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SubjectRe: [PATCH 01/15] drivers: phy: add generic PHY framework
Date
[Fixed address of devicetree mailing list and added more people on CC.]

For reference, full thread can be found under following link:
http://thread.gmane.org/gmane.linux.ports.arm.kernel/252813

Best regards,
Tomasz

On Tuesday 23 of July 2013 09:29:32 Tomasz Figa wrote:
> 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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