linux-kernel - Re: [PATCH v6 2/2] iio: accel: Add support for the Bosch-Sensortec BMI088

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Date:   Mon, 25 Jan 2021 08:59:43 +0100
From:   Mike Looijmans <mike.looijmans@...ic.nl>
To:     Jonathan Cameron <jic23@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>
CC:     linux-iio <linux-iio@...r.kernel.org>,
        Dan Robertson <dan@...obertson.com>,
        Gaëtan André <rvlander@...tanandre.eu>,
        Jonathan Bakker <xc-racer2@...e.ca>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/2] iio: accel: Add support for the Bosch-Sensortec
 BMI088

See below


Met vriendelijke groet / kind regards,

Mike Looijmans
System Expert


TOPIC Embedded Products B.V.
Materiaalweg 4, 5681 RJ Best
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T: +31 (0) 499 33 69 69
E: mike.looijmans@...icproducts.com
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On 24-01-2021 14:23, Jonathan Cameron wrote:
> On Sun, 24 Jan 2021 00:21:13 +0100
> Linus Walleij <linus.walleij@...aro.org> wrote:
>
>> On Sat, Jan 23, 2021 at 4:35 PM Jonathan Cameron <jic23@...nel.org> wrote:
>>> [Me]
>>>> Next, I think it is better to let suspend/resume, i.e. system PM
>>>> reuse runtime PM since you're implementing that. This is why
>>>> we invented PM runtime force resume and force suspend.
>>> Here the driver is turning more off for full suspend than in the
>>> runtime path.  If that results in significant extra delay then
>>> it's not appropriate to have that in the runtime suspend path.
>> I see the point.
>>
>> The resume path calls bmi088_accel_enable() which incurs
>> a 5ms delay.
>>
>> The runtime resume path incurs a 1 ms delay.
>>
>> The runtime autosuspend kicks in after 2 ms.

It's set to 2 seconds as I understand it. This to support reading a 
single value every second or so.

>>
>>> Maybe the simplification of not doing the deeper power saving
>>> mode is worth the extra power cost or extra delay, but
>>> I'm not yet convinced.
>> I would personally set the autosuspend to ~20ms and just use
>> one path and take a hit of 5 ms whenever we go down between
>> measures if it is a system that is for human interaction, but for
>> control systems this more complex set-up may be better for
>> response latencies.
>>
>> The current approach may be better tuned to perfection and
>> we are all perfectionists :D
>>
>> I'm just worrying a little about bugs and maintainability.
> Fully understood.  Though for things like this I like to leave
> it at the discretion of the driver author as fairly safe they
> are a user of the device.
>
> May well make sense to go with the longer times as you
> suggest though!  Over to you Mike :)

I've been digging in the datasheet and it's really unclear how you're 
supposed to control the two power registers.

I think it's best to just put both control values into on/off state at 
the same time. I also prefer the simplicity of Linus' suggestion. I'll 
do some testing to see if the device behaves properly.

-- 
Mike Looijmans