Re: [PATCH v2 2/2] one-bit-adc-dac: Add initial version of one bit ADC-DAC

[Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>]

On Fri, 21 Jan 2022 10:24:18 +0000
"Pop, Cristian" <Cristian.Pop@xxxxxxxxxx> wrote:

> Hi Jonathan,
> 
> The additional functionality it is a little bit unclear to me.
> 
> > -----Original Message-----
> > From: Jonathan Cameron <jic23@xxxxxxxxxx>
> > Sent: Sunday, January 16, 2022 1:52 PM
> > To: Pop, Cristian <Cristian.Pop@xxxxxxxxxx>
> > Cc: linux-iio@xxxxxxxxxxxxxxx; linux-gpio@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; robh+dt@xxxxxxxxxx
> > Subject: Re: [PATCH v2 2/2] one-bit-adc-dac: Add initial version of one bit
> > ADC-DAC
> > 
> > [External]
> > 
> > On Tue, 11 Jan 2022 13:59:19 +0200
> > Cristian Pop <cristian.pop@xxxxxxxxxx> wrote:
> >   
> > > This allows remote reading and writing of the GPIOs. This is useful in
> > > application that run on another PC, at system level, where multiple
> > > iio devices and GPIO devices are integrated together.  
> > Hi Cristian,
> > 
> > So I fully understand why this can be useful, but it is a software only
> > construction so I'm not convinced that it makes sense to necessarily
> > configure it via device tree.   A better fit may well be configfs.
> > (note I always meant to add configfs controls for iio_hwmon but haven't
> > found the time to do it yet...)
> > 
> > As it currently stands, doing only polled / sysfs reads this driver doesn't do
> > enough to justify its existence. You could just do all this in userspace using
> > the existing gpio interfaces.  So to be useful I'd expect it to at least do
> > triggered buffer capture.  
> 
> What do you mean by triggered buffer capture? Should I save previous GPIO
> states into a buffer? What should I do with them?

As in using IIO triggers in conjunction with a Kfifo. So that you can do
in kernel capture of a series of samples triggered at the same time as
captures from ADCs.  Then it's easy to line the data up afterwards in
userspace.

Same in the output direction.  This is only really interesting if it is being
done in sync with something else - otherwise you could just use the usespace
gpio interfaces directly.

> 
> A useful use case that I see:
> - Is to register a function callback (in kernel space, maybe user space that
>   should be used by a different driver), and called in the interrupt handler
>   when the state of an input GPIO changes.

Ok. But if you can just do that directly using the gpio interaces as it will
require a bit of custom driver code anyway.

> - Output to a GPIO from a buffer using a clock, obtaining a PWM like signal.
Again, why is the IIO bit useful?  It's only useful if you are doing it
in sync with other IIO stuff and I don't just mean using the IIO in kernel
interfaces for accessing a channel in a polled mode (effectively call read_raw
/write_raw because whereever you can do that you can just the gpio directly.

