Re: [PATCH 2/2] iio: magnetometer: Add driver support for PNI RM3100

[Phil Reid]

On 26/09/2018 4:09 PM, Song Qiang wrote:
> 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...
The one transaction is better.
Reduces i2c traffic by 6 bytes and ensure the measurements are tightly
coupled in time for the 3 axis.

> 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...

yes that does the job.


--
Regards
Phil Reid