linux-kernel - Re: [PATCH 01/15] drivers: phy: add generic PHY framework

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Date:	Tue, 23 Jul 2013 11:17:40 +0530
From:	Kishon Vijay Abraham I <kishon@...com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	<kyungmin.park@...sung.com>, <balbi@...com>, <jg1.han@...sung.com>,
	<s.nawrocki@...sung.com>, <kgene.kim@...sung.com>,
	<grant.likely@...aro.org>, <tony@...mide.com>, <arnd@...db.de>,
	<swarren@...dia.com>, <devicetree@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-samsung-soc@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<linux-usb@...r.kernel.org>, <linux-media@...r.kernel.org>,
	<linux-fbdev@...r.kernel.org>, <akpm@...ux-foundation.org>,
	<balajitk@...com>, <george.cherian@...com>, <nsekhar@...com>
Subject: Re: [PATCH 01/15] drivers: phy: add generic PHY framework

Hi,

On Monday 22 July 2013 08:14 PM, Alan Stern wrote:
> On Mon, 22 Jul 2013, Kishon Vijay Abraham I wrote:
> 
>>> 	The PHY and the controller it is attached to are both physical
>>> 	devices.
>>>
>>> 	The connection between them is hardwired by the system
>>> 	manufacturer and cannot be changed by software.
>>>
>>> 	PHYs are generally described by fixed system-specific board
>>> 	files or by Device Tree information.  Are they ever discovered
>>> 	dynamically?
>>
>> No. They are created just like any other platform devices are created.
> 
> Okay.  Are PHYs _always_ platform devices?

Not always. It can be any other device also.
> 
>>> 	Is the same true for the controllers attached to the PHYs?
>>> 	If not -- if both a PHY and a controller are discovered 
>>> 	dynamically -- how does the kernel know whether they are 
>>> 	connected to each other?
>>
>> No differences here. Both PHY and controller will have dt information or hwmod
>> data using which platform devices will be created.
>>>
>>> 	The kernel needs to know which controller is attached to which
>>> 	PHY.  Currently this information is represented by name or ID
>>> 	strings embedded in platform data.
>>
>> right. It's embedded in the platform data of the controller.
> 
> It must also be embedded in the PHY's platform data somehow.  
> Otherwise, how would the kernel know which PHY to use?
> 
>>> 	The PHY's driver (the supplier) uses the platform data to 
>>> 	construct a platform_device structure that represents the PHY.  
>>
>> Currently the driver assigns static labels (corresponding to the label used in
>> the platform data of the controller).
>>> 	Until this is done, the controller's driver (the client) cannot 
>>> 	use the PHY.
>>
>> right.
>>>
>>> 	Since there is no parent-child relation between the PHY and the 
>>> 	controller, there is no guarantee that the PHY's driver will be
>>> 	ready when the controller's driver wants to use it.  A deferred
>>> 	probe may be needed.
>>
>> right.
>>>
>>> 	The issue (or one of the issues) in this discussion is that 
>>> 	Greg does not like the idea of using names or IDs to associate
>>> 	PHYs with controllers, because they are too prone to
>>> 	duplications or other errors.  Pointers are more reliable.
>>>
>>> 	But pointers to what?  Since the only data known to be 
>>> 	available to both the PHY driver and controller driver is the
>>> 	platform data, the obvious answer is a pointer to platform data
>>> 	(either for the PHY or for the controller, or maybe both).
>>
>> hmm.. it's not going to be simple though as the platform device for the PHY and
>> controller can be created in entirely different places. e.g., in some cases the
>> PHY device is a child of some mfd core device (the device will be created in
>> drivers/mfd) and the controller driver (usually) is created in board file. I
>> guess then we have to come up with something to share a pointer in two
>> different files.
> 
> The ability for two different source files to share a pointer to a data 
> item defined in a third source file has been around since long before 
> the C language was invented.  :-)
> 
> In this case, it doesn't matter where the platform_device structures 
> are created or where the driver source code is.  Let's take a simple 
> example.  Suppose the system design includes a PHY named "foo".  Then 
> the board file could contain:
> 
> struct phy_info { ... } phy_foo;
> EXPORT_SYMBOL_GPL(phy_foo);
> 
> and a header file would contain:
> 
> extern struct phy_info phy_foo;
> 
> The PHY supplier could then call phy_create(&phy_foo), and the PHY 
> client could call phy_find(&phy_foo).  Or something like that; make up 
> your own structure tags and function names.

Alright. Thanks for the hint :-)

Thanks
Kishon
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