On Mon, Jun 24, 2013 at 05:35:42PM +0200, Sebastian Andrzej Siewior wrote: > On 06/24/2013 05:18 PM, Felipe Balbi wrote: > >> It is a little. The first USB packet has 512 vs 502 bytes on the > >> wire. > > > > you wouldn't notice the difference. The DMA engine is the one > > which would read the sgtable to figure where the data is scattered, > > at the end of the day, SW only knows of a single 1044bytes URB and > > controller is required to generate proper USB packets out of that. > > The DMA engines (intern or extern) do not merge transfers. Some of them > are clever enough to transfer a single 1024 as two 512 requests. But > two 256 bytes requests are not merged into one 512 request. I believe > the device side of dwc3 is able to do so if marked properly but musb > host side won't be able to do so. right, that's why we use a temporary buffer to copy data to and do the transfer. -- balbi
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