Jonathan
> 
> > 
> > When we have talked about handling digital signals int he past we discussed
> > whether the IIO channel description would also benefit from tighter packing
> > than the current minimum of a byte per channel.  Perhaps we don't
> > technically 'need' it here but if we do add it in future it will be hard to retrofit
> > into this driver without big userspace ABI changes (i.e. breaking all existing
> > users).
> > 
> > I've not repeated stuff Andy got it in his review (assuming I remembered it
> > was something Andy raised).
> > 
> > Conclusion:
> > 1) Creation interface needs a rethink or strong argument why it belongs in
> > DT.
> > 2) Driver needs to do more to justify it's existence.
> > 
> > Jonathan
> >   
> > >
> > > Signed-off-by: Cristian Pop <cristian.pop@xxxxxxxxxx>
> > > ---
> > >  drivers/iio/addac/Kconfig           |   8 +
> > >  drivers/iio/addac/Makefile          |   1 +
> > >  drivers/iio/addac/one-bit-adc-dac.c | 229
> > > ++++++++++++++++++++++++++++
> > >  3 files changed, 238 insertions(+)
> > >  create mode 100644 drivers/iio/addac/one-bit-adc-dac.c
> > >
> > > diff --git a/drivers/iio/addac/Kconfig b/drivers/iio/addac/Kconfig
> > > index 138492362f20..5f311f4a747e 100644
> > > --- a/drivers/iio/addac/Kconfig
> > > +++ b/drivers/iio/addac/Kconfig
> > > @@ -17,4 +17,12 @@ config AD74413R
> > >  	  To compile this driver as a module, choose M here: the
> > >  	  module will be called ad74413r.
> > >
> > > +config ONE_BIT_ADC_DAC
> > > +	tristate "ONE_BIT_ADC_DAC driver"
> > > +	help
> > > +	  Say yes here to build support for ONE_BIT_ADC_DAC driver.  
> > 
> > Needs a lot more detail here.  Though naming it to be explicitly about GPIO to
> > IIO binding would help.
> >   
> > > +
> > > +	  To compile this driver as a module, choose M here: the
> > > +	  module will be called one-bit-adc-dac.
> > > +
> > >  endmenu
> > > diff --git a/drivers/iio/addac/Makefile b/drivers/iio/addac/Makefile
> > > index cfd4bbe64ad3..0a33f0706b55 100644
> > > --- a/drivers/iio/addac/Makefile
> > > +++ b/drivers/iio/addac/Makefile
> > > @@ -5,3 +5,4 @@
> > >
> > >  # When adding new entries keep the list in alphabetical order
> > >  obj-$(CONFIG_AD74413R) += ad74413r.o
> > > +obj-$(CONFIG_ONE_BIT_ADC_DAC) += one-bit-adc-dac.o
> > > diff --git a/drivers/iio/addac/one-bit-adc-dac.c
> > > b/drivers/iio/addac/one-bit-adc-dac.c
> > > new file mode 100644
> > > index 000000000000..5680de594429
> > > --- /dev/null
> > > +++ b/drivers/iio/addac/one-bit-adc-dac.c
> > > @@ -0,0 +1,229 @@
> > > +// SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
> > > +/*
> > > + * one-bit-adc-dac  
> > 
> > Improve description here as well.  It's really just a wrapper for a gpio in an IIO
> > channel, so just say that.  Fine to say the representation is 1 bit ADC or DAC
> > channels as well, but I think the GPIO part will be more obvious to the casual
> > reader.
> >   
> > > + *
> > > + * Copyright 2022 Analog Devices Inc.
> > > + */
> > > +
> > > +#include <linux/device.h>
> > > +#include <linux/module.h>
> > > +#include <linux/iio/iio.h>
> > > +#include <linux/platform_device.h>
> > > +#include <linux/gpio/consumer.h>
> > > +
> > > +enum ch_direction {
> > > +	CH_IN,
> > > +	CH_OUT,
> > > +};
> > > +
> > > +struct one_bit_adc_dac_state {
> > > +	int			in_num_ch;
> > > +	int			out_num_ch;
> > > +	struct platform_device  *pdev;  
> > 
> > Not used after probe so don't keep it around.
> >   
