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. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
