netdev - Re: [PATCH] net: ethernet: fec: Replace interrupt driven MDIO with polled IO

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Message-Id: <20200414.163830.2151404177947586721.davem@davemloft.net>
Date:   Tue, 14 Apr 2020 16:38:30 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     andrew@...n.ch
Cc:     netdev@...r.kernel.org, fugang.duan@....com, Chris.Healy@....aero,
        cphealy@...il.com
Subject: Re: [PATCH] net: ethernet: fec: Replace interrupt driven MDIO with
 polled IO

From: Andrew Lunn <andrew@...n.ch>
Date: Tue, 14 Apr 2020 02:45:51 +0200

> Measurements of the MDIO bus have shown that driving the MDIO bus
> using interrupts is slow. Back to back MDIO transactions take about
> 90uS, with 25uS spent performing the transaction, and the remainder of
> the time the bus is idle.
> 
> Replacing the completion interrupt with polled IO results in back to
> back transactions of 40uS. The polling loop waiting for the hardware
> to complete the transaction takes around 27uS. Which suggests
> interrupt handling has an overhead of 50uS, and polled IO nearly
> halves this overhead, and doubles the MDIO performance.
> 
> Suggested-by: Chris Heally <cphealy@...il.com>
> Signed-off-by: Andrew Lunn <andrew@...n.ch>

Where are we with this?

Andrew, do you intend to submit a version that via iopoll.h does
cpu relax and usleeps?