net: ethernet: fec: Replace interrupt driven MDIO with polled IO
authorAndrew Lunn <andrew@lunn.ch>
Sat, 2 May 2020 15:25:04 +0000 (17:25 +0200)
committerDavid S. Miller <davem@davemloft.net>
Sat, 2 May 2020 23:41:34 +0000 (16:41 -0700)
commitf166f890c8f026a931e1bb80f51561a1d2f41b27
tree54f90ef565b1d345bfcb5dfc5d313d45af177879
parent40b94224c339e44f689e713875c6c27c9c1270a7
net: ethernet: fec: Replace interrupt driven MDIO with polled IO

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 28us. Which suggests
interrupt handling has an overhead of 50us, and polled IO nearly
halves this overhead, and doubles the MDIO performance.

Care has to be taken when setting the MII_SPEED register, or it can
trigger an MII event> That then upsets the polling, due to an
unexpected pending event.

Suggested-by: Chris Heally <cphealy@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
drivers/net/ethernet/freescale/fec.h
drivers/net/ethernet/freescale/fec_main.c