net: Add up_before_preup variable for CAN devices

Historically, we have tried to up interfaces before running preup, so
that the kernel setups up the device and makes things like ethtool work
(some hardware cannot be correct probed until then). However this ends
up breaking other hardware, so a variable has been introduced to allow
the up prior to preup to be disabled: up_before_preup_IFVAR=no

X-Gentoo-Bug: 389475
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=389475
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/net.example.Linux.in b/doc/net.example.Linux.in
index 0ee83a6e..d48983ee 100644
--- a/doc/net.example.Linux.in
+++ b/doc/net.example.Linux.in
@@ -1102,6 +1102,16 @@
 #
 # ${IFACE} is set to the interface being brought up/down
 # ${IFVAR} is ${IFACE} converted to variable name bash allows
+#
+# For historical & compatbility reasons, preup is actually normally called in
+# the follow sequence: up ; preup ; up
+# In that case, the first up causes the kernel to initialize the device, so
+# that it is available for use in the preup function.  However, for some other
+# hardware, eg CAN devices, some configuration is needed before trying to up
+# the interface will actually work. For such harware, there are the
+# 'up_before_preup' variables, that skips the first up call.
+#up_before_preup_IFVAR=no
+#up_before_preup=no
 
 #preup() {
 #	# Test for link on the interface prior to bringing it up.  This