Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
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 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 (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 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 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. 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. 99 100 - You're connected to an HP MP port 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 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  http://www.dig64.org/specifications/agreement 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  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  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.