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Date:   Wed, 8 Jan 2020 15:55:34 +0100
From:   Ahmad Fatoum <>
Cc:     Sascha Hauer <>
Subject: [BUG] pfifo_fast may cause out-of-order CAN frame transmission


I've run into an issue of CAN frames being sent out-of-order on an i.MX6 Dual
with Linux v5.5-rc5. Bisecting has lead me down to this commit:

ba27b4cdaaa ("net: dev: introduce support for sch BYPASS for lockless qdisc")

With it, using pfifo_fast, every few hundred frames, FlexCAN's .ndo_start_xmit is
passed frames in an order different from how userspace stuffed them into the same

Reverting it fixes the issue as does booting with maxcpus=1 or using pfifo
instead of pfifo_fast.

According to [1], such reordering shouldn't be happening.

Details on my setup:
Kernel version: v5.5-rc5, (occurs much more often with LOCKDEP turned on)
CAN-Bitrate: 250 kbit/s
CAN frames are generated with:
cangen canX -I2 -L1 -Di -i -g0.12 -p 100
which keeps polling after ENOBUFS until socket is writable, sends out a CAN
frame with one incrementing payload byte and then waits 120 usec before repeating.

Please let me know if any additional info is needed.



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