1SERIAL 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
43CONSOLE 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
70EARLY 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
83TROUBLESHOOTING 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.
152