udev: add a few comments about variables used to recognize events

diff --git a/udev/10-dm.rules.in b/udev/10-dm.rules.in
index 512d156ddde6b2aadd5e417bee2109f10b2eb0c2..3a157d53825379a93c5afed28f33ca5647cea4af 100644 (file)
--- a/udev/10-dm.rules.in
+++ b/udev/10-dm.rules.in
@@ -77,11 +77,29 @@ LABEL="dm_flags_done"
 # before (e.g. in initrd). If udev is used in initrd, we require the udev init
 # script to not remove the existing udev database so we can reuse the information
 # stored at the time of device activation in the initrd.
+# The DM_ACTIVATION variable tells when any device stacked above should be
+# (re)activated as well.
 ACTION!="add", GOTO="dm_no_coldplug"
 ENV{DM_UDEV_RULES_VSN}!="1", ENV{DM_UDEV_PRIMARY_SOURCE_FLAG}!="1", GOTO="dm_disable"
 ENV{DM_ACTIVATION}="1"
 LABEL="dm_no_coldplug"
 
+# Putting it together, following table is used to recognize genuine and spurious events.
+# N.B. Spurious events are generated based on use of the WATCH udev
+# rule or by triggering an event manually by "udevadm trigger" call
+# or by "echo <event_name> > /sys/block/dm-X/uevent".
+#
+#        EVENT              DM_UDEV_PRIMARY_SOURCE_FLAG   DM_ACTIVATION
+# ======================================================================
+# add event (genuine)                  0                        0
+# change event (genuine)               1                        1
+# add event (spurious)
+#   |_ dev still not active            0                        0
+#   \_ dev already active              1                        1
+# change event (spurious)
+#   |_ dev still not active            0                        0
+#   \_ dev already active              1                        0
+
 # "dm" sysfs subdirectory is available in newer versions of DM
 # only (kernels >= 2.6.29). We have to check for its existence
 # and use dmsetup tool instead to get the DM name, uuid and