netdev - tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved accuracy at high rates)

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Date:	Wed, 29 May 2013 15:13:30 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Stephen Hemminger <stephen@...workplumber.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...hat.com>, j.vimal@...il.com,
	Michal Soltys <soltys@....info>,
	Mike Frysinger <vapier@...too.org>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Patrick McHardy <kaber@...sh.net>,
	Jiri Pirko <jpirko@...hat.com>
Cc:	brouer@...hat.com,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Dave Taht <dave.taht@...il.com>, netdev@...r.kernel.org,
	bloat@...ts.bufferbloat.net, Dan Siemon <dan@...erfire.com>,
	Jim Gettys <jg@...edesktop.org>,
	Steven Barth <cyrus@...nwrt.org>, Felix Fietkau <nbd@....name>,
	Jiri Benc <jbenc@...hat.com>
Subject: tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved
 accuracy at high rates)

I recently discovered that the (traffic control) tc linklayer
calculations for ATM/ADSL have been broken by:
 commit 56b765b79 (htb: improved accuracy at high rates).

Thus, people shaping on ADSL links, using e.g.:
 tc class add ... htb rate X ceil Y linklayer atm overhead 10

Will no-longer get ATM cell tax/overhead adjusted.

How can we solve/fix this?
Perhaps we can change to use the "stab" system instead (as it does
not seem to be broken by the commit).

But how do we facilitate a change to use "stab" system (for all the
scripts using the old option)?

Can we change the iproute2/tc command to handle this transparently, or
should we give an error/warning if someone uses "tc" and "linklayer" on
a kernel above v.3.8. ?

History:
 - My linklayer ATM changes appeared in kernel 2.6.24 (and iproute2 2.6.25)
 - The STAB changes appeared in kernel 2.6.27

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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