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Add Ottawa Linux Symposium 2011 and 2012 index pages.
author Rob Landley <rob@landley.net>
date Fri, 26 Jul 2013 15:23:43 -0500
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1 <html>
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2 <title>The history of hotplug</title>
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3 <body>
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4
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5 <h1><b>The history of hotplug.</b></h1>
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6
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7 <h2>What is hotplug, what problems does it solve, why do we have the current
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8 set of hotplug mechanisms, and what legacy mechanisms did the current
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9 hotplug implementation obsolete?</h2>
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11 <ul>
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12 <li><a href="#before">Before hotplug</a></li>
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13 <li><a href="#removable">Removable media</a></li>
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14 <li><a href="#modules">Modules</a></li>
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15
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16 <h2><a name="before"><b>Before hotplug: static everything.</b></h2>
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17
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18 <p>Originally, the kernel had no hotplug capability. A kernel without hotplug
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19 manages a fixed set of hardware, all of which is detected and initialized at
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20 boot time, and all of which remains present until the system is shut down.
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21 This is very simple, but also very limited.</p>
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22
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23 <p>This meant device drivers statically linked into the kernel image,
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24 and a /dev directory filled of device nodes for every potential device
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25 when the system was installed. A program that wanted to probe for available
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26 hardware sifted through /dev and opened devices it found there, elminating
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27 the ones which gave an -ENODEV error.</p>
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28
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29 <p>One reason for this was simplicity, but equally important was that early PCs
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30 were not designed around hotpluggable hardware, and Linux started out on a PC.
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31 When the PC was introduced, users couldn't even switch keyboards while the
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32 machine was on without risking hardware damage.</p>
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33
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34 <p>[FOOTNOTE]Of course users widely ignored this constraint whenever they could. Users
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35 hotplugged keyboards, serial, and parallel devices all the time, no matter
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36 what the manufacturer said, and by the late 80's most hardware developers had
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37 adapted to reality and buffered their more vulnerable external I/O ports. But
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38 the number of keyboard, serial, and parallel ports on the machine remained
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39 fixed, and each port could handle only one device at a time, so drivers focused
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40 on handling ports and left figuring out what device was behind an I/O port to
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41 the userspace application trying to talk to that device.[/FOOTNOTE]</p>
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42
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43 <h2><a name="removable">Removable media</h2>
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44
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45 <p>The original PC did have one type of early hotplug: it had removable storage
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46 media in the form of floppy drives (and later, CD-ROM drives, zip disks,
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47 and DVD drives). The first stirrings of hotplug support came from Linux's
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48 need to cope with removable media.</p>
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49
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50 <p>With removable media, the contents of the corresponding block devices
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51 changed, including even the size of the media represented by those block
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52 devices. Since filesystems could depend on those block devices, and processes
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53 depended on those filesystems, in extreme cases ejecting a floppy could
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54 lead to a kernel panic.</p>
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55
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56 <p>The kernel's response to this was to ignore as much of the problem as
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57 possible, and work around the rest. Removable media was treated as a special
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58 case, and the kernel grew workarounds rather than any real systematic
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59 solution to a larger generic problem.</p>
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60
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61 <p>Since the drives themselves stayed around awaiting the insertion of
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62 new media, media were treated as a property of drives, and most drives could
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63 have exactly one instance of removable media in them at a time anyway.
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64 [FOOTNOTE]There were "jukebox" style multi-CD changers, but they were poorly
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65 supported and mostly treated like a single drive with multiple
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66 partitions.[/FOOTNOTE]
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67 So device drivers used device nodes representing the drive instead
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68 of the media, and when the drive contained no media the driver would respond to
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69 attempts to access the drive's device node with error codes.</p>
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70
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71 <p>Poor hardware support for hotplug continued to be a problem: most removable
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72 media provided no notification mechanism to inform the system
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73 when media was inserted or removed. The driver could probe the device to
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74 see what media it contained at any given moment, but no interrupt was
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75 generated to signal changes. Thus the kernel
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76 had no way to respond to the a block device's removal except via extensive
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77 error handling after the fact.</p>
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78
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79 <p>Applications using removable media probed for them or
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80 received error codes on attempted access to an empty drive, and the kernel
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81 developer's advice about dealing with the problems of removing a mounted
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82 volume was "don't do that": inserting or removing media when the system didn't
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83 expect it was dismissed as user error.</p>
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84
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85 <h3>A workaround: drive locking</h2>
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86
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87 <p>To avoid being surprised by the unexpected removal of media containing
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88 a mounted filesystem, some drives grew the ability for software to "lock"
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89 a drive, preventing it from ejecting its media until it was unlocked.
