serial.txt 6.4 KB

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