Re: [PATCH] usb: phy: Move R-Car Gen2 driver registration to postcore_inictall

[Valentine <valentine.barshak@cogentembedded.com>]

On 11/01/2013 06:17 PM, Alan Stern wrote:
> On Fri, 1 Nov 2013, Valentine wrote:
>
>>> You need to tell usb_hcd_pci_probe() to wait for the PHY.  That seems
>>> to be the proper solution to your problem.
>>>
>>> The difficulty is that you have a discoverable device (the PCI EHCI
>>> controller) which needs to wait for a platform device (the PHY).  The
>>> kernel doesn't have a good way to describe such a constraint between
>>> two different kinds of device like that, as far as I know.
>>
>> Thanks, unfortunately this doesn't help.
>> I'm not sure how this problem should be addressed using USB HCD PCI deferred probing.
>
> I'm not sure either.  It requires further discussion, and it is an
> important problem.  Not a trivial one, as you seem to think.
>
>> However, at the same time I see that six usb phy drivers use subsys_initcall and one
>> uses postcore_initcall to adjust the initialization order.
>>
>> The same approach is used with other drivers quite often. Take I2C, for example.
>> I'm not sure why we can't use it here with the R-Car Gen2 phy.
>
> The fact that other drivers do it doesn't mean it is the right thing to
> do.  (Besides, I2C is different from PCI because it isn't discoverable,
> right?)
>

I'm not using postcore_init with PCI. I'm doing it for a platform USB phy driver.
Anyway why can't we use postcore_initcall to register a driver for a discoverable device?

> Greg KH can explain further, if you ask him.

Yes, I'd like to hear from Greg as well.
Explanaitions and suggestions are greatly appreciated.

>
>> This driver is used only with R-Car SoC and the approach is trivial and  working fine.
>>
>> Why can't we use it instead of trying to create a bigger mess in the USB HCD PCI driver,
>> which is used on quite a number of platforms, to workaround the PHY initialization order
>> that is only relevant to R-Car Gen2 SoC?
>
> Because other platforms are likely to experience the same problem in
> the future.  And as you pointed out, other platforms already _are_
> experiencing this problem (although perhaps for other drivers).
>
> We should implement a proper solution.  One that can be used
> everywhere, not an initcall-order hack.

That would be great, but I don't think we can implement a universal solution for all of those drivers.
The solution that may suit R-Car may not be good for other USB phy drivers since most likely they have
their init order fixed-up for other reasons not related to PCI USB hosts.

Fixing all the drivers so that they all can use deferred probing is, in my opinion, a bit out of scope
here.

The USB HDC PCI deferred probing could be used on R-Car. But I'm not sure how to make a particular
PCI USB HDC device attached to a particular PCI host controller on a particular SoC defer its probing
while waiting for the USB phy. At the same time other identical PCI USB HCD devices (with the same PCI id)
on the same SoC should be probed as usual.

We can't use PCI ids here to distinguish between PCI USB HCD devices. Neither can we use PCI busses
to distinguish between PCI host controllers, since bus numbers are assigned dynamically.

It looks that it's quite hard to do that without bigger hacks in the PCI USB HCD driver that are
most likely not to be used on any other platforms except for R-Car.

I just can't see why changing init order is considered such a bad hack in this case,
while it is being used fine with other USB phy drivers.

Can't we use it for R-Car until a universal solution to the deferred probing is found?

Probably, moving the whole phy subsystem to the earlier initialization stage would be a good-enough
solution since that many PHY drivers have already been using it?
You know, just like the PCI host drivers are initialized at subsys_initcall, or is it considered wrong?

>
> Alan Stern
>

Thanks,
Val.