Subject: Re: [PATCH v2 1/5] drivers: phy: add generic PHY framework

From: kishon <[email-masked]>
Date: Mon, 25 Feb 2013 12:11:48 +0530

Hi,

On Sunday 24 February 2013 04:14 AM, Rob Landley wrote:
> On 02/18/2013 11:53:14 PM, Kishon Vijay Abraham I wrote:
>> The PHY framework provides a set of APIs for the PHY drivers to
>> create/destroy a PHY and APIs for the PHY users to obtain a reference
>> to the
>> PHY with or without using phandle. To obtain a reference to the PHY
>> without
>> using phandle, the platform specfic intialization code (say from board
>> file)
>> should have already called phy_bind with the binding information. The
>> binding
>> information consists of phy's device name, phy user device name and an
>> index.
>> The index is used when the same phy user binds to mulitple phys.
>
> Given that this has a separately selectable config option, I'm guessing
> that it's useful all by itself even in the absence of a driver using

Not really. It has to be thought of like a bridge that connects the 
device (can be USB/SATA/..) and the PHY. The PHY driver will register 
the PHY in this framework and the device controller driver can obtain a 
reference to this PHY from this framework.
> this phy? (Or it gives user visibility to the phy buried in an E1000 or
> SATA drive or some such?)

The PHY and the device are generally independent entities and there has 
to be a way to bind them in-order for the device controller to use the 
PHY. For e.g., MUSB in OMAP4 uses a PHY which is different from PHY in 
OMAP3 and it's going to be different from a PHY used in other SoCs. So 
in-order for MUSB to use the correct PHY, there has to be a framework 
that maintains the list of PHY and returns the correct PHY when a device 
controller driver requests for it. So whenever the PHY driver gets 
probed, it registers itself with this framework and then the MUSB can 
get the reference to the PHY from this framework. The use of this 
framework is more prevalent when there are multiple MUSB controllers 
each using a different PHY in a single SoC.

This holds true for other device controllers as well.
>
>> +1. Introduction
>> +
>> +*PHY* is the abbreviation for physical layer. It is used to connect a
>> device
>> +to the physical medium e.g., the USB controller has a PHY to provide
>> functions
>> +such as serialization, de-serialization, encoding, decoding and is
>> responsible
>> +for obtaining the required data transmission rate. Note that some USB
>> +controller has PHY functionality embedded into it and others use an
>> external
>> +PHY. Other peripherals that uses a PHY include Wireless LAN, Ethernet,
>> +SATA etc.
>
> I've usually heard the word "transciever" used to describe these.

hmm.. IMO there is a thin line differentiating "transceiver" and "PHY" 
and can be used interchangeably. Since it's been referred as "PHY" in 
data sheets and TRMs, I preferred to call it PHY.
>
>> +The intention of creating this framework is to bring the phy drivers
>> spread
>> +all over the Linux kernel to drivers/phy to increase code re-use and to
>> +increase code maintainability.
>> +
>> +This framework will be of use only to devices that uses external PHY
>> (PHY
>> +functionality is not embedded within the controller).
>> +
>> +2. Creating the PHY
>> +
>> +The PHY driver should create the PHY in order for other peripheral
>> controllers
>> +to make use of it. The PHY framework provides 2 APIs to create the PHY.
>
> Given that a PHY is a chip (random example
> http://ark.intel.com/products/47620/Intel-82579LM-Gigabit-Ethernet-PHY),
> you seem to be saying that software should manifest a piece of hardware
> out of thin air through sheer willpower. I'm pretty sure I've
> misunderstood this phrasing.
>
>> +struct phy *phy_create(struct device *dev, struct phy_descriptor *desc);
>> +struct phy *devm_phy_create(struct device *dev, struct phy_descriptor
>> *desc);
>> +
>> +The PHY drivers can use one of the above 2 APIs to create the PHY by
>> passing
>
> Um, the driver should _bind_ to the phy, maybe? Allocate? Initialize?

There is actually difference between allocating/initializing and 
binding. Binding is to bind the device controller with the PHY. My 
previous explanation might help to clarify it.
>
>> +6. Destroying the PHY
>
> I've run drivers like that. I try not to, though.
>
>> +7. Current Status
>> +
>> +Currently only USB in OMAP is made to use this framework. However
>> using the
>> +USB PHY library cannot be completely removed because it is
>> intertwined with
>> +OTG. Once we move OTG out of PHY completely, using the old PHY
>> library can be
>> +completely removed. SATA in OMAP will also more likely use this new
>> framework
>> +and we should have a patch for it soon.
>
> Does this paragraph belong in the documentation? (Git commit, sure, but
> I've seen a lot of stale paragraphs like these hang around a
> surprisingly long time.)

hmm.. there are a few others who raised concern on having this 
paragraph. I've planned to remove it in my next version.

Thanks
Kishon