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

[Grygorii Strashko]

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