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90 (Pressing the eject button still didn't generate an interrupt, it simply had
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91 no effect until the software unlocked the drive.) This let the operating
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92 system force users to eject removable media from software (via the "eject"
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93 command) rather than by pressing the button on the drive, allowing the OS to
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94 safely use the block device at the expense of annoying users.</p>
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95
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96 <p>By locking the drive to prevent unauthorized eject, the hotplug-less kernel
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97 avoided having to unmount filesystems on short notice. This meant the kernel
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98 didn't have to promptly flush buffers when data was written to the device (to
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99 minimize unmount time), and that the kernel could veto attempts to unmount a
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100 filesystem that was still in use for any reason, such as due to any process
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101 having open files in that filesystem or that filesystem containing any
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102 process's current directory. (Yes, even though a process's current directory
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103 could be deleted, it couldn't be unmounted. Not for any deep technical reason;
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104 support for it simply hadn't been implemented. Removable media was a poorly
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105 supported afterthought.)</p>
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106
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107 <p>Of course some drives (most notably PC floppy drives) had no provision for
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108 locking the drive; ejecting a floppy was a manual process controlled by a
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109 purely mechanical button. Users that didn't remember to manually unmount a
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110 floppy lost data, and were largely mocked as clueless by traditional Unix
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111 developers (or else PC hardware was mocked for not having drive locking
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112 support). The "mtools" package provided a popular workaround, reading and
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113 writing FAT files directly through a floppy disk's unmounted block device,
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114 probing for media before each command, and flushing all data after each
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115 command. (It even accepted dos-style names for floppy drives.)</p>
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116
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117 <p>As late as Linux 1.0, support for eject was still a special case for CDROM
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118 drives (/include/linux/cdrom.h had a "#define CROMEJECT"), and door locking
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119 support was a special case for SCSI (/drivers/scsi/sci_ioctl.h #defined
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120 SCSI_IOCTL_DOORLOCK). As late as Linux 2.4, the underlying problems
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121 with dynamically unplugging block devices were considered too hard to
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122 properly solve.</p>
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123
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124 <h2><a name="modules">Modules</h2>
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125
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126 <p>The first serious hotplug mechanism in Linux was modules, because modules
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127 allow device drivers to be loaded after the kernel boots and unloaded again
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128 before shutdown.</p>
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129
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130 <p>Hotplug was a side effect, since most hardware used by Linux still wasn't
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131 hotpluggable. The primary motivation for modules was reducing the memory
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132 footprint of kernels, which was increasing due to the proliferation of device
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133 drivers. A generically configured kernel, such as those in the emerging Linux
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134 distributions, needed to be built with drivers for every piece of hardware it
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135 might encounter, but most systems it ran on would use only a small subset of
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136 those drivers.</p>
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137
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138 <p>Modules meant that device drivers could probe for hardware present when the
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139 module was inserted, after the kernel booted. A module that failed to find any
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140 devices (of the kind it contained a device driver for) could refuse to load, so
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141 attempting to load all modules was a simple way of probing for available
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142 hardware. Modules could even be removed and re-inserted to scan for and handle
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143 new hardware.</p>
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144
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145 <p>This was an improvement, but not a complete solution. Using modules as the
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146 primary hotplug mechanism quickly revealed numerous deficiencies: the
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147 granularity was wrong, there were insufficient notification mechanisms, and
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148 this approach didn't handle configuration issues like device nodes.</p>
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149
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150 <p>The granularity is wrong because a module encapsulates a device driver, and
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151 one instance of a driver can manage multiple instances of a device. When
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152 inserting a second instance of a device into a system, removing the module to
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153 reinsert it (and thus find the new instance of the device) takes the first
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154 instance of the device offline. This is an extremely unpleasant side effect,
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155 and not always possible if the device is in use.</p>
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156
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157 <pre>
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158 More to come...
