1==============
2Serial Devices
3==============
4
5Serial Device Naming
6====================
7
8    As of 2.6.10, serial devices on ia64 are named based on the
9    order of ACPI and PCI enumeration.  The first device in the
10    ACPI namespace (if any) becomes /dev/ttyS0, the second becomes
11    /dev/ttyS1, etc., and PCI devices are named sequentially
12    starting after the ACPI devices.
13
14    Prior to 2.6.10, there were confusing exceptions to this:
15
16	- Firmware on some machines (mostly from HP) provides an HCDP
17	  table[1] that tells the kernel about devices that can be used
18	  as a serial console.  If the user specified "console=ttyS0"
19	  or the EFI ConOut path contained only UART devices, the
20	  kernel registered the device described by the HCDP as
21	  /dev/ttyS0.
22
23	- If there was no HCDP, we assumed there were UARTs at the
24	  legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so
25	  the kernel registered those as /dev/ttyS0 and /dev/ttyS1.
26
27    Any additional ACPI or PCI devices were registered sequentially
28    after /dev/ttyS0 as they were discovered.
29
30    With an HCDP, device names changed depending on EFI configuration
31    and "console=" arguments.  Without an HCDP, device names didn't
32    change, but we registered devices that might not really exist.
33
34    For example, an HP rx1600 with a single built-in serial port
35    (described in the ACPI namespace) plus an MP[2] (a PCI device) has
36    these ports:
37
38      ==========  ==========     ============    ============   =======
39      Type        MMIO           pre-2.6.10      pre-2.6.10     2.6.10+
40		  address
41				 (EFI console    (EFI console
42                                 on builtin)     on MP port)
43      ==========  ==========     ============    ============   =======
44      builtin     0xff5e0000        ttyS0           ttyS1         ttyS0
45      MP UPS      0xf8031000        ttyS1           ttyS2         ttyS1
46      MP Console  0xf8030000        ttyS2           ttyS0         ttyS2
47      MP 2        0xf8030010        ttyS3           ttyS3         ttyS3
48      MP 3        0xf8030038        ttyS4           ttyS4         ttyS4
49      ==========  ==========     ============    ============   =======
50
51Console Selection
52=================
53
54    EFI knows what your console devices are, but it doesn't tell the
55    kernel quite enough to actually locate them.  The DIG64 HCDP
56    table[1] does tell the kernel where potential serial console
57    devices are, but not all firmware supplies it.  Also, EFI supports
58    multiple simultaneous consoles and doesn't tell the kernel which
59    should be the "primary" one.
60
61    So how do you tell Linux which console device to use?
62
63	- If your firmware supplies the HCDP, it is simplest to
64	  configure EFI with a single device (either a UART or a VGA
65	  card) as the console.  Then you don't need to tell Linux
66	  anything; the kernel will automatically use the EFI console.
67
68	  (This works only in 2.6.6 or later; prior to that you had
69	  to specify "console=ttyS0" to get a serial console.)
70
71	- Without an HCDP, Linux defaults to a VGA console unless you
72	  specify a "console=" argument.
73
74    NOTE: Don't assume that a serial console device will be /dev/ttyS0.
75    It might be ttyS1, ttyS2, etc.  Make sure you have the appropriate
76    entries in /etc/inittab (for getty) and /etc/securetty (to allow
77    root login).
78
79Early Serial Console
80====================
81
82    The kernel can't start using a serial console until it knows where
83    the device lives.  Normally this happens when the driver enumerates
84    all the serial devices, which can happen a minute or more after the
85    kernel starts booting.
86
87    2.6.10 and later kernels have an "early uart" driver that works
88    very early in the boot process.  The kernel will automatically use
89    this if the user supplies an argument like "console=uart,io,0x3f8",
90    or if the EFI console path contains only a UART device and the
91    firmware supplies an HCDP.
92
93Troubleshooting Serial Console Problems
94=======================================
95
96    No kernel output after elilo prints "Uncompressing Linux... done":
97
98	- You specified "console=ttyS0" but Linux changed the device
99	  to which ttyS0 refers.  Configure exactly one EFI console
100	  device[3] and remove the "console=" option.
101
102	- The EFI console path contains both a VGA device and a UART.
103	  EFI and elilo use both, but Linux defaults to VGA.  Remove
104	  the VGA device from the EFI console path[3].
105
106	- Multiple UARTs selected as EFI console devices.  EFI and
107	  elilo use all selected devices, but Linux uses only one.
108	  Make sure only one UART is selected in the EFI console
109	  path[3].
110
111	- You're connected to an HP MP port[2] but have a non-MP UART
112	  selected as EFI console device.  EFI uses the MP as a
113	  console device even when it isn't explicitly selected.
114	  Either move the console cable to the non-MP UART, or change
115	  the EFI console path[3] to the MP UART.
116
117    Long pause (60+ seconds) between "Uncompressing Linux... done" and
118    start of kernel output:
119
120	- No early console because you used "console=ttyS<n>".  Remove
121	  the "console=" option if your firmware supplies an HCDP.
122
123	- If you don't have an HCDP, the kernel doesn't know where
124	  your console lives until the driver discovers serial
125	  devices.  Use "console=uart,io,0x3f8" (or appropriate
126	  address for your machine).
127
128    Kernel and init script output works fine, but no "login:" prompt:
129
130	- Add getty entry to /etc/inittab for console tty.  Look for
131	  the "Adding console on ttyS<n>" message that tells you which
132	  device is the console.
133
134    "login:" prompt, but can't login as root:
135
136	- Add entry to /etc/securetty for console tty.
137
138    No ACPI serial devices found in 2.6.17 or later:
139
140	- Turn on CONFIG_PNP and CONFIG_PNPACPI.  Prior to 2.6.17, ACPI
141	  serial devices were discovered by 8250_acpi.  In 2.6.17,
142	  8250_acpi was replaced by the combination of 8250_pnp and
143	  CONFIG_PNPACPI.
144
145
146
147[1]
148    http://www.dig64.org/specifications/agreement
149    The table was originally defined as the "HCDP" for "Headless
150    Console/Debug Port."  The current version is the "PCDP" for
151    "Primary Console and Debug Port Devices."
152
153[2]
154    The HP MP (management processor) is a PCI device that provides
155    several UARTs.  One of the UARTs is often used as a console; the
156    EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
157    The external connection is usually a 25-pin connector, and a
158    special dongle converts that to three 9-pin connectors, one of
159    which is labelled "Console."
160
161[3]
162    EFI console devices are configured using the EFI Boot Manager
163    "Boot option maintenance" menu.  You may have to interrupt the
164    boot sequence to use this menu, and you will have to reset the
165    box after changing console configuration.
166