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

[Arnd Bergmann]

On Friday, September 2, 2016 10:21:23 AM CEST Alan Stern wrote:
> On Fri, 2 Sep 2016, Felipe Balbi wrote:
> 
> > Hi,
> > 
> > Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> 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.

Speaking of that flag, I suppose we need the same logic to know where
to look for USB devices attached to a dwc3 host when we need to describe
them in DT. By default we look for child device nodes under the
node of the HCD device node, but that would be wrong here too.

	Arnd