linux-kernel - [Patch v1] driver/eth/usb/asix.ko: Support for 88772C in Rev-RMII

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Message-ID: <000201dbfbe7$7d4ac1b0$77e04510$@newagemicro.com>
Date: Wed, 23 Jul 2025 11:35:57 -0400
From: <msobchuk@...agemicro.com>
To: <linux-kernel@...r.kernel.org>,
	<linux-usb@...r.kernel.org>
Cc: "Dave Tyree" <dtyree@...ercogs.com>,
	"Mike Sobchuk" <msobchuk@...ercogs.com>
Subject: [Patch v1] driver/eth/usb/asix.ko: Support for 88772C in Rev-RMII

This Patch adds support to the asix.ko driver for the AX88772C in
Reverse-RMII mode.

The 'asix.ko' driver is designed to support 88772A, B and C.

The AX88772C supports running in Reverse-RMII mode to enable MAC to MAC
communication.

The base 'asix.ko' driver does NOT currently support the AX88772C in
Reverse-RMII mode.

This is due to several reasons:

1.	The AX88772C PHY register "LINK STATUS" Link value never is set to
1. 

a.	This prevents the drivers from setting the interface 'UP'

2.	The AX88772C PHY-ID is hard coded to 0x10, this configures the chip
to use the internal PHY not an external PHY.

a.	To use the AX88772C in Reverse-RMII mode, the chip must be
configured to use the external 'PHY'.

Asix provides a dedicated AX88772C driver which supports Reverse-RMII. This
'C' driver was references to add Reverse-RMII support to the base asix.ko
driver.

To resolve the issue, the following changes were made:

1.	MDIO_READ prior to configuring chip to determine what operational
mode it is in (Internal, RMII, or Reverse-RMII)
2.	If in Reverse-RMII mode, force chip to be configured as if it had an
external PHY.
3.	If in Reverse-RMII mode, set the interface status to UP regardless
of the value of the 'LINK STATUS' bit.

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