> > > +	struct gpio_descs       *in_gpio_descs;
> > > +	struct gpio_descs       *out_gpio_descs;
> > > +	const char		**labels;
> > > +};
> > > +
> > > +static int one_bit_adc_dac_read_raw(struct iio_dev *indio_dev,
> > > +	const struct iio_chan_spec *chan, int *val, int *val2, long info) {
> > > +	struct one_bit_adc_dac_state *st = iio_priv(indio_dev);
> > > +	struct gpio_descs *descs;
> > > +
> > > +	switch (info) {
> > > +	case IIO_CHAN_INFO_RAW:
> > > +		if (chan->output)
> > > +			descs = st->out_gpio_descs;
> > > +		else
> > > +			descs = st->in_gpio_descs;
> > > +		*val = gpiod_get_value_cansleep(descs->desc[chan-
> > >channel]);
> > > +
> > > +		return IIO_VAL_INT;
> > > +	default:
> > > +		return -EINVAL;
> > > +	}
> > > +}
> > > +
> > > +static int one_bit_adc_dac_write_raw(struct iio_dev *indio_dev,
> > > +			    struct iio_chan_spec const *chan,
> > > +			    int val,
> > > +			    int val2,
> > > +			    long info)
> > > +{
> > > +	struct one_bit_adc_dac_state *st = iio_priv(indio_dev);
> > > +	int channel = chan->channel;
> > > +
> > > +	if (!chan->output)
> > > +		return 0;  
> > 
> > -EINVAL;  It's an error that should be reported to userspace..
> >   
> > > +
> > > +	switch (info) {
> > > +	case IIO_CHAN_INFO_RAW:
> > > +		gpiod_set_value_cansleep(st->out_gpio_descs-
> > >desc[channel], val);
> > > +
> > > +		return 0;
> > > +	default:
> > > +		return -EINVAL;
> > > +	}
> > > +}
> > > +
> > > +static int one_bit_adc_dac_read_label(struct iio_dev *indio_dev,
> > > +	const struct iio_chan_spec *chan, char *label) {
> > > +	struct one_bit_adc_dac_state *st = iio_priv(indio_dev);
> > > +	int ch;
> > > +
> > > +	if (chan->output)
> > > +		ch = chan->channel + st->in_num_ch;
> > > +	else
> > > +		ch = chan->channel;
> > > +
> > > +	return sprintf(label, "%s\n", st->labels[ch]); }
> > > +
> > > +static const struct iio_info one_bit_adc_dac_info = {
> > > +	.read_raw = &one_bit_adc_dac_read_raw,
> > > +	.write_raw = &one_bit_adc_dac_write_raw,
> > > +	.read_label = &one_bit_adc_dac_read_label, };
> > > +
> > > +static int one_bit_adc_dac_set_ch(struct iio_chan_spec *channels,
> > > +					int num_ch,
> > > +					enum ch_direction direction)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < num_ch; i++) {
> > > +		channels[i].type = IIO_VOLTAGE;
> > > +		channels[i].indexed = 1;
> > > +		channels[i].channel = i;
> > > +		channels[i].info_mask_separate =  
> > BIT(IIO_CHAN_INFO_RAW);  
> > > +		channels[i].output = direction;
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int one_bit_adc_dac_set_channel_label(struct iio_dev *indio_dev,
> > > +						struct iio_chan_spec  
> > *channels,  
> > > +						int num_channels)
> > > +{
> > > +	struct device *device = indio_dev->dev.parent;
> > > +	struct one_bit_adc_dac_state *st = iio_priv(indio_dev);
> > > +	struct fwnode_handle *fwnode;
> > > +	struct fwnode_handle *child;
> > > +	struct iio_chan_spec *chan;
> > > +	const char *label;
> > > +	int crt_ch = 0, child_num, i = 0;
> > > +
> > > +	fwnode = dev_fwnode(device);
> > > +	child_num = device_get_child_node_count(device);
> > > +
> > > +	st->labels = devm_kzalloc(device, sizeof(*st->labels) * child_num,  
> > GFP_KERNEL);  
> > > +	if (!st->labels)
> > > +		return -ENOMEM;
> > > +
> > > +	i = child_num;
> > > +	fwnode_for_each_child_node(fwnode, child) {
> > > +		if (fwnode_property_read_u32(child, "reg", &crt_ch))
> > > +			continue;
> > > +
> > > +		if (crt_ch >= num_channels)
> > > +			continue;
> > > +
