Re: DMA over USB

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On Wednesday 02 July 2014 11:02 AM, Greg KH wrote:
> On Tue, Jul 01, 2014 at 10:29:39PM -0700, Greg KH wrote:
>> On Wed, Jul 02, 2014 at 10:40:21AM +0530, Raghavendra wrote:
>>> Hello,
>>>
>>> I have a query regarding DMA(Direct Memory Access) for the usb devices.
>>>
>>> The understanding of DMA actions over PCI is straight forward. PCI
>>> devices support bus mastering capability, such that the PCI devices
>>> could take the ownership of the bus and perform access to the memory
>>> directly, and a software support exists for the same in Linux.
>>>
>>> As far as USB devices are concerned, they don’t have the bus mastering
>>> capability like the PCI devices.
>>> But the USB URB structure have a field named 'dma_addr_t transfer_dma',
>>> used for DMA access. The USB driver allocate the DMA buffers coherently
>>> and pass the DMA address to the URBs during its initialization.
>>> As far as Linux is concerned, how the DMA action being taking place for
>>> USB devices. As per my understanding, the USB host controller is taking
>>> care of the DMA operations. But I require a little more insight into it.
>> Why, what exactly are you concerned about?  What are you trying to do?
> Also, you _have_ read the USB DMA documentation, right?
>
> What about the documentation in this area is unclear?
Yes, I have read the documentation and its fine. I am mostly concerned 
about the all the heavy lifting happening in the background.
>
> thanks,
>
> greg k-h
>
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