netdev - Re: [PATCH net-next v2 1/3] net: ethernet: fec: Replace interrupt driven MDIO with polled IO

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Date:   Sat, 18 Apr 2020 14:06:44 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>, David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Heiner Kallweit <hkallweit1@...il.com>, fugang.duan@....com,
        Chris Heally <cphealy@...il.com>
Subject: Re: [PATCH net-next v2 1/3] net: ethernet: fec: Replace interrupt
 driven MDIO with polled IO



On 4/17/2020 5:03 PM, Andrew Lunn wrote:
> 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>

You should support both modes and let an user decide whether they want 
to use interrupts or polling mode by specifying or omitting the 
'interrupts' property in their Device Tree.
-- 
Florian