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 ==============
 Serial Devices
 ==============
 
 Serial Device Naming
 ====================
 
     As of 2.6.10, serial devices on ia64 are named based on the
     order of ACPI and PCI enumeration.  The first device in the
     ACPI namespace (if any) becomes /dev/ttyS0, the second becomes
     /dev/ttyS1, etc., and PCI devices are named sequentially
     starting after the ACPI devices.
 
     Prior to 2.6.10, there were confusing exceptions to this:
 
         - Firmware on some machines (mostly from HP) provides an HCDP
           table[1] that tells the kernel about devices that can be used
           as a serial console.  If the user specified "console=ttyS0"
           or the EFI ConOut path contained only UART devices, the
           kernel registered the device described by the HCDP as
           /dev/ttyS0.
 
         - If there was no HCDP, we assumed there were UARTs at the
           legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so
           the kernel registered those as /dev/ttyS0 and /dev/ttyS1.
 
     Any additional ACPI or PCI devices were registered sequentially
     after /dev/ttyS0 as they were discovered.
 
     With an HCDP, device names changed depending on EFI configuration
     and "console=" arguments.  Without an HCDP, device names didn't
     change, but we registered devices that might not really exist.
 
     For example, an HP rx1600 with a single built-in serial port
     (described in the ACPI namespace) plus an MP[2] (a PCI device) has
     these ports:
 
       ==========  ==========     ============    ============   =======
       Type        MMIO           pre-2.6.10      pre-2.6.10     2.6.10+
                   address
                                  (EFI console    (EFI console
                                  on builtin)     on MP port)
       ==========  ==========     ============    ============   =======
       builtin     0xff5e0000        ttyS0           ttyS1         ttyS0
       MP UPS      0xf8031000        ttyS1           ttyS2         ttyS1
       MP Console  0xf8030000        ttyS2           ttyS0         ttyS2
       MP 2        0xf8030010        ttyS3           ttyS3         ttyS3
       MP 3        0xf8030038        ttyS4           ttyS4         ttyS4
       ==========  ==========     ============    ============   =======
 
 Console Selection
 =================
 
     EFI knows what your console devices are, but it doesn't tell the
     kernel quite enough to actually locate them.  The DIG64 HCDP
     table[1] does tell the kernel where potential serial console
     devices are, but not all firmware supplies it.  Also, EFI supports
     multiple simultaneous consoles and doesn't tell the kernel which
     should be the "primary" one.
 
     So how do you tell Linux which console device to use?
 
         - If your firmware supplies the HCDP, it is simplest to
           configure EFI with a single device (either a UART or a VGA
           card) as the console.  Then you don't need to tell Linux
           anything; the kernel will automatically use the EFI console.
 
           (This works only in 2.6.6 or later; prior to that you had
           to specify "console=ttyS0" to get a serial console.)
 
         - Without an HCDP, Linux defaults to a VGA console unless you
           specify a "console=" argument.
 
     NOTE: Don't assume that a serial console device will be /dev/ttyS0.
     It might be ttyS1, ttyS2, etc.  Make sure you have the appropriate
     entries in /etc/inittab (for getty) and /etc/securetty (to allow
     root login).
 
 Early Serial Console
 ====================
 
     The kernel can't start using a serial console until it knows where
     the device lives.  Normally this happens when the driver enumerates
     all the serial devices, which can happen a minute or more after the
     kernel starts booting.
 
     2.6.10 and later kernels have an "early uart" driver that works
     very early in the boot process.  The kernel will automatically use
     this if the user supplies an argument like "console=uart,io,0x3f8",
     or if the EFI console path contains only a UART device and the
     firmware supplies an HCDP.
 
 Troubleshooting Serial Console Problems
 =======================================
 
     No kernel output after elilo prints "Uncompressing Linux... done":
 
         - You specified "console=ttyS0" but Linux changed the device
           to which ttyS0 refers.  Configure exactly one EFI console
           device[3] and remove the "console=" option.
 
         - The EFI console path contains both a VGA device and a UART.
           EFI and elilo use both, but Linux defaults to VGA.  Remove
           the VGA device from the EFI console path[3].
 
         - Multiple UARTs selected as EFI console devices.  EFI and
           elilo use all selected devices, but Linux uses only one.
           Make sure only one UART is selected in the EFI console
           path[3].
 
         - You're connected to an HP MP port[2] but have a non-MP UART
           selected as EFI console device.  EFI uses the MP as a
           console device even when it isn't explicitly selected.
           Either move the console cable to the non-MP UART, or change
           the EFI console path[3] to the MP UART.
 
     Long pause (60+ seconds) between "Uncompressing Linux... done" and
     start of kernel output:
 
         - No early console because you used "console=ttyS<n>".  Remove
           the "console=" option if your firmware supplies an HCDP.
 
         - If you don't have an HCDP, the kernel doesn't know where
           your console lives until the driver discovers serial
           devices.  Use "console=uart,io,0x3f8" (or appropriate
           address for your machine).
 
     Kernel and init script output works fine, but no "login:" prompt:
 
         - Add getty entry to /etc/inittab for console tty.  Look for
           the "Adding console on ttyS<n>" message that tells you which
           device is the console.
 
     "login:" prompt, but can't login as root:
 
         - Add entry to /etc/securetty for console tty.
 
     No ACPI serial devices found in 2.6.17 or later:
 
         - Turn on CONFIG_PNP and CONFIG_PNPACPI.  Prior to 2.6.17, ACPI
           serial devices were discovered by 8250_acpi.  In 2.6.17,
           8250_acpi was replaced by the combination of 8250_pnp and
           CONFIG_PNPACPI.
 
 
 
 [1]
     http://www.dig64.org/specifications/agreement
     The table was originally defined as the "HCDP" for "Headless
     Console/Debug Port."  The current version is the "PCDP" for
     "Primary Console and Debug Port Devices."
 
 [2]
     The HP MP (management processor) is a PCI device that provides
     several UARTs.  One of the UARTs is often used as a console; the
     EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
     The external connection is usually a 25-pin connector, and a
     special dongle converts that to three 9-pin connectors, one of
     which is labelled "Console."
 
 [3]
     EFI console devices are configured using the EFI Boot Manager
     "Boot option maintenance" menu.  You may have to interrupt the
     boot sequence to use this menu, and you will have to reset the
     box after changing console configuration.

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