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Date:   Wed, 26 Sep 2018 16:09:45 +0800
From:   Song Qiang <songqiang1304521@gmail.com>
To:     Phil Reid <preid@electromag.com.au>
Cc:     Jonathan Cameron <jonathan.cameron@huawei.com>, jic23@kernel.org,
        knaack.h@gmx.de, lars@metafoo.de, pmeerw@pmeerw.net,
        robh+dt@kernel.org, mark.rutland@arm.com,
        rtresidd@electromag.com.au, linux-iio@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] iio: magnetometer: Add driver support for PNI RM3100
Message-ID: <20180926080752.GA12183@Eros>
References: <20180925031724.21399-1-songqiang1304521@gmail.com>
 <20180925031724.21399-2-songqiang1304521@gmail.com>
 <20180925143054.00007fcb@huawei.com>
 <57d8a9d6-c402-068e-0a21-bbd6c20a7816@electromag.com.au>
 <20180926014951.GA7094@Eros>
 <4de691f2-dbc9-6124-efa6-66f2dd7e0f2b@electromag.com.au>
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On Wed, Sep 26, 2018 at 10:30:34AM +0800, Phil Reid wrote:
> On 26/09/2018 9:49 AM, Song Qiang wrote:
> > On Tue, Sep 25, 2018 at 10:36:54PM +0800, Phil Reid wrote:
> > > On 25/09/2018 9:30 PM, Jonathan Cameron wrote:
> > > > > +static irqreturn_t rm3100_trigger_handler(int irq, void *p)
> > > > > +{
> > > > > +	struct iio_poll_func *pf = p;
> > > > > +	struct iio_dev *indio_dev = pf->indio_dev;
> > > > > +	struct rm3100_data *data = iio_priv(indio_dev);
> > > > > +	struct regmap *regmap = data->regmap;
> > > > > +	u8 buffer[9];
> > > > > +	int ret;
> > > > > +	int i;
> > > > > +
> > > > > +	mutex_lock(&data->lock);
> > > > > +	ret = rm3100_wait_measurement(data);
> > > > > +	if (ret < 0) {
> > > > > +		mutex_unlock(&data->lock);
> > > > > +		goto done;
> > > > > +	}
> > > > > +
> > > > > +	ret = regmap_bulk_read(regmap, RM3100_REG_MX2, buffer, sizeof(buffer));
> > > > > +	mutex_unlock(&data->lock);
> > > > > +	if (ret < 0)
> > > > > +		goto done;
> > > > > +
> > > > > +	/* Convert XXXYYYZZZxxx to XXXxYYYxZZZx. x for padding. */
> > > > > +	for (i = 0; i < 3; i++)
> > > > > +		memcpy(data->buffer + i * 4, buffer + i * 3, 3);
> > > > Firstly X doesn't need copying.
> > > > Secondly the copy of Y actually overwrites the value of Z
> > > > XXXYYYZZZxxx
> > > > XXXxYYYZZxxx
> > > > XXXxYYYxYZZx
> > > > 
> > > > I think...
> > > > 
> > > > > +
> > > > > +	iio_push_to_buffers_with_timestamp(indio_dev, data->buffer,
> > > > > +			iio_get_time_ns(indio_dev));
> > > 
> > > memcpy target is a different buffer so should be ok.
> > > 
> > > But that raises the question of does it need to be?
> > > 'buffer' could be 12 bytes long and just shuffle Z then Y.
> > > Do the unused bytes need to be zeroed? or does libiio mask them anyway?
> > > 
> > > 
> > > 
> > > -- 
> > > Regards
> > > Phil Reid
> > 
> > Hi Phil,
> > 
> > This is interesting, last patch I submitted uses three transactions and
> > shuffles X, Y and Z respectively. You said it should be better to use one
> > transactions, I thought it makes point, and one transaction may reduce
> > IO pressure of the i2c bus. :)
> > And that's not necessary for unused bytes to be zero. I'm not familiar
> > with libiio, actually just been studying it, can't say anything about
> > it.
> > 
> > yours,
> > Song Qiang
> > 
> > 
> G'day Song,
> 
> yes the one transaction suggestion was to reduce pressure on the bus.
> I think also with 3 calls you can up up with other devices taking over
> the i2c / spi bus in between.
> 
> We've got a devkit for this part, but haven't got to wiring it up to our system as yet.
> We're looking at using the i2c interface which could push things at max samplerate, so yes I'm
> keen to see bus pressure reduced as much as possible.
> 
> I was thinking something like the following:
> 
> u8 buffer[12];
> regmap_bulk_read(regmap, RM3100_REG_MX2, buffer, 9);
> 
> buffer[10] = buffer[8]; // or memcpy or some other fancy shuffle code.
> buffer[9]  = buffer[7];
> buffer[8]  = buffer[6];
> 
> buffer[6]  = buffer[5];
> buffer[5]  = buffer[4];
> buffer[4]  = buffer[3];
> 
> iio_push_to_buffers_with_timestamp(indio_dev, buffer, iio_get_time_ns(indio_dev));
> 
> but I'm unsure if this would be needed:
> buffer[7] = 0
> buffer[3] = 0
> 
> What you've got does the job I think.
> 
> I haven't dug into the datasheet in great detail, and my iio knownledge is limited.
> Are you sure the RM3100_CHANNEL scantype endianness is IIO_LE.
> rm3100_read_mag looks to be doing big endian conversion and the datasheet agrees with that.
> 
> 
> -- 
> Regards
> Phil Reid
> 

Hi Phil,

You're absolutely right!
This should be big endian, I think I probably just want something there
when I was writing this code, planned to change it later, but apparently
I've forgotten it...

AFAIK, filling places we do not need with 0 is not needed, we just
extract valid data from valid bit field(24 here).

Both one transaction and three transactions way have their point, but
this is a OS, probably the spiltted one is better, I need some real
thinking about this...

I could have use the same buffer to read from the sensor and send it to
userspace like this:

int i = 0;
ret = regmap_bulk_read(regmap, RM3100_REG_MX2, 9);
if(ret)
	...
/* Convert XXXYYYZZZxxx to XXXxYYYxZZZx. */
for (i = 0; i < 2; i++)
	memcpy(buffer + (2 - i) * 4, buffer + (2 - i) * 3), 3);

This code snippet will use the same buffer, actually that's what I was
using the first time. Jonathan must thinks so, from what he commented,
he assumed I was using the same buffer, also what you want.
But I changed this due to Peter's comment, maybe not a big deal, he
suggests to use sizeof(buffer), this makes me use an additional size 9
buffer. I thought this doesn't matter too much, just some additional
space from the stack, but now I think maybe less memory using would be
better...
After all, this length 9 seems like never shouldn't be changed...

yours,
Song Qiang