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mdev: support for serializing hotplug

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function                                             old     new   delta
mdev_main                                            503     647    +144
This commit is contained in:
Denis Vlasenko
2008-07-15 22:33:13 +00:00
parent 97faf531c8
commit 018bee6afb
2 changed files with 51 additions and 3 deletions
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23
docs/mdev.txt
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@ -95,3 +95,26 @@ properly initialize a device. Place all such firmware files into the
filename of the firmware which mdev will load out of /lib/firmware/ and into
the kernel via the sysfs interface. The exact filename is hardcoded in the
kernel, so look there if you need to know how to name the file in userspace.
------------
SEQUENCING
------------
Kernel does not serialize hotplug events. It increments SEQNUM environmental
variable for each successive hotplug invocation. Normally, mdev doesn't care.
This may reorder hotplug and hot-unplug events, with typical symptoms of
device nodes sometimes not created as expected.
However, if /dev/mdev.seq file is found, mdev will compare its
contents with SEQNUM. It will retry up to two seconds, waiting for them
to match. If they match exactly (not even trailing '\n' is allowed),
or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
with SEQNUM+1.
IOW: this will serialize concurrent mdev invocations.
If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
setting mdev to be the hotplug handler. This writes single '\n' to the file.
NB: mdev recognizes /dev/mdev.seq consisting of single '\n' characher
as a special case. IOW: this will not make your first hotplug event
to stall for two seconds.