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

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Date:   Tue, 14 Nov 2017 08:28:35 +0100
From:   Martin Kepplinger <martink@...teo.de>
To:     harinath Nampally <harinath922@...il.com>
Cc:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: mma8452: add power_mode sysfs configuration

Am 14.11.2017 05:56 schrieb harinath Nampally:
> Hi Martin,
> 
>> 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!
> OK then we can map 'low_noise' to high resolution mode. But I am afraid
> I can't test the functionality because I don't have proper instruments 
> to
> measure the current draw(in microAmps) accurately.
> 
>> 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.
> I think 'oversampling' is already implemented, as I see
> 'case IIO_CHAN_INFO_OVERSAMPLING_RATIO:'
> being handled which is basically setting the all 4 different power 
> modes.
> If we also add 'power_mode', I think it would be like having two
> different user interfaces for
> same functionality. So I don't see much of value adding 'power_mode' as 
> well.
> Please correct me if I am wrong.
> 
> Thanks,
> Harinath
> 

You're right. I should've looked more closely. oversampling is there and 
seems to
work. No need to blow up this driver or let alone extend an ABI now. 
Let's drop
this patch.

thanks
                                  martin


> On Sun, Nov 12, 2017 at 7:28 AM, Martin Kepplinger <martink@...teo.de> 
> wrote:
>> 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?