On Monday 29 October 2012 13:44:45 Mauro Carvalho Chehab wrote: > Thanks for digging into it and getting more data. Do you know if this change > it also needed with USB devices that do DMA (isoc and/or bulk)? Or the USB > core already handles that? > I'm not a huge expert - the linux-pm list (cc'd) will have people around who know more. If I've understood correctly, though, the USB core should take care of pm_qos requests if they're needed for the hardware; remember that if the HCD has enough buffering, there's no need for a pm_qos request. It's only needed for devices like the SAA7134 where the buffer is small (1K split into pieces) compared to the sample data rate (27 megabytes/second raw video). For the benefit of the linux-pm list; this all starts with me providing a patch to have the saa7134 driver request reduced cpu_dma_latency when streaming, as I've seen buffer exhaustion. We've got far enough to know that the value I chose was wrong for the saa7134, but Mauro also wants guidance on whether USB devices (not host controllers) also need to request reduced latency. -- Simon Farnsworth Software Engineer ONELAN Ltd http://www.onelan.com
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