Date:	Tue, 23 Jul 2013 16:53:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tomasz Figa <tomasz.figa@...il.com>
cc:	Tomasz Figa <t.figa@...sung.com>,
	Greg KH <gregkh@...uxfoundation.org>,
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	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 01/15] drivers: phy: add generic PHY framework

On Tue, 23 Jul 2013, Tomasz Figa wrote:

> > That's what I was going to suggest too.  The struct phy is defined in
> > the board file, which already knows about all the PHYs that exist in
> > the system.  (Or perhaps it is allocated dynamically, so that when many
> > board files are present in the same kernel, only the entries listed in
> > the board file for the current system get created.) 
> 
> Well, such dynamic allocation is a must. We don't accept non-multiplatform 
> aware code anymore, not even saying about multiboard.
> 
> > Then the
> > structure's address is stored in the platform data and made available
> > to both the provider and the consumer.
> 
> Yes, technically this can work. You would still have to perform some kind 
> of synchronization to make sure that the PHY bound to this structure is 
> actually present. This is again technically doable (e.g. a list of 
> registered struct phys inside PHY core).

The synchronization takes place inside phy_get.  If phy_create hasn't
been called for this structure by the time phy_get runs, phy_get will 
return an error.

> > Even though the struct phy is defined (or allocated) in the board file,
> > its contents don't get filled in until the PHY driver provides the
> > details.
> 
> You can't assure this. Board file is free to do whatever it wants with 
> this struct. A clean solution would prevent this.

I'm not sure what you mean here.  Of course I can't prevent a board 
file from messing up a data structure.  I can't prevent it from causing 
memory access violations either; in fact, I can't prevent any bugs in 
other people's code.

Besides, why do you say the board file is free to do whatever it wants 
with the struct phy?  Currently the struct phy is created by the PHY 
provider and the PHY core, right?  It's not even mentioned in the board 
file.

> > > It's technically correct, but quality of this solution isn't really
> > > nice, because it's a layering violation (at least if I understood
> > > what you mean). This is because you need to have full definition of
> > > struct phy in board file and a structure that is used as private data
> > > in PHY core comes from platform code.
> > 
> > You don't have to have a full definition in the board file.  Just a
> > partial definition -- most of the contents can be filled in later, when
> > the PHY driver is ready to store the private data.
> > 
> > It's not a layering violation for one region of the kernel to store
> > private data in a structure defined by another part of the kernel.
> > This happens all the time (e.g., dev_set_drvdata).
> 
> Not really. The phy struct is something that _is_ private data of PHY 
> subsystem, not something that can store private data of PHY subsystem 
> (sure it can store private data of particular PHY driver, but that's 
> another story) and only PHY subsystem should have access to its contents.

If you want to keep the phy struct completely separate from the board
file, there's an easy way to do it.  Let's say the board file knows
about N different PHYs in the system.  Then you define an array of N
pointers to phys:

struct phy *(phy_address[N]);

In the platform data for both PHY j and its controller, store
&phy_address[j].  The PHY provider passes this cookie to phy_create:

	cookie = pdev->dev.platform_data;
	ret = phy_create(phy, cookie);

and phy_create simply stores: *cookie = phy.  The PHY consumer does
much the same the same thing:

	cookie = pdev->dev.platform_data;
	phy = phy_get(cookie);

phy_get returns *cookie if it isn't NULL, or an ERR_PTR otherwise.

> By the way, we need to consider other cases here as well, for example it 
> would be nice to have a single phy_get() function that works for both non-
> DT and DT cases to make the consumer driver not have to worry whether it's 
> being probed from DT or not.

You ought to be able to adapt this scheme to work with DT.  Maybe by 
having multiple "phy_address" arrays.

Alan Stern

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