Date:	Tue, 23 Jul 2013 09:29:32 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Kishon Vijay Abraham I <kishon@...com>,
	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-discuss@...ts.ozlabs.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 Alan,

On Monday 22 of July 2013 10:44:39 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?

They can be i2c, spi or any other device types as well.

> > > 	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?

By using a PHY lookup as Stephen and I suggested in our previous replies. 
Without any extra data in platform data. (I have even posted a code 
example.)

> > > 	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.
> 
> It's still possible to have conflicts, but now two PHYs with the same
> name (or a misspelled name somewhere) will cause an error at link time.

This is incorrect, sorry. First of all it's a layering violation - you 
export random driver-specific symbols from one driver to another. Then 
imagine 4 SoCs - A, B, C, D. There are two PHY types PHY1 and PHY2 and 
there are two types of consumer drivers (e.g. USB host controllers). Now 
consider following mapping:

SoC	PHY	consumer
A	PHY1	HOST1
B	PHY1	HOST2
C	PHY2	HOST1
D	PHY2	HOST2

So we have to be able to use any of the PHYs with any of the host drivers. 
This means you would have to export symbol with the same name from both 
PHY drivers, which obviously would not work in this case, because having 
both drivers enabled (in a multiplatform aware configuration) would lead 
to linking conflict.

Best regards,
Tomasz

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