DMA over USB

[Raghavendra]

On Wednesday 02 July 2014 11:55 AM, Greg KH wrote:
> On Wed, Jul 02, 2014 at 11:39:36AM +0530, Raghavendra wrote:
>> 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.
> The code is there for your theoretical research :)
Ok thanks Greg. I shall look forward to it.

Regards,
Raghavendra

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