Re: EHCI and MUSB do not discover devices without CONFIG_PM

[Greg KH <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>]

On Tue, Nov 28, 2017 at 09:57:51AM +0100, Ladislav Michl wrote:
> Hi Greg,
> 
> On Tue, Nov 28, 2017 at 08:33:28AM +0100, Greg KH wrote:
> > On Mon, Nov 27, 2017 at 11:08:33PM +0100, Ladislav Michl wrote:
> > > Hi there,
> > > 
> > > USB hosts do not discover any connected device on OMAP3 based board
> > > with CONFIG_PM=n. Just enabling this option is enough to restore working
> > > behaviour. Nothing unusual in log. Tested 4.14.2 and 4.15-rc1. I know
> > > a lot of stuff depends on CONFIG_PM, but is this expected behaviour?
> > > Neither EHCI nor MUSB is working without CONFIG_PM.
> > 
> > What bus type is your controllers on?  PCI?  platform?  Something else?
> 
> Platform controllers inside OMAP3630 Soc.
> 
> > And yes, perhaps this is to be expected, why would you not want
> > CONFIG_PM to be enabled?  :)
> 
> For a start, I know Linux is general purpose OS and I know I cannot expect
> low latency or low jitter when dealing with interrupts.

Well, it's the best latency of any other OS out there :)

Anyway, if you want guaranteed response time, you are going to have to
use the RT patchset, no matter what.  Otherwise you have the potential
to have bad jitter at times.

> Original problem is described here:
> https://www.spinics.net/lists/linux-omap/msg140081.html
> 
> Shortly, with CONFIG_PM jitter of GPIO interrupt is about 350us which
> renders IR receiver unuseable - is cannot reliably decode IR protocol
> (gpio-ir-recv is used). With CONFIG_PM disabled, jitter is around 30us
> and that's enough to make IR decoders work.

bit-banging an ir decoder, ugh, you are in for a world of hurt.  Can't
you put a chip on the device that does this for you in hardware?

Anyway, good luck!

greg k-h
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