linux-kernel - Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev

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Date:   Fri, 2 Sep 2016 10:21:23 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Felipe Balbi <balbi@...nel.org>
cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Leo Li <pku.leo@...il.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Catalin Marinas <catalin.marinas@....com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        David Fisher <david.fisher1@...opsys.com>,
        "Thang Q. Nguyen" <tqnguyen@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Stuart Yoder <stuart.yoder@....com>,
        Scott Wood <oss@...error.net>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent
 dev

On Fri, 2 Sep 2016, Felipe Balbi wrote:

> Hi,
> 
> Russell King - ARM Linux <linux@...linux.org.uk> writes:
> > On Fri, Sep 02, 2016 at 12:43:39PM +0200, Arnd Bergmann wrote:
> >> On Thursday, September 1, 2016 5:14:28 PM CEST Leo Li wrote:
> >> > 
> >> > Hi Felipe and Arnd,
> >> > 
> >> > It has been a while since the last response to this discussion, but we
> >> > haven't reached an agreement yet!  Can we get to a conclusion on if it
> >> > is valid to create child platform device for abstraction purpose?  If
> >> > yes, can this child device do DMA by itself?
> >> 
> >> I'd say it's no problem for a driver to create child devices in order
> >> to represent different aspects of a device, but you should not rely on
> >> those devices working when used with the dma-mapping interfaces.
> >
> > That's absolutely right.  Consider the USB model - only the USB host
> > controller can perform DMA, not the USB devices themselves.  All DMA
> > mappings need to be mapped using the USB host controller device struct
> > not the USB device struct.
> >
> > The same _should_ be true everywhere else: the struct device representing
> > the device performing DMA must be the one used to map the transfer.
> 
> How do we fix dwc3 in dual-role, then?
> 
> Peripheral-side dwc3 is easy, we just require a glue-layer to be present
> and use dwc3.ko's parent device (which will be the PCI device or OF
> device). But for host side dwc3, the problem is slightly more complex
> because we're using xhci-plat.ko by just instantiating a xhci-platform
> device so xhci-plat can probe.
> 
> xhci core has no means to know if its own device or the parent of its
> parent should be used for DMA. Any ideas?

In theory, you can store a flag somewhere in the platform device,
something that would tell xhci-hcd that it has to use the parent's
parent for DMA purposes.

I know it would be somewhat of a hack, but ought to work.

Alan Stern