RE: [PATCH v2 2/2] net: ethernet: Add driver for Sunplus SP7021

Hi Andrew,


From data provided by ASIC engineer, MAC of SP7021 only
reads the 4 registers of PHY:
0: Control register
1: Status register
4: Auto-negotiation advertisement register
5: Auto-negotiation link partner ability register

It does not read any other registers of PHY.


Best regards,
Wells Lu

quoted

Hi Andrew,

I set phy-id registers to 30 and 31 and found the read-back values of
mdio read commands from CPU are all 0x0000.

I consulted with an ASIC engineer. She confirmed that if phy-id of a
mdio command from CPU does not match any phy-id registers, the mdio
command will not be sent out.

She explained if phy-id of a mdio command does not match any phy-id
registers (represent addresses of external PHYs), why MAC needs to
send a command to non-existing PHY?

Reads or writes on a real PHY which Linux is driving can have side effects. There is a
link statue register which latches. Read it once, you get the last status, read it again,
you get the current status. If the MAC hardware is reading this register as well a Linux,
bad things will happen. A read on the interrupt status register often clears the interrupts.
So Linux will not see the interrupts.

So you need to make sure you hardware is not touching a PHY which Linux uses. Which is
why i suggested using MDIO bus address 31, which generally does not have a PHY at that
address.

	  Andrew