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

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Message-ID: <SG2PR06MB091923EEBAA67EC1D1E48B6DD8730@SG2PR06MB0919.apcprd06.prod.outlook.com>
Date:	Thu, 12 May 2016 10:31:09 +0000
From:	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:	Roger Quadros <rogerq@...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,

> 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.

R-Car Gen3 SoC (r8a7795 / arm64) will be this type.
(Both USB 2.0 host/function controllers connect to AXI bus.)

> Any objections?

I don't have any objections because I'm just focus on R-Car Gen3 SoC for now.
If someone needs for PCI case, I think it is possible to add such a code somehow later.

Best regards,
Yoshihiro Shimoda

> cheers,
> -roger