linux-kernel - Re: Fwd: Re: [PATCH] USB: add USB test and measurement class driver

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Date:	Thu, 28 Aug 2008 20:22:53 +0200
From:	Marcel Janssen <korgull@...e.nl>
To:	Greg KH <greg@...ah.com>
Cc:	Marcel Janssen <marcel.janssen@...esy.nl>,
	Stefan Kopp <stefan_kopp@...lent.com>,
	Felipe Balbi <me@...ipebalbi.com>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fwd: Re: [PATCH] USB: add USB test and measurement class driver

On Thursday 28 August 2008 18:21:09 Greg KH wrote:
> On Thu, Aug 28, 2008 at 11:37:24AM +0200, Marcel Janssen wrote:
> > Hello Greg,
> >
> > I've got me a bit confused there :-)
> >
> > We currently only use the suspend/resume in our device which works on a
> > received request from the PC.
> > Basically sending suspend will make the device enter this mode. When the
> > PC enters this mode, it's also clear it will send a resume when resuming
> > from standby. But, how is this supposed to work when the PC is not in
> > sleep mode.....how will the device wake up ?
>
> It should "automatically" wake up if something happens to it.  The USB
> spec calls this out as to how this should work.  If the devices don't
> support this, we don't have to add support for it :)

Actually this is a good point.

The USB spec mentions that the device should suspend when a constant idle 
state is detected for more than 3.0ms.

I either have not seen that happening before or I missed something when 
writing my firmware. I'll keep an eye on my USB analyzer though :-)
Although it's a good thing to keep to the spec, I also believe that in the 
case of a test and measurement device suspending the device may be an 
unwanted behaviour but perhaps I do misinterpret this part of the spec here.

Anyway, this is no issue for self-powered devices, which is probably the 
majority in this class because most test & measurement devices need a good 
power supply and USB just isn't fit for that unless you're able to make a 
clean power from that (which is possible of course).
Our device is both USB and self powered, but when in USB powered mode it will 
only suspend when the host goes in sleep mode.

To be honest I would put suspend mode far down on the priority list :-)

regards,
Marcel

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