). 78 79Early Serial Console 80==================== 81 82 The kernel can't start using a serial console until it knows where 83 the device lives. Normally this happens when the driver enumerates 84 all the serial devices, which can happen a minute or more after the 85 kernel starts booting. 86 87 2.6.10 and later kernels have an "early uart" driver that works 88 very early in the boot process. The kernel will automatically use 89 this if the user supplies an argument like "console=uart,io,0x3f8", 90 or if the EFI console path contains only a UART device and the 91 firmware supplies an HCDP. 92 93Troubleshooting Serial Console Problems 94======================================= 95 96 No kernel output after elilo prints "Uncompressing Linux... done": 97 98 - You specified "console=ttyS0" but Linux changed the device 99 to which ttyS0 refers. Configure exactly one EFI console 100 device[3] and remove the "console=" option. 101 102 - The EFI console path contains both a VGA device and a UART. 103 EFI and elilo use both, but Linux defaults to VGA. Remove 104 the VGA device from the EFI console path[3]. 105 106 - Multiple UARTs selected as EFI console devices. EFI and 107 elilo use all selected devices, but Linux uses only one. 108 Make sure only one UART is selected in the EFI console 109 path[3]. 110 111 - You're connected to an HP MP port[2] but have a non-MP UART 112 selected as EFI console device. EFI uses the MP as a 113 console device even when it isn't explicitly selected. 114 Either move the console cable to the non-MP UART, or change 115 the EFI console path[3] to the MP UART. 116 117 Long pause (60+ seconds) between "Uncompressing Linux... done" and 118 start of kernel output: 119 120 - No early console because you used "console=ttyS<n>". Remove 121 the "console=" option if your firmware supplies an HCDP. 122 123 - If you don't have an HCDP, the kernel doesn't know where 124 your console lives until the driver discovers serial 125 devices. Use "console=uart,io,0x3f8" (or appropriate 126 address for your machine). 127 128 Kernel and init script output works fine, but no "login:" prompt: 129 130 - Add getty entry to /etc/inittab for console tty. Look for 131 the "Adding console on ttyS<n>" message that tells you which 132 device is the console. 133 134 "login:" prompt, but can't login as root: 135 136 - Add entry to /etc/securetty for console tty. 137 138 No ACPI serial devices found in 2.6.17 or later: 139 140 - Turn on CONFIG_PNP and CONFIG_PNPACPI. Prior to 2.6.17, ACPI 141 serial devices were discovered by 8250_acpi. In 2.6.17, 142 8250_acpi was replaced by the combination of 8250_pnp and 143 CONFIG_PNPACPI. 144 145 146 147[1] 148 http://www.dig64.org/specifications/agreement 149 The table was originally defined as the "HCDP" for "Headless 150 Console/Debug Port." The current version is the "PCDP" for 151 "Primary Console and Debug Port Devices." 152 153[2] 154 The HP MP (management processor) is a PCI device that provides 155 several UARTs. One of the UARTs is often used as a console; the 156 EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart". 157 The external connection is usually a 25-pin connector, and a 158 special dongle converts that to three 9-pin connectors, one of 159 which is labelled "Console." 160 161[3] 162 EFI console devices are configured using the EFI Boot Manager 163 "Boot option maintenance" menu. You may have to interrupt the 164 boot sequence to use this menu, and you will have to reset the 165 box after changing console configuration. 166