Messages in this thread | | | Date | Thu, 27 Jan 2022 00:54:05 +0100 | From | Andrew Lunn <> | Subject | Re: [PATCH net-next 1/2] net: dsa: mv88e6xxx: Improve performance of busy bit polling |
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On Thu, Jan 27, 2022 at 12:12:38AM +0100, Tobias Waldekranz wrote: > Avoid a long delay when a busy bit is still set and has to be polled > again. > > Measurements on a system with 2 Opals (6097F) and one Agate (6352) > show that even with this much tighter loop, we have about a 50% chance > of the bit being cleared on the first poll, all other accesses see the > bit being cleared on the second poll. > > On a standard MDIO bus running MDC at 2.5MHz, a single access with 32 > bits of preamble plus 32 bits of data takes 64*(1/2.5MHz) = 25.6us. > > This means that mv88e6xxx_smi_direct_wait took 26us + CPU overhead in > the fast scenario, but 26us + 1500us + 26us + CPU overhead in the slow > case - bringing the average close to 1ms. > > With this change in place, the slow case is closer to 2*26us + CPU > overhead, with the average well below 100us - a 10x improvement. > > This translates to real-world winnings. On a 3-chip 20-port system, > the modprobe time drops by 88%: > > Before: > > root@coronet:~# time modprobe mv88e6xxx > real 0m 15.99s > user 0m 0.00s > sys 0m 1.52s > > After: > > root@coronet:~# time modprobe mv88e6xxx > real 0m 2.21s > user 0m 0.00s > sys 0m 1.54s > > Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Andrew
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