linux-kernel - Re: [PATCH v7 10/14] usb: otg: add hcd companion support

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Date:	Thu, 12 May 2016 12:31:39 +0300
From:	Roger Quadros <rogerq@...com>
To:	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Rob Herring <robh@...nel.org>
CC:	"peter.chen@...escale.com" <peter.chen@...escale.com>,
	"balbi@...nel.org" <balbi@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"jun.li@...escale.com" <jun.li@...escale.com>,
	"mathias.nyman@...ux.intel.com" <mathias.nyman@...ux.intel.com>,
	"tony@...mide.com" <tony@...mide.com>,
	"Joao.Pinto@...opsys.com" <Joao.Pinto@...opsys.com>,
	"abrestic@...omium.org" <abrestic@...omium.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v7 10/14] usb: otg: add hcd companion support

Hi,

On 12/05/16 11:34, Roger Quadros wrote:
> On 12/05/16 07:00, Yoshihiro Shimoda wrote:
>> Hi,
>>
>>> From: Alan Stern
>>> Sent: Wednesday, May 11, 2016 11:47 PM
>>>
>>> On Wed, 11 May 2016, Roger Quadros wrote:
>>>
>>>>> What I mean is if you have 2 EHCI controllers with 2 companion
>>>>> controllers, don't you need to know which companion goes with which EHCI
>>>>> controller? Just like you do for the otg-controller property.
>>>>>
>>>>
>>>> That is a very good point. I'm not very sure and it seems that current code won't work
>>>> with multiple EHCI + companion instances.
>>
>> I may misunderstand this topic, but if I use the following environment, it works correctly.
>>
>> < My environment >
>> - an otg controller: Sets hcd-needs-companion.
>> - ehci0 and ohci0 and a function: They connect to the otg controller using "otg-controller" property.
>> - ehci1 and ohci1: No "otg-controller" property.
>> - ehci2 and ohci2: No "otg-controller" property.
>>
>> In this environment, all hosts works correctly.
>> Also I think if we have 2 otg controlelrs, it should be work because otg_dev instance differs.
> 
> The topic is about more than one otg controllers and how to tie the right ehci and ohci
> to the correct otg_dev instance especially in cases where we can't depend on probe order.
> 
>> Or, does this topic assume an otg controller handles 2 EHCI controllers?
>> I'm not sure such environment actually exists.
> 
> No it is not about that.
> 
>>
>>>> Alan, does USB core even know which EHCI and OHCI are linked to the same port
>>>> or the handoff is software transparent?
>>>
>>> The core knows.  It doesn't use the information for a whole lot of
>>> things, but it does use it in a couple of places.  Search for
>>> "companion" in core/hcd-pci.c and you'll see.
>>
>> Thank you for the information. I didn't know this code.
>> If my understanding is correct, the core/hcd-pci.c code will not be used by non-PCI devices.
> 
> That is correct.
> 
>> In other words, nobody sets "hcd->self.hs_companion" if we use such a device.
>> So, I will try to add such a code if needed.
> 
> I think OTG core would have to rely on USB core in providing the right companion device,
> just like we rely on it for the primary vs shared HCD case.
> 

OK, it is not so simple.

EHCI and companion port handoff is really meant to be software transparent.

non-PCI devices really don't have knowledge of which OHCI instance is companion to the EHCI.
With device tree we could provide this mapping but for non-device tree case we can't do
anything.

So my suggestion would be to keep dual role implementation limited to one instance for
EHCI + companion case for non-DT.
For PCI case I don't see how dual role can be implemented. I don't think we have any
dual-role PCI cards.
For DT case we could have a DT binding to tie the EHCI and companion and use that
in the OTG framework.

Any objections?

cheers,
-roger