linux-kernel - Re: [PATCH] iio: mma8452: add power

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Date:   Sun, 12 Nov 2017 13:28:00 +0100
From:   Martin Kepplinger <martink@...teo.de>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     harinath922@...il.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: mma8452: add power_mode sysfs configuration

On 2017-11-11 01:33, Jonathan Cameron wrote:
> On Mon, 6 Nov 2017 08:19:58 +0100
> Martin Kepplinger <martink@...teo.de> wrote:
> 
>> This adds the power_mode sysfs interface to the device as documented in
>> sysfs-bus-iio.
>>
>> ---
>>
>> Note that I explicitely don't sign off on this.
>>
>> This is a starting point for anybody who can test it and check for correct
>> API usage, and ABI correctness, as documented in Documentation/ABI/testing/sys-bus-iio
>> (grep it for "power_mode"). The ABI doc probably would need an addition
>> too, if the 4 power modes here seem generally useful (there are only
>>  2 listed there)!
>>
>> So, if you can test this, feel free to set up a proper patch or
>> two, and I'm happy to review.
>>
>> Please note that this patch is quite old. It really should be that simple
>> as far as my understanding back then. We always list the available frequencies
>> of the given power mode we are in, for example, already, and everything
>> basically is in place except for the user interface.
> 
> Hmm. A lot of devices support something along these lines.  The issue
> has always been - how is userspace to figure out what to do with it?
> It's all very vague...
> 
> Funnily enough - this used to be really common, but is becoming less so
> now - presumably because no one was using it much (or maybe I am reading
> too much into that ;)
> 
> Now the question is whether it can be tied to better defined things?
> 
> Here low noise restricts the range to 4g.  Issue is that we don't actually
> have writeable _available attributes (which correspond to range in this case).
> 

Does it? Isn't it merely less oversampling.

> Low power mode... This one is apparently oversampling.  If possible support
> it as that as we have well defined interfaces for that.
> 
> Jonathan.

Ah, I remember; the oversampling settings was actually a reason why I
hadn't submitted the patch :) The oversampling API would definitely be
more accurate.

I would like "oversampling" more than this "power_mode" too. For this
driver it would be far more complicated to implement though. I doubt
that it'll be done. power_mode is basically already there implicitely,
and given that there *is* the ABI, we could offer it for free.

But given your concerns, I would strip down this patch to only offer the
already documented "low_noise" and "low_power" modes. It wouldn't be
worth it to extend the ABI just because of this!

Users would have a simple switch if they don't really *want* to know the
details. I think it can be useful to just say "I don't care about power
consuption. Be as accurate as possible" or "I just want this think to
work. Use a little power as possible." Sure it's vage, but would it be
useless?