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1 | ============== |
2 | Serial Devices | |
3 | ============== | |
4 | ||
5 | Serial Device Naming | |
6 | ==================== | |
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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 | ||
db9a0975 MCC |
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 | ========== ========== ============ ============ ======= | |
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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 | |
db9a0975 | 49 | ========== ========== ============ ============ ======= |
1da177e4 | 50 | |
db9a0975 MCC |
51 | Console Selection |
52 | ================= | |
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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 | ||
db9a0975 MCC |
79 | Early Serial Console |
80 | ==================== | |
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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 | ||
db9a0975 MCC |
93 | Troubleshooting Serial Console Problems |
94 | ======================================= | |
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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 | |
339d541a | 125 | devices. Use "console=uart,io,0x3f8" (or appropriate |
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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 | ||
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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 | ||
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145 | |
146 | ||
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147 | [1] |
148 | http://www.dig64.org/specifications/agreement | |
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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 | ||
db9a0975 MCC |
153 | [2] |
154 | The HP MP (management processor) is a PCI device that provides | |
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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 | ||
db9a0975 MCC |
161 | [3] |
162 | EFI console devices are configured using the EFI Boot Manager | |
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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. |