linux-kernel - Re: [PATCH] i2c-mux-pca954x: Disable mux after 200ms timeout

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Date:	Tue, 26 Nov 2013 18:00:19 +0100
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Mike Looijmans <mike.looijmans@...ic.nl>
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH] i2c-mux-pca954x: Disable mux after 200ms timeout


> I had looked a bit in that direction, but I think there's currently
> no way for a driver to say "I won't be needing the bus for a while".
> Something like that would be critical for such a pm system to work.

Yes. I wasn't sure if something already existed.

> In any case, it doesn't sound like something I can sell - it's
> understandable for my employer that I spent an extra hour or so to
> clean up and submit the code to upstream, but this appears to go
> into a different class of rework.
>
> So where do you want to go with this? Should I rework the patch to
> make the timeout optional, or should I simply forget about
> integrating at all?

I understand your constraints, yet from a maintenance perspective I
shouldn't have such code in a driver. So, out-of-tree that is for now.

> In fact, on the customer's board, the pca mux is powered by a supply
> so the whole mux can be powered-down too, for which I also needed to
> patch the pca driver to reset its state when the powersupply
> reported that it was going down. I think that particular patch isn't
> of much use to the community though, or is it?

If it uses standard pm callbacks, I'd think this makes sense.

> Most hardware can control power and clocks to the I2C controller,
> which would indeed account for some power savings. But again, that
> would require drivers to provide estimations as to when they will
> need the bus. And it would require much more information on the bus
> controller too, though I suspect that to be the easier part.

There are drivers gating off the clocks simply after every transfer. I
don't know your HW details and workloads, but I wondered if you can
unconditionally switch off the core, do some pinmuxing...

> > Anyway, thanks for letting me know about your requirements (they should
> > have been in the original commit message, though ;))
> 
> I'm new to Linux kernel development, so please forgive me...

That was just a pointer, no complaint. You did a fine job in supplying
information around your patch, so thanks.


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