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

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Message-ID: <20200415002045.GF611399@lunn.ch>
Date:   Wed, 15 Apr 2020 02:20:45 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     David Miller <davem@...emloft.net>
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

On Tue, Apr 14, 2020 at 04:38:30PM -0700, David Miller wrote:
> 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?

Hi David

I do want to test such a version, yes.

  Andrew