Re: [PATCH v2 net-next 1/2] net: dsa: mv88e6xxx: Improve performance of busy bit polling

[Andrew Lunn <andrew@lunn.ch>]

On Fri, Jan 28, 2022 at 11:49:37AM +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