Documentation/ia64/serial.txt - arm/linux - 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/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.
	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/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.