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

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Date:	Thu, 12 May 2016 11:34:04 +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

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.

cheers,
-roger