From: Raghavendra
> Thank you for responding Peter,
> > Raghavendra wrote:
> >> I have a query regarding DMA(Direct Memory Access) for the usb devices.
> > USB devices never do DMA.
> >
> >> As far as Linux is concerned, how the DMA action being taking place for
> >> USB devices.
> > It doesn't take place.
> >
> >> As per my understanding, the USB host controller is taking care of
> >> the DMA operations.
> > That's correct. When an URB is submitted by host software the host
> > controller asks the device for data, and when data arrives from the
> > device the host controller writes the data into memory using DMA.
> As I understand, the USB host controllers(on x86) are basically PCI
> devices and PCI devices are capable of bus mastering, thus performing DMA.
> But, if we take the case of a non x86 platform (say ARM), the USB host
> controller may or may not be capable of performing DMA. In that case how
> far the cases are valid if we try to push the data from the USB device
> through DMA
> For example, if we take the case of a simple USB mouse
> driver(usbmouse.c), it uses DMA to obtain the data. What if this driver
> is associated with a host controller which doesnt have the support for
> DMA. What will happen in that scenario?
It is extremely unlikely that a USB host controller will be unable to
do DMA - in the sense that it transfers USB data to/from host memory
buffers.
However the host buffer addresses are not made available to the USB
target hardware - so it can't select which buffer is used.
It is just possible that the memory the host controller uses is resident
on (say) a PCI card. But the normal bandwidth requirements make that
unlikely.
Another exception might be for slow devices (keyboard/mouse) where the
amount of data is very limited - but that won't be a host.
David
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