> > > +		if (fwnode_property_read_string(child, "label", &label))
> > > +			continue;
> > > +
> > > +		chan = &channels[crt_ch];  
> > ? Not used.
> >   
> > > +		st->labels[--i] = label;  
> > 
> > I've no idea how this works...  Should be looking for the chan->channel value
> > as that's what your read uses to index.
> >   
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int one_bit_adc_dac_parse_dt(struct iio_dev *indio_dev) {
> > > +	struct one_bit_adc_dac_state *st = iio_priv(indio_dev);
> > > +	struct iio_chan_spec *channels;
> > > +	int ret, in_num_ch = 0, out_num_ch = 0;
> > > +
> > > +	st->in_gpio_descs = devm_gpiod_get_array_optional(&st->pdev-
> > >dev, "in", GPIOD_IN);
> > > +	if (IS_ERR(st->in_gpio_descs))
> > > +		return PTR_ERR(st->in_gpio_descs);
> > > +
> > > +	if (st->in_gpio_descs)
> > > +		in_num_ch = st->in_gpio_descs->ndescs;
> > > +
> > > +	st->out_gpio_descs = devm_gpiod_get_array_optional(&st->pdev-
> > >dev, "out", GPIOD_OUT_LOW);
> > > +	if (IS_ERR(st->out_gpio_descs))
> > > +		return PTR_ERR(st->out_gpio_descs);
> > > +
> > > +	if (st->out_gpio_descs)
> > > +		out_num_ch = st->out_gpio_descs->ndescs;
> > > +	st->in_num_ch = in_num_ch;
> > > +	st->out_num_ch = out_num_ch;
> > > +
> > > +	channels = devm_kcalloc(indio_dev->dev.parent, in_num_ch +  
> > out_num_ch,  
> > > +				sizeof(*channels), GFP_KERNEL);
> > > +	if (!channels)
> > > +		return -ENOMEM;
> > > +
> > > +	ret = one_bit_adc_dac_set_ch(&channels[0], in_num_ch, CH_IN);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	ret = one_bit_adc_dac_set_ch(&channels[in_num_ch],  
> > out_num_ch, CH_OUT);  
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	ret = one_bit_adc_dac_set_channel_label(indio_dev, channels,  
> > in_num_ch + out_num_ch);  
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	indio_dev->channels = channels;
> > > +	indio_dev->num_channels = in_num_ch + out_num_ch;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int one_bit_adc_dac_probe(struct platform_device *pdev) {
> > > +	struct iio_dev *indio_dev;
> > > +	struct one_bit_adc_dac_state *st;
> > > +	int ret;
> > > +
> > > +	indio_dev = devm_iio_device_alloc(&pdev->dev, sizeof(*st));
> > > +	if (!indio_dev)
> > > +		return -ENOMEM;
> > > +
> > > +	st = iio_priv(indio_dev);
> > > +	st->pdev = pdev;
> > > +	indio_dev->name = "one-bit-adc-dac";
> > > +	indio_dev->modes = INDIO_DIRECT_MODE;
> > > +	indio_dev->info = &one_bit_adc_dac_info;
> > > +
> > > +	ret = one_bit_adc_dac_parse_dt(indio_dev);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return devm_iio_device_register(indio_dev->dev.parent,  
> > indio_dev); }  
> > > +
> > > +static const struct of_device_id one_bit_adc_dac_dt_match[] = {
> > > +	{ .compatible = "one-bit-adc-dac" },
> > > +	{}
> > > +};
> > > +MODULE_DEVICE_TABLE(of, one_bit_adc_dac_dt_match);
> > > +
> > > +static struct platform_driver one_bit_adc_dac_driver = {
> > > +	.driver = {
> > > +		.name = "one-bit-adc-dac",
> > > +		.of_match_table = one_bit_adc_dac_dt_match,
> > > +	},
> > > +	.probe = one_bit_adc_dac_probe,
> > > +};
> > > +module_platform_driver(one_bit_adc_dac_driver);
> > > +
> > > +MODULE_AUTHOR("Cristian Pop <cristian.pop@xxxxxxxxxx>");
> > > +MODULE_DESCRIPTION("One bit ADC DAC converter");  
> > MODULE_LICENSE("Dual  
> > > +BSD/GPL");
> > > \ No newline at end of file  
> > 
> > Make sure to eyeball your patches before sending as this sort of thing should
> > be caught at that stage...
> >   
>