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

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Date:	Mon, 16 May 2016 16:13:26 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Roger Quadros <rogerq@...com>
Cc:	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Rob Herring <robh@...nel.org>,
	"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 Mon, May 16, 2016 at 11:01:27AM +0300, Roger Quadros wrote:
> On 16/05/16 05:13, Peter Chen wrote:
> > On Thu, May 12, 2016 at 03:13:48PM +0300, Roger Quadros wrote:
> >> Hi,
> >>
> >> On 12/05/16 13:31, Yoshihiro Shimoda wrote:
> >>> Hi,
> >>>
> >>>> From: Roger Quadros
> >>>> Sent: Thursday, May 12, 2016 6:32 PM
> >>>>
> >>>> 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.
> >>>
> >>> Thank you for the reply. I understood it.
> >>>
> >>>>>>>> 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.
> >>>
> >>> R-Car Gen2 SoCs (r8a779[0134] / arm32) has USB 2.0 host controllers via PCI bus and
> >>> one high speed function controller via AXI bus.
> >>> One of channel can be used as host or function.
> >>>
> >>>> For DT case we could have a DT binding to tie the EHCI and companion and use that
> >>>> in the OTG framework.
> >>
> >> After looking at the code it seems we don't need this special binding as we are already
> >> linking the EHCI controller and companion controller to the single otg controller instance
> >> using the otg-controller property.
> >>

[...]
> > 
> > Then, how you know this EHCI + companion controller special case during otg adds
> > hcd, it needs special handling, right?
> 
> We know the special case by using the hcd_needs_companion flag.
> 

You had said "we don't need this..", ok, yes, we do need it.

-- 

Best Regards,
Peter Chen