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?
> > 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.
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.
> > 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?
>
> phy_create is the API by which the PHY's driver (the supplier) hook into the
> PHY framework. It's like the controller driver will always interact with the
> PHY driver through the PHY framework.
Alan Stern
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