[IA64] Document serial device names on ia64

Document how Linux/ia64 names serial devices and how to use them
as console devices.
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

Documentation/ia64/serial.txt

+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:
+
+                                  pre-2.6.10      pre-2.6.10
+                    MMIO         (EFI console    (EFI console
+                   address        on builtin)     on MP port)    2.6.10
+                  ==========      ==========      ==========     ======
+      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.
+ 
+
+
+[1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf
+    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.