Date:	Mon, 15 Apr 2013 17:56:10 +0530
From:	Kishon Vijay Abraham I <kishon@...com>
To:	Grant Likely <grant.likely@...retlab.ca>
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Subject: Re: [PATCH v3 1/6] drivers: phy: add generic PHY framework

Hi,

On Monday 15 April 2013 05:04 PM, Grant Likely wrote:
> On Wed, 20 Mar 2013 14:42:00 +0530, Kishon Vijay Abraham I <kishon@...com> wrote:
>> The PHY framework provides a set of APIs for the PHY drivers to
>> create/destroy a PHY and APIs for the PHY users to obtain a reference to the
>> PHY with or without using phandle. To obtain a reference to the PHY without
>> using phandle, the platform specfic intialization code (say from board file)
>> should have already called phy_bind with the binding information. The binding
>> information consists of phy's device name, phy user device name and an index.
>> The index is used when the same phy user binds to mulitple phys.
>>
>> PHY drivers should create the PHY by passing phy_descriptor that has
>> describes the PHY (label, type etc..) and ops like init, exit, suspend, resume,
>> poweron, shutdown.
>>
>> The documentation for the generic PHY framework is added in
>> Documentation/phy.txt and the documentation for the sysfs entry is added
>> in Documentation/ABI/testing/sysfs-class-phy.
>>
>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>
> Hi Kishon,
>
> For review purposes, I'll skip most of the implementation and focus on
> the data structures. I think you need to take another look at the
> approach your using. The kernel already provides a lot of support for
> implementing devices and binding them to drivers that you should be
> able to use...
>
>
> [...]
>> +/**
>> + * struct phy - represent the phy device
>> + * @dev: phy device
>> + * @label: label given to phy
>> + * @type: specifies the phy type
>> + * @of_node: device node of the phy
>> + * @ops: function pointers for performing phy operations
>> + */
>> +struct phy {
>> +	struct device		dev;
>> +	const char		*label;
>> +	int			type;
>> +	struct bus_type		*bus;
>> +	struct device_node	*of_node;
>> +	struct phy_ops		*ops;
>> +};
>
> Alright, so the core of the functionality is this 'struct phy' which
> tracks all the instances of PHY devices. As I understand it each
> physical phy will have a 'struct phy' instance tracking it. That makes
> sense.
>
> struct phy embeds a struct device. Also good. The linux driver model
> knows all about devices and how to handle them. However, most of the
> rest of this structure looks to be reinventing stuff the driver model
> already does.
>
> 'label' seems unnecessary. struct device embeds a struct kobject, which
> means it has a name and shows up in sysfs. Is there a reason that the
> device name cannot be used directly?

hmm.. the label name is supposed to be a simpler name than device name.
Say for instance "omap-usb2.1.auto" device name can simply be 
"omap-usb2-phy". Further the device name while using dt will have 
register address in it. However it's used only for displaying the label 
in sysfs entry (/sys/class/phy/<phy>/label).
>
> 'type' I just don't understand. I don't see any users of it in this
> patch. I only see where it is set.

yeah. Was planning to remove this in my next version (btw the latest is 
version 5).
>
> 'bus' absolutely should not be here. The bus type should be set in the
> struct device by this subsystem when the device gets registered. There
> is no reason to have a pointer to it here.

right. I had removed it in version 5 of this patch series.
>
> 'of_node' is already in struct device

I wasn't sure if we can manually assign the of_node of one device to 
of_node of an another device. Here the of_node comes from _phy provider_.
>
> Finally, it really doesn't look right for a device object to have an
> 'ops' structure. The whole point of the driver model is that a struct
> device doesn't encapsulate any behaviour. A device gets registers to a
> bus type, and then the driver core will associate a struct device_driver

We have decided not to implement the PHY layer as a separate bus layer. 
The PHY provider can be part of any other bus. Making the PHY layer as a 
bus will make the PHY provider to be part of multiple buses which will 
lead to bad design. All we are trying to do here is keep the pool of PHY 
devices under PHY class in this layer and help any controller that wants 
to use the PHY to get it.
> to a device that it is able to drive, and the struct device_driver is
> supposed to contain any ops structure used to control the device.
>
>> +
>> +/**
>> + * struct phy_bind - represent the binding for the phy
>> + * @dev_name: the device name of the device that will bind to the phy
>> + * @phy_dev_name: the device name of the phy
>> + * @index: used if a single controller uses multiple phys
>> + * @auto_bind: tells if the binding is done explicitly from board file or not
>> + * @phy: reference to the phy
>> + * @list: to maintain a linked list of the binding information
>> + */
>> +struct phy_bind {
>> +	const char	*dev_name;
>> +	const char	*phy_dev_name;
>> +	int		index;
>> +	int		auto_bind:1;
>> +	struct phy	*phy;
>> +	struct list_head list;
>> +};
>
> How are PHYs attached to the system. Would the PHY be a direct child of
> the host controller device, or would a PHY hang off another bus (like
> i2c) for control? (this is how Ethernet phys work; they hang off the
> MDIO bus, but may be attached to any Ethernet controller).

This layer does not have any restriction on how the PHY is attached to 
the system. For all the sample cases (USB OTG controller in OMAP 
platforms) that follows this patch, the PHY is attached to the platform 
bus and the PHY provider is a platform driver. All this framework does 
is maintain a list of PHY's added in the system and helps the controller 
to find the appropriate PHY.
We are currently not looking at having Ethernet use this layer because 
it itself has a comprehensive PHY layer and merging it with this will 
take some effort. However other subsystems like USB, SATA, video etc.. 
can make use of this.
>
> Since there is at most a 1:N relationship between host controllers and
> PHYs, there shouldn't be any need for a separate structure to describe
> binding. Put the inding data into the struct phy itself. Each host
> controller can have a list of phys that it is bound to.

No. Having the host controller to have a list of phys wont be a good 
idea IMHO. The host controller is just an IP and the PHY to which it 
will be connected can vary from board to board, platform to platform. So 
ideally this binding should come from platform initialization code/dt data.

Thanks
Kishon
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