RE: [PATCH 03/16] iio: adc: max1027: Push only the requested samples

["Sa, Nuno" <Nuno.Sa@analog.com>]

> -----Original Message-----
> From: Jonathan Cameron <jic23@kernel.org>
> Sent: Monday, August 30, 2021 12:08 PM
> To: Sa, Nuno <Nuno.Sa@analog.com>
> Cc: Miquel Raynal <miquel.raynal@bootlin.com>; Lars-Peter Clausen
> <lars@metafoo.de>; Thomas Petazzoni
> <thomas.petazzoni@bootlin.com>; linux-iio@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH 03/16] iio: adc: max1027: Push only the requested
> samples
> 
> [External]
> 
> On Fri, 20 Aug 2021 07:10:48 +0000
> "Sa, Nuno" <Nuno.Sa@analog.com> wrote:
> 
> > > -----Original Message-----
> > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > Sent: Wednesday, August 18, 2021 1:11 PM
> > > To: Jonathan Cameron <jic23@kernel.org>; Lars-Peter Clausen
> > > <lars@metafoo.de>
> > > Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>; linux-
> > > iio@vger.kernel.org; linux-kernel@vger.kernel.org; Miquel Raynal
> > > <miquel.raynal@bootlin.com>
> > > Subject: [PATCH 03/16] iio: adc: max1027: Push only the requested
> > > samples
> > >
> > > [External]
> > >
> > > When a triggered scan occurs, the identity of the desired channels
> is
> > > known in indio_dev->active_scan_mask. Instead of reading and
> > > pushing to
> > > the IIO buffers all channels each time, scan the minimum amount
> of
> > > channels (0 to maximum requested chan, to be exact) and only
> > > provide the
> > > samples requested by the user.
> > >
> > > For example, if the user wants channels 1, 4 and 5, all channels
> from
> > > 0 to 5 will be scanned but only the desired channels will be pushed
> to
> > > the IIO buffers.
> > >
> > > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> > > ---
> > >  drivers/iio/adc/max1027.c | 25 +++++++++++++++++++++----
> > >  1 file changed, 21 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/iio/adc/max1027.c b/drivers/iio/adc/max1027.c
> > > index b753658bb41e..8ab660f596b5 100644
> > > --- a/drivers/iio/adc/max1027.c
> > > +++ b/drivers/iio/adc/max1027.c
> > > @@ -360,6 +360,9 @@ static int max1027_set_trigger_state(struct
> > > iio_trigger *trig, bool state)
> > >  	struct max1027_state *st = iio_priv(indio_dev);
> > >  	int ret;
> > >
> > > +	if (bitmap_empty(indio_dev->active_scan_mask, indio_dev-
> > > >masklength))
> > > +		return -EINVAL;
> > > +
> >
> > I'm not sure this can actually happen. If you try to enable the buffer
> > with no scan element, it should give you an error before you reach
> > this point...
> >
> > >  	if (state) {
> > >  		/* Start acquisition on cnvst */
> > >  		st->reg = MAX1027_SETUP_REG |
> > > MAX1027_CKS_MODE0 |
> > > @@ -368,9 +371,12 @@ static int max1027_set_trigger_state(struct
> > > iio_trigger *trig, bool state)
> > >  		if (ret < 0)
> > >  			return ret;
> > >
> > > -		/* Scan from 0 to max */
> > > -		st->reg = MAX1027_CONV_REG | MAX1027_CHAN(0) |
> > > -			  MAX1027_SCAN_N_M | MAX1027_TEMP;
> > > +		/*
> > > +		 * Scan from 0 to the highest requested channel. The
> > > temperature
> > > +		 * could be avoided but it simplifies a bit the logic.
> > > +		 */
> > > +		st->reg = MAX1027_CONV_REG |
> > > MAX1027_SCAN_0_N | MAX1027_TEMP;
> > > +		st->reg |= MAX1027_CHAN(fls(*indio_dev-
> > > >active_scan_mask) - 2);
> > >  		ret = spi_write(st->spi, &st->reg, 1);
> > >  		if (ret < 0)
> > >  			return ret;
> > > @@ -391,11 +397,22 @@ static irqreturn_t
> > > max1027_trigger_handler(int irq, void *private)
> > >  	struct iio_poll_func *pf = private;
> > >  	struct iio_dev *indio_dev = pf->indio_dev;
> > >  	struct max1027_state *st = iio_priv(indio_dev);
> > > +	unsigned int scanned_chans = fls(*indio_dev-
> > > >active_scan_mask);
> > > +	u16 *buf = st->buffer;
> >
> > I think sparse will complain here. buffer is a __be16 restricted
> > type so you should not mix those...
> > > +	unsigned int bit;
> > >
> > >  	pr_debug("%s(irq=%d, private=0x%p)\n", __func__, irq,
> > >
> private);in/20210818_miquel_raynal_bring_software_triggers_support
> _to_max1027_like_adcs.mbx
> > >
> > >  	/* fill buffer with all channel */
> > > -	spi_read(st->spi, st->buffer, indio_dev->masklength * 2);
> > > +	spi_read(st->spi, st->buffer, scanned_chans * 2);
> > > +
> > > +	/* Only keep the channels selected by the user */
> > > +	for_each_set_bit(bit, indio_dev->active_scan_mask,
> > > +			 indio_dev->masklength) {
> > > +		if (buf[0] != st->buffer[bit])
> > > +			buf[0] = st->buffer[bit];
> >
> > Since we are here, when looking into the driver, I realized
> > that st->buffer is not DMA safe. In IIO, we kind of want to enforce
> > that all buffers that are passed to spi/i2c buses are safe... Maybe
> > this is something you can include in your series.
> 
> Why is it not?  st->buffer is result of a devm_kmalloc_array() call and
> that should provide a DMA safe buffer as I understand it.
> 

That's a good question. I'm not sure how I came to that conclusion which
is clearly wrong. Though I think the buffer might share the line with the
mutex...

- Nuno Sá