DMA over USB

[arrao@cdac.in (Raghavendra)]

On Wednesday 02 July 2014 11:42 AM, Greg KH wrote:
> On Wed, Jul 02, 2014 at 11:32:47AM +0530, Raghavendra wrote:
>> 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.
> Why?  What exactly are you "concerned" about?  What hardware controller?
> Is something not working properly?
Everything is fine. This is more of a theoretical understanding.
>
> greg k-h


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