Date:	Tue, 23 Jul 2013 16:50:52 +0200
From:	Tomasz Figa <t.figa@...sung.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Tomasz Figa <tomasz.figa@...il.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	broonie@...nel.org, 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@...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,
	olof@...om.net, Stephen Warren <swarren@...dotorg.org>,
	b.zolnierkie@...sung.com,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 01/15] drivers: phy: add generic PHY framework

On Tuesday 23 of July 2013 10:37:05 Alan Stern wrote:
> On Tue, 23 Jul 2013, Tomasz Figa wrote:
> > On Tuesday 23 of July 2013 09:29:32 Tomasz Figa wrote:
> > > Hi Alan,
> 
> Thanks for helping to clarify the issues here.
> 
> > > > Okay.  Are PHYs _always_ platform devices?
> > > 
> > > They can be i2c, spi or any other device types as well.
> 
> In those other cases, presumably there is no platform data associated
> with the PHY since it isn't a platform device.  Then how does the
> kernel know which controller is attached to the PHY?  Is this spelled
> out in platform data associated with the PHY's i2c/spi/whatever parent?
> 
> > > > > > 	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.)
> 
> I don't understand, because I don't know what "a PHY lookup" does.

I have provided a code example in [1]. Feel free to ask questions about 
those code snippets.

[1] http://thread.gmane.org/gmane.linux.ports.arm.kernel/252813/focus=20889

> > > > 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
> 
> No, that's not what I said.  Neither the PHY driver nor the controller
> driver exports anything to the other.  Instead, both drivers use data
> exported by the board file.

It's still a random, driver-specific global symbol exported from board file 
to drivers.

> > > 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.
> 
> You're right; the scheme was too simple.  Instead, the board file must
> export two types of data structures, one for PHYs and one for
> controllers.  Like this:
> 
> struct phy_info {
> 	/* Info for the controller attached to this PHY */
> 	struct controller_info	*hinfo;
> };
> 
> struct controller_info {
> 	/* Info for the PHY which this controller is attached to */
> 	struct phy_info		*pinfo;
> };
> 
> The board file for SoC A would contain:
> 
> struct phy_info phy1 = {&host1);
> EXPORT_SYMBOL(phy1);
> struct controller_info host1 = {&phy1};
> EXPORT_SYMBOL(host1);
> 
> The board file for SoC B would contain:
> 
> struct phy_info phy1 = {&host2);
> EXPORT_SYMBOL(phy1);
> struct controller_info host2 = {&phy1};
> EXPORT_SYMBOL(host2);
> 
> And so on.  This explicitly gives the connection between PHYs and
> controllers.  The PHY providers would use &phy1 or &phy2, and the PHY
> consumers would use &host1 or &host2.

This could work assuming that only one SoC and one board is supported in 
single kernel image. However it's not the case.

We've used to support multiple boards since a long time already and now for 
selected platforms we even support multiplatform, i.e. multiple SoCs in 
single zImage. Such solution will not work.

Best regards,
Tomasz

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