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159
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160 Ad-hoc mechanisms to rescan busses
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161
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162
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163 module manages muliple devices; insert a second ethernet card.
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164 Need a way to tell module to rescan devices.
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165 Still need a separate notification mechanism to trigger module loading.
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166 Module loading either has to happen _after_ device insertion, or module
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167 has to be told to rescan after module loaded.
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168 Doesn't handle unplug.
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169 Notification problem worse: ideally need to know before device goes away
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170 so flush buffers, umount filesystems, close file handles, etc. Cleanup.
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171 Unloading module.
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172 Doesn't handle /dev entries.
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173 Fill up /dev with every possible device, there could be thousands of them.
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174 (Every possible partition on every possible hard drive...)
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175 How does userspace know which ones are active? (In a static /dev, presence
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176 of an entry gives no information about whether or not the device is there.
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177 Iterate through the lot and test. Very slow, timeouts, generates spurious
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178 activity that can have unwanted side effects like spinning up drives...)
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179
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180
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181 <h2>PCMCIA</h2> The arrival of PCMCIA (a hotpluggable 16-bit expansion card bus for early laptops) provided the first , USB, laptop docking stations,
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182
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183 static everything.
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184 modules
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185 The /proc directory
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186 Rescan scsi bus via /proc/scsi
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187 http://bash.cyberciti.biz/diskadmin/rescan-scsi-bus.sh.php
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188 laptops (pcmcia)
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189 USB
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190 Added in 2.2.7, written by Linus Torvalds throwing out earlier work.
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191 devfs
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192 Having the driver detect the presence of hardware is backwards. Driver
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193 loads in response to device being plugged in, but the driver detects the
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194 existence of the device... chicken and egg problem.
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195
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196 The biggest problem with devfs
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197 is that device nodes and physical devices aren't the same thing. Some device
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198 nodes have no corresponding hardware (such as /dev/null), and some devices
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199 have multiple device nodes (such as partitioned hard drives). Devfs had no
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200 way to tell userspace what actual devices the system had separate from what
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201 drivers were loaded, which was especially problematic when a newly inserted
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202 device required some action from userspace (loading modules or firmware) before
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203 device nodes could be created for it. And some USB devices can be driven
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204 entirely from userspace, with no kernel device driver
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205
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206 A device that requires some action to
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207 be taken (such as a userspace program loading firmware into it) between
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208 device insertion and device node creation, Suppose device that requires firmware to
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209 be loaded (by userspace) before a device driver can probe it and create
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210 appropriate /dev nodes
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211 of information was missing from userspace.</p>
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212
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213
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214
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215 sysfs
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216 Finally, the modern approach.
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217 /sbin/hotplug vs netlink
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218
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219 Also, devfs provides /dev entries, but that's the wrong layer. Some
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220 hardware provides multiple /dev entries (partitioned hard drives), some
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221 dev entries have no underlying hardware (/dev/zero, /dev/null, network
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222 block devices), and some devices have no /dev entry (ethernet, for
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223 historical reasons).
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224
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225
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226
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227
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228
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229
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230 <h2>Static drivers, modules, and hotplug.</h2>
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231
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232 A kernel without any hotplug capability manages a fixed set of hardware,
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233 all of which is detected and initialized at boot time, and all of which remains
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234 present until the system is shut down.
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235
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236 The Linux module loading mechanism allowed drivers to be loaded after the
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237 system boots
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238
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239
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240
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241 Early atte
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242
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243 Hotplug allows the kernel to dynamically respond to the addition or removal of
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244 hardware. At boot time,
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245
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246 /sbin/hotplug or netlink.
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247 Support for hotplug in Linux evolved out of Linus's rewrite of USB
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248 </body>
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249 </html>
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250
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251 note:
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252 What's the probe for removable media? (ioctl()?)
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253
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254 http://kerneltrap.org/node/1893
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255 http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev_vs_devfs
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256 http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ
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257 http://kerneltrap.org/node/5340
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258 http://kerneltrap.org/node/6744
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259 </pre>