On 09/02/2016 02:08 PM, 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? > Wouldn't be possible to use dma_mask for such purposes? Like, case 1: dwc3-omap (dma_mask=X) -> dwc3 (dma_mask = NULL) -> xhci-plat (NULL) and then it might be possible to find proper parent by traversing DD hierarchy. or : dwc3 (dma_mask = X) -> xhci-plat (NULL) or : xhci-plat (dma_mask = X) of course, it might be needed to skip DMA configuration for devices which parents have been configured for dma already (easy for xhci-plat, but can be not easy for dwc3). just thinking.. Also, I'd like to note that problem become more complex when scsi layer is used on top USB ;( -- regards, -grygorii -- 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
