linux-kernel - Re: How to handle access to multiple PHYs through MDIO

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Date:	Fri, 16 Jan 2015 17:47:02 -0800
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Ray Jui <rjui@...adcom.com>,
	Kishon Vijay Abraham I <kishon@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: How to handle access to multiple PHYs through MDIO

Hi Ray,

On 16/01/15 17:10, Ray Jui wrote:
> Hi,
> 
> Our SoC, Cygnus, uses a generic MDC/MDIO controller to talk to various
> PHYs, including 2 x Ethernet GPHY, 2 x PCIe Serdes, and 3 x USB PHYs. In
> this case, how should I work out a generic PHY driver to handle this?

Interesting, I have typically seen separate MDIO controllers for at
least Ethernet and USB/PCIe/SATA.

> 
> I notice that most generic PHY drivers are in drivers/phy/*, but
> Ethernet seems to have its own interface of talking to a PHY through
> MDIO (drivers/net/phy/*).

That's right, the Ethernet PHY library predates the generic PHY library
from Kishon and they have little to no common ground.

> 
> I need a single driver to handle these so there isn't any race condition
> for this single MDIO access in our system.

How about the following design:

- you create a MDIO bus controller library in e.g:
drivers/phy/cygnus-mdio.c which offers generic generic read/write
operations with a prototype looking like this:

	int mdio_read(void *device, enum device_type, int reg, int addr);
	- int mdio_write(void *device, enum device_type, int reg, int addr, int
value)
	- where device_type is MDIO_DEV_SERDES or MDIO_DEV_GPHY

	- these reads and writes are protected by a local spinlock which is not
exposed to the caller, it just needs to know that it gets serialized
access to the controller

- you write a MDIO controller in drivers/phy/ for the USB and PCIe PHYs
which uses this library and interfaces with Kishon's PHY Library operations

- you create a MDIO bus controller driver in drivers/net/phy/ which also
uses this library and registers with Linux using mdiobus_register()

This is imho the easiest way to achieve what you want here, however, you
could also stash all of what I describe above in a single MDIO bus
driver in drivers/phy/ and ifdef out what is relevant based on your
kernel configuration, up to you, there could be some tricky dependencies
to solve though.
-- 
Florian
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