linux-kernel - Re: [PATCH v2 2/4] iio: accel: adxl345_core: Introduce set_mode and data

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Date:   Wed, 10 May 2017 21:07:35 +0800
From:   Eva Rachel Retuya <eraretuya@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     linux-iio@...r.kernel.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net, dmitry.torokhov@...il.com,
        michael.hennerich@...log.com, daniel.baluta@...il.com,
        amsfield22@...il.com, florian.vaussard@...g-vd.ch,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] iio: accel: adxl345_core: Introduce set_mode and
 data_ready functions

On Tue, May 02, 2017 at 05:32:07PM +0100, Jonathan Cameron wrote:
> On 02/05/17 12:39, Eva Rachel Retuya wrote:
> > On Mon, May 01, 2017 at 01:22:52AM +0100, Jonathan Cameron wrote:
> > Hello Jonathan,
> > [...]
> >>> +static int adxl345_set_mode(struct adxl345_data *data, u8 mode)
> >>> +{
> >>> +	struct device *dev = regmap_get_device(data->regmap);
> >>> +	int ret;
> >>> +
> >>> +	ret = regmap_write(data->regmap, ADXL345_REG_POWER_CTL, mode);
> >>> +	if (ret < 0) {
> >>> +		dev_err(dev, "Failed to set power mode, %d\n", ret);
> >>> +		return ret;
> >> drop the return ret here and just return ret at the end of the function.
> >> One of the static checkers will probably moan about this otherwise.
> > 
> > OK.
> > 
> >>> +	}
> >>> +
> >>> +	return 0;
> >>> +}
> >>> +
> >>> +static int adxl345_data_ready(struct adxl345_data *data)
> >>> +{
> >> So this is a polling the dataready bit.  Will ensure we always
> >> get fresh data when a read occurs.  Please add a comment to
> >> that effect as that's not always how devices work.
> > 
> > OK.
> > 
> >>> +	struct device *dev = regmap_get_device(data->regmap);
> >>> +	int tries = 5;
> >>> +	u32 val;
> >>> +	int ret;
> >>> +
> >>> +	do {
> >>> +		/*
> >>> +		 * 1/ODR + 1.1ms; 11.1ms at ODR of 0.10 Hz
> >>> +		 * Sensor currently operates at default ODR of 100 Hz
> >>> +		 */
> >>> +		usleep_range(1100, 11100);
> >> That's a huge range to allow... I'm not following the argument for why.
> >> Or do we have a stray 0?
> >>
> > 
> > Not a stray 0. Range is from 1.1ms to 11.1ms, this represents the
> > wake-up time when going to standby/other power saving modes ->
> > measurement mode. I'm going to clarify the comment on why it is needed
> > on the next revision.
> The point about a range sleep is to allow the kernel flexibility in scheduling
> so as to avoid waking the processor from lower power states when high precision is
> not needed.
> 
> If the thing you are talking about needs the maximum time sometimes then you
> should set the minimum value to that and add a bit to avoid unnecessary processor
> wake ups.

Thank you for explaining it. I will keep this in mind while working on
the 3rd revision.

Eva