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
128
129 [1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf
130 The table was originally defined as the "HCDP" for "Headless
131 Console/Debug Port." The current version is the "PCDP" for
132 "Primary Console and Debug Port Devices."
133
134 [2] The HP MP (management processor) is a PCI device that provides
135 several UARTs. One of the UARTs is often used as a console; the
136 EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
137 The external connection is usually a 25-pin connector, and a
138 special dongle converts that to three 9-pin connectors, one of
139 which is labelled "Console."
140
141 [3] EFI console devices are configured using the EFI Boot Manager
142 "Boot option maintenance" menu. You may have to interrupt the
143 boot sequence to use this menu, and you will have to reset the
144 box after changing console configuration.
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