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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

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.
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