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Update CD symlinks section to use upstream rule_generator rules instead of generating our own. Fixes part of #1912.

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git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@7923 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689
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Bryan Kadzban 2007-02-17 23:30:52 +00:00
parent aeb956f6e3
commit 84dbfdac1d
2 changed files with 65 additions and 70 deletions
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5
chapter01/changelog.xml
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@ -39,6 +39,11 @@
<listitem>
<para>2007-02-17</para>
<itemizedlist>
<listitem>
<para>[bryan] - Change from writing CD symlink rules files directly
to configuring the file installed by Udev's rule_generator. Fixes
part of <ulink url="&lfs-ticket-root;1912">#1912</ulink>.</para>
</listitem>
<listitem>
<para>[bryan] - Update contents of lfs-bootscripts.</para>
</listitem>

130
chapter07/symlinks.xml
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@ -15,84 +15,74 @@
<title>CD-ROM symlinks</title>
<para>Some software that you may want to install later (e.g., various
media players) expect the /dev/cdrom and /dev/dvd symlinks to exist.
Also, it may be convenient to put references to those symlinks into
<filename>/etc/fstab</filename>. For each of your CD-ROM devices,
find the corresponding directory under
media players) expect the <filename class="symlink">/dev/cdrom</filename>
and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
references to those symlinks into <filename>/etc/fstab</filename>. Udev
comes with a script that will generate rules files to create these symlinks
for you, depending on the capabilities of each device, but you need to
decide which of two modes of operation you wish to have the script use.</para>
<para>First, the script can operate in <quote>by-path</quote> mode, where
the rules it creates depend on the physical path to the CD or DVD device.
Second, it can operate in <quote>by-id</quote> mode, where the rules it
creates depend on identification strings stored in the CD or DVD device
itself. The path is determined by Udev's <command>path_id</command> script,
and the identification strings are read from the hardware by its
<command>ata_id</command> or <command>scsi_id</command> programs, depending
on which type of device you have.</para>
<para>There are advantages to each approach; the correct approach to use
will depend on what kinds of device changes may happen. If you expect the
physical path to the device (that is, the ports and/or slots that it plugs
into) to change, for example because you plan on moving the drive to a
different IDE port or a different USB connector, then you should use the
<quote>by-id</quote> mode. On the other hand, if you expect the device's
identification to change, for example because it may die, and you would
replace it with a different device with the same capabilities and which
is plugged into the same connectors, then you should use the
<quote>by-path</quote> mode.</para>
<para>If either type of change is possible with your drive, then choose a
mode based on the type of change you expect to happen more often.</para>
<!-- If you use by-id mode, the symlinks will survive even the transition
to libata for IDE drives, but that is not for the book. -->
<important>External devices (for example, a USB-connected CD drive) should
not use by-path persistence, because each time the device is plugged into a
new external port, its physical path will change. All externally-connected
devices will have this problem if you write Udev rules to recognize them
by their physical path; the problem is not limited to CD and DVD drives.</important>
<para>If you wish to see the values that the Udev scripts will use, then
for the appropriate CD-ROM device, find the corresponding directory under
<filename class="directory">/sys</filename> (e.g., this can be
<filename class="directory">/sys/block/hdd</filename>) and
run a command similar to the following:</para>
<screen role="nodump"><userinput>udevtest /block/hdd</userinput></screen>
<screen role="nodump"><userinput>udevtest /sys/block/hdd</userinput></screen>
<para>Look at the lines containing the output of various *_id programs.</para>
<para>Look at the lines containing the output of various *_id programs.
The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
is not empty, otherwise it will use a combination of ID_MODEL and
ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
<para>There are two approaches to creating symlinks. The first one is to
use the model name and the serial number, the second one is based on the
location of the device on the bus. If you are going to use the first
approach, create a file similar to the following:</para>
<para>If you choose the <quote>by-path</quote> mode, then the rules files
installed by default with Udev will work. If you choose the <quote>by-id</quote>
mode, then you will have to modify the
<filename>/etc/udev/rules.d/75-cd-aliases-generator.rules</filename> file,
as follows:</para>
<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/70-persistent-cd.rules &lt;&lt; EOF
<literal>
# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", \
ENV{ID_REVISION}=="PS05", ENV{GENERATED}="1", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", \
ENV{ID_SERIAL}=="5VO1306DM00190", ENV{GENERATED}="1", SYMLINK+="cdrom1 dvd"
</literal>
EOF</userinput></screen>
<screen><userinput>sed -i -e 's/write_cd_aliases/&amp; by-id/' \
/etc/udev/rules.d/75-cd-aliases-generator.rules</userinput></screen>
<note>
<para>Be aware that Udev does not recognize the backslash for line
continuation. The examples in this book work properly because both
the backslash and newline are ignored by the shell. This makes the
shell send each rule to cat on only one line. (The shell ignores
this sequence because the EOF string used in the here-document
redirection is not enclosed in either double or single quotes. For
more details, see the bash(1) manpage, and search it for "Here
Documents".)</para>
<para>If modifying Udev rules with an editor, be sure to leave each
rule on one physical line.</para>
</note>
<para>This way, the symlinks will stay correct even if you move the drives
to different positions on the IDE bus, but the
<filename>/dev/cdrom</filename> symlink won't be created if you replace
the old SAMSUNG CD-ROM with a new drive.</para>
<!-- The symlinks in the first approach survive even the transition
to libata for IDE drives, but that is not for the book. -->
<para>The SUBSYSTEM==&quot;block&quot; key is needed in order to avoid
matching SCSI generic devices. Without it, in the case with SCSI
CD-ROMs, the symlinks will sometimes point to the correct
<filename>/dev/srX</filename> devices, and sometimes to
<filename>/dev/sgX</filename>, which is wrong.</para>
<para>The ENV{GENERATED}="1" key is needed to prevent the Udev
75-cd-aliases-generator.rules file from overriding your custom
rules.</para>
<para>The second approach yields:</para>
<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/70-persistent-cd.rules &lt;&lt; EOF
<literal>
# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", \
ENV{GENERATED}="1", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", \
ENV{GENERATED}="1", SYMLINK+="cdrom1 dvd"
</literal>
EOF</userinput></screen>
<para>This way, the symlinks will stay correct even if you replace drives
with different models, but place them to the old positions on the IDE
bus. The ENV{ID_TYPE}==&quot;cd&quot; key makes sure that the symlink
disappears if you put something other than a CD-ROM in that position on
the bus.</para>
<para>Of course, it is possible to mix the two approaches.</para>
<para>Note that it is not necessary to create the rules files or symlinks
at this time, because you have bind-mounted the host's
<filename class="directory">/dev</filename> directory into the LFS system,
and we assume the symlinks exist and are correct on the host. The rules
will be created, along with the symlinks, the first time you boot your LFS
system.</para>
</sect2>