Documentation/ia64/serial.txt - kernel/common - Git at Google

 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.

     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/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.
	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.

	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/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.