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

[Alan Stern <stern@rowland.harvard.edu>]

On Sun, 21 Jul 2013, Sylwester Nawrocki wrote:

> > What's wrong with the platform_data structure, why can't that be used
> > for this?
> 
> At the point the platform data of some driver is initialized, e.g. in
> board setup code the PHY pointer is not known, since the PHY supplier
> driver has not initialized yet.  Even though we wanted to pass pointer
> to a PHY through some notifier call, it would have been not clear
> which PHY user driver should match on such notifier.  A single PHY
> supplier driver can create M PHY objects and this needs to be mapped
> to N PHY user drivers.  This mapping needs to be defined somewhere by
> the system integrator.  It works well with device tree, but except that
> there seems to be no other reliable infrastructure in the kernel to
> define links/dependencies between devices, since device identifiers are
> not known in advance in all cases.
> 
> What Tomasz proposed seems currently most reasonable to me for non-dt.
> 
> > Or, if not, we can always add pointers to the platform device structure,
> > or even the main 'struct device' as well, that's what it is there for.
> 
> Still we would need to solve a problem which platform device structure
> gets which PHY pointer.

Can you explain the issues in more detail?  I don't fully understand 
the situation.

Here's what I think I know:

	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?

	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?

	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.

	The PHY's driver (the supplier) uses the platform data to 
	construct a platform_device structure that represents the PHY.  
	Until this is done, the controller's driver (the client) cannot 
	use the PHY.

	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.

	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).

Probably some of the details above are wrong; please indicate where I
have gone astray.  Also, I'm not clear about the role played by various 
APIs.  For example, where does phy_create() fit into this picture?

Alan Stern