Re: [PATCH v2] TTY: serial, add pm function

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On Thu, Jan 17, 2013 at 3:32 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

> Also, why does this driver need something that the hundreds of other
> serial drivers we have in-kernel today do not?  What makes it special
> over everything else?

So (and I think Alan just seconded this) I think several vendors
already have different ugly out-of-tree hacks to solve exactly this.

The reason you haven't seen it before is that there is no AT
command to power save a US Robotics 33k8 modem.
And these are handled from userspace.

The embedded systems have:

- In-kernel driver using the TTY

- Control over both ends of the link (we have the equivalent
  of an AT command setting the 33k8 into retention).

- Critical (as in really critical) power requirements.

Blueetooth chips on megabit serial links seem to be pretty
common actually. Maybe people even have modems on these.

I think the reason you haven't seen it before is that either
people are scared of the TTY layer (whyyyy would they be that)
or that they are in the out-of-tree business. We're trying to be
in the in-tree business, so to be there we need to put the
infrastructure in place to be able to be there.

But I'm looking around for a *simple* UART-connected device
with a low-power/retention state to use as example in
question, so we don't have to conflate the CG2900 upstreaming
issue with the TTY/UART PM issue. Anyone have hints
on a suitable device?

Linus Walleij
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