Documentation/ia64/serial.txt

   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/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 [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.