Re: [PATCH v4 3/3] iio: magnetometer: Add driver support for PNI RM3100

[Song Qiang]

On 2018/10/21 äå10:14, Jonathan Cameron wrote:
> On Thu, 18 Oct 2018 16:24:15 +0800
> Song Qiang <songqiang1304521@xxxxxxxxx> 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;
> > > > +	unsigned long scan_mask = *indio_dev->active_scan_mask;
> > > > +	unsigned int mask_len = indio_dev->masklength;
> > > > +	struct rm3100_data *data = iio_priv(indio_dev);
> > > > +	struct regmap *regmap = data->regmap;
> > > > +	int ret, i, bit;
> > > > +
> > > > +	mutex_lock(&data->lock);
> > > > +	switch (scan_mask) {
> > > > +	case BIT(0) | BIT(1) | BIT(2):
> > > > +		ret = regmap_bulk_read(regmap, RM3100_REG_MX2, data->buffer, 9);
> > > > +		mutex_unlock(&data->lock);
> > > > +		if (ret < 0)
> > > > +			goto done;
> > > > +	break;
> > > > +	case BIT(0) | BIT(1):
> > > > +		ret = regmap_bulk_read(regmap, RM3100_REG_MX2, data->buffer, 6);
> > > > +		mutex_unlock(&data->lock);
> > > > +		if (ret < 0)
> > > > +			goto done;
> > > > +	break;
> > > > +	case BIT(1) | BIT(2):
> > > > +		ret = regmap_bulk_read(regmap, RM3100_REG_MY2, data->buffer, 6);
> > > > +		mutex_unlock(&data->lock);
> > > > +		if (ret < 0)
> > > > +			goto done;
> > > > +	break;
> > > What about BIT(0) | BIT(2)?
> > >
> > > Now you can do it like you have here and on that one corner case let the iio core
> > > demux code take care of it, but then you will need to provide available_scan_masks
> > > so the core knows it needs to handle this case.
> > >   
> > This confused me a little. The available_scan_masks I was using is {BIT(0) |
> > BIT(1) | BIT(2), 0x0}. Apparently in this version of patch I would like it to
> > handle every circumstances like BIT(0), BIT(0) | BIT(2), BIT(1) | BIT(2), etc.
> > Since Phil mentioned he would like this to reduce bus usage as much as we can
> > and I want it, too, I think these three circumstances can be read consecutively
> > while others can be read one axis at a time. So I plan to let BIT(0) | BIT(2)
> > fall into the 'default' section, which reads axis one by one.
> >
> > My question is, since this handles every possible combination, do I still need
> > to list every available scan masks in available_scan_masks?
> Ah. I see, I'd missed that the default was picking up that case as well as the
> single axes.   It would be interesting to sanity check if it is quicker on
> a 'typical' platform to do the all axis read for the BIT(0) | BIT(2) case
> and drop the middle value (which would be done using available scan_masks)
> or to just do two independent reads.
>
> (I would guess it is worth reading the 'dead' axis).
>
> > All other problems will be fixed in the next patch.
> >
> > yours,
> >
> > Song Qiang
> >
> >
> > ...
> Thanks,
>
> Jonathan

I tested this two ways of getting data with the following code snippet:


ÂÂÂ u8 buffer[9];
ÂÂÂ struct timeval timebefore, timeafter;

ÂÂÂ do_gettimeofday(&timebefore);
ÂÂÂ ret = regmap_bulk_read(regmap, RM3100_REG_MX2, buffer, 9);
ÂÂÂ if (ret < 0)
ÂÂÂ ÂÂÂ goto unlock_return;
ÂÂÂ do_gettimeofday(&timeafter);
ÂÂÂ printk(KERN_INFO "read with dead axis time: %ld",
ÂÂÂ ÂÂÂÂÂÂ timeafter.tv_sec * 1000000 + timeafter.tv_usec -
ÂÂÂ ÂÂÂÂÂÂ timebefore.tv_sec * 1000000 - timebefore.tv_usec);
ÂÂÂ do_gettimeofday(&timebefore);

ÂÂÂ ret = regmap_bulk_read(regmap, RM3100_REG_MX2, buffer, 3);
ÂÂÂ if (ret < 0)
ÂÂÂ ÂÂÂ goto unlock_return;
ÂÂÂ ret = regmap_bulk_read(regmap, RM3100_REG_MZ2, buffer + 6, 3);
ÂÂÂ if (ret < 0)
ÂÂÂ ÂÂÂ goto unlock_return;
ÂÂÂ do_gettimeofday(&timeafter);
ÂÂÂ printk(KERN_INFO "read two single axis time: %ld",
ÂÂÂ ÂÂÂÂÂÂ timeafter.tv_sec * 1000000 + timeafter.tv_usec -
ÂÂÂ ÂÂÂÂÂÂ timebefore.tv_sec * 1000000 - timebefore.tv_usec);


And get this result:

[Â 161.264777] read with dead axis time: 883
[Â 161.270621] read two single axis time: 1359
[Â 161.575134] read with dead axis time: 852
[Â 161.580973] read two single axis time: 1356
[Â 161.895704] read with dead axis time: 854
[Â 161.903744] read two single axis time: 3540
[Â 162.223600] read with dead axis time: 853
[Â 162.229451] read two single axis time: 1363
[Â 162.591227] read with dead axis time: 850
[Â 162.597630] read two single axis time: 1555
[Â 162.920102] read with dead axis time: 852
[Â 162.926467] read two single axis time: 1534
[Â 163.303121] read with dead axis time: 881
[Â 163.308997] read two single axis time: 1390
[Â 163.711004] read with dead axis time: 861


It seems like you're right! Reading consecutively 9 bytes does save a lot time 
compared to read 3 bytes twice.