DMA over USB

[arrao@cdac.in (Raghavendra)]

On Wednesday 02 July 2014 11:05 AM, Chan Kim wrote:
> Hi Raghavendra,
> In my case, we used USB host design from opencore. And the driver (I don't know where it came from. it's similar to sl811-hcd.c from cypress) writes or reads the data to /from the USB host. Not using DMA. I think if the USB host has DMA capability, tha pio read/write should be chnaged to triggering DMA.
> http://lxr.free-electrons.com/source/drivers/usb/host/sl811-hcd.c  see line 234.
> Chan
>
>
>
> ?? ?? : "Raghavendra" <arrao@cdac.in>
> ?? ?? : 2014-07-02 14:20:42 ( +09:00 )
> ?? ?? : kernelnewbies <Kernelnewbies@kernelnewbies.org>
> ?? : 
> ?? : DMA over USB
>
> 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.
>
> Further more, if it is possible for the USB devices, then can this 
> support be also extended towards low-end protocols such as I2C or SPI?
>
> Thanks,
> Raghavendra
>
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Thanks Chan.

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