Re: [PATCH] saa7134: Add pm_qos_request to fix video corruption

Re: [PATCH] saa7134: Add pm_qos_request to fix video corruption

[Simon Farnsworth]

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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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Re: [PATCH] saa7134: Add pm_qos_request to fix video corruption

[Alan Stern]

On Mon, 29 Oct 2012, Simon Farnsworth wrote:

> 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.

The USB core is not PM-QOS aware.  It relies on the PM core to tell it 
when devices may safely be runtime-suspended.

Alan Stern