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

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Message-ID: <51EC170E.3080201@gmail.com>
Date:	Sun, 21 Jul 2013 19:14:54 +0200
From:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Sascha Hauer <s.hauer@...gutronix.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Kishon Vijay Abraham I <kishon@...com>,
	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-discuss@...ts.ozlabs.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
Subject: Re: [PATCH 01/15] drivers: phy: add generic PHY framework

On 07/21/2013 05:48 PM, Greg KH wrote:
> On Sun, Jul 21, 2013 at 12:22:48PM +0200, Sascha Hauer wrote:
>> On Sat, Jul 20, 2013 at 07:59:10PM -0700, Greg KH wrote:
>>> On Sat, Jul 20, 2013 at 10:32:26PM -0400, Alan Stern wrote:
>>>> On Sat, 20 Jul 2013, Greg KH wrote:
>>>>
>>>>>>>> That should be passed using platform data.
>>>>>>>
>>>>>>> Ick, don't pass strings around, pass pointers.  If you have platform
>>>>>>> data you can get to, then put the pointer there, don't use a "name".
>>>>>>
>>>>>> I don't think I understood you here :-s We wont have phy pointer
>>>>>> when we create the device for the controller no?(it'll be done in
>>>>>> board file). Probably I'm missing something.
>>>>>
>>>>> Why will you not have that pointer?  You can't rely on the "name" as the
>>>>> device id will not match up, so you should be able to rely on the
>>>>> pointer being in the structure that the board sets up, right?
>>>>>
>>>>> Don't use names, especially as ids can, and will, change, that is going
>>>>> to cause big problems.  Use pointers, this is C, we are supposed to be
>>>>> doing that :)
>>>>
>>>> Kishon, I think what Greg means is this:  The name you are using must
>>>> be stored somewhere in a data structure constructed by the board file,
>>>> right?  Or at least, associated with some data structure somehow.
>>>> Otherwise the platform code wouldn't know which PHY hardware
>>>> corresponded to a particular name.
>>>>
>>>> Greg's suggestion is that you store the address of that data structure
>>>> in the platform data instead of storing the name string.  Have the
>>>> consumer pass the data structure's address when it calls phy_create,
>>>> instead of passing the name.  Then you don't have to worry about two
>>>> PHYs accidentally ending up with the same name or any other similar
>>>> problems.
>>>
>>> Close, but the issue is that whatever returns from phy_create() should
>>> then be used, no need to call any "find" functions, as you can just use
>>> the pointer that phy_create() returns.  Much like all other class api
>>> functions in the kernel work.
>>
>> I think the problem here is to connect two from the bus structure
>> completely independent devices. Several frameworks (ASoC, soc-camera)
>> had this problem and this wasn't solved until the advent of devicetrees
>> and their phandles.
>> phy_create might be called from the probe function of some i2c device
>> (the phy device) and the resulting pointer is then needed in some other
>> platform devices (the user of the phy) probe function.
>> The best solution we have right now is implemented in the clk framework
>> which uses a string matching of the device names in clk_get() (at least
>> in the non-dt case).
>
> I would argue that clocks are wrong here as well, as others have already
> pointed out.
>
> 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.

--
Regards,
Sylwester
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