Re: [PATCH 2/2] iio: adc: Add driver for TI ADS1100 and ADS1000 chips

[Jonathan Cameron]

On Sun, 26 Feb 2023 09:24:02 +0100
Mike Looijmans <mike.looijmans@xxxxxxxx> wrote:

> Met vriendelijke groet / kind regards,
> 
> Mike Looijmans
> System Expert
> 
> 
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> On 25-02-2023 18:01, Jonathan Cameron wrote:
> > On Sat, 25 Feb 2023 12:03:05 +0100
> > Mike Looijmans <mike.looijmans@xxxxxxxx> wrote:
> >  
> >> Comments below - mailserver has a top-post fetish and will inject a
> >> signature here somewhere...
> >>
> >> No further comment from me means "agree, will implement in v2"...  
> > One 'nice to have' when replying where you have chunks like that
> > is to just crop them out so it's easier to spot the interesting bits.
> >
> > I've done that below.
> >  
> >> On 18-02-2023 17:48, Jonathan Cameron wrote:  
> >>> On Fri, 17 Feb 2023 10:31:28 +0100
> >>> Mike Looijmans <mike.looijmans@xxxxxxxx> wrote:
> >>>     
> >>>> The ADS1100 is a 16-bit ADC (at 8 samples per second).
> >>>> The ADS1000 is similar, but has a fixed data rate.
> >>>>
> >>>> Signed-off-by: Mike Looijmans <mike.looijmans@xxxxxxxx>  
> >>> Hi Mike,
> >>>
> >>> I probably overlapped a lot in here with what Andy said, so just ignore
> >>> anything you've already fixed.
> >>>
> >>> Biggest thing is HARDWAREGAIN.
> >>> That is very rarely used with ADCs.  It exists to support cases where
> >>> the gain is not directly coupled to the thing being measured (light
> >>> sensitivity of a time of flight sensor for example).  Userspace expects
> >>> to use SCALE to both control amplifier gains and to multiply with
> >>> the _raw value to get a value in the real world units.
> >>>
> >>> It's a bit fiddly to do the computation, but typically at start up time
> >>> you work out what the combination of each PGA gain and the reference
> >>> voltage means for the scales available and stash that somewhere to use later.  
> >> Complicating factor with this ADC is that the final gain value depends
> >> on the sampling rate as well as the power supply voltage. Which would
> >> lead to the "available" list being different after changing the sample
> >> rate and confusing userspace. If a userspace app would read the
> >> available sample rates and gains, and then proceeded to set them, the
> >> results would be weird, as the order in which they were set would
> >> matter. Setting the gain after setting the sample rate would likely
> >> result in an EINVAL error because the selected gain is no longer applicable.
> >>
> >> To me it seemed more logical to provide the analog amplification and
> >> sample rate as separate, unrelated values.  
> > It may be logical, but it isn't how the IIO ABI has ever worked and it doesn't
> > extend to more complex cases.  It's in general true that a PGA will result
> > in a change to the scale that userspace needs to apply. There are devices
> > where it doesn't. There are lots of things that at first glance 'could'
> > affect scale but often do it in complex non linear ways that userspace
> > simply can't know about - hence if we are pushing calculations into userspace
> > we need it to just be (_raw + _offset) * _scale.
> > Note that there is some wiggle room with how raw "raw" is.
> >
> > There are two solutions that have been taken in drivers.
> > 1) The above software flow is broken as any ABI write in IIO is allowed
> >     to affect other ABI elements.  This is less than ideal though.
> > 2) Let the scale free float.  So the attempt is to keep as close as possible
> >     to the set value as other things change (i.e. the sampling frequency).
> >     The scale_avail reflects current settings of everything else, and indeed
> >     changes with other ABI wirtes (this is quite common) but the interface is
> >     made more intuitive by matching as closely as possible. Thus if you change the
> >     sampling rate and the scale changes then you attempt to modify the PGA
> >     to keep it approximately the same.  Obviously it clamps at end points
> >     but nothing we can do about that.
> >
> > However, having said that, I don't 'think' we need either here...
> > A common thing to do for scale vs sampling rate (which is typically
> > either oversampling based, or based on another SAR cycle) is to just shift
> > the raw data to incorporate it - a common sensor design is to justify it
> > so that the unused bits are the LSBs - so may be a case of just not shifting
> > it to compensate for the datarate change.. That's not true here if I read
> > the datasheet correctly, but a simple
> > sysfs raw read value == raw_value << (16 - bits for data rate) should fix that.  
> 
> I agree, a bit of shifting (pun intended) is by far the better solution.
> 
> > Interesting the datasheet argues they deliberately right aligned and sign extended
> > to allow oversampling without shifts, though counter argument is they made everyone
> > who wants a real scale apply a shift.  I guess it depends on the expected use case.  
> 
> My guess is that the chip internally always runs at 128Hz and simply 
> adds the sampled values together in a 16-bit register for the lower 
> sampling rates. Someone came up with that datasheet text later on.
> 
> >>>> +
> >>>> +static const struct iio_chan_spec ads1100_channel = {
> >>>> +	.type = IIO_VOLTAGE,
> >>>> +	.differential = 0,
> >>>> +	.indexed = 0,
> >>>> +	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
> >>>> +	.info_mask_shared_by_all =
> >>>> +				BIT(IIO_CHAN_INFO_SCALE) |
> >>>> +				BIT(IIO_CHAN_INFO_HARDWAREGAIN) |
> >>>> +				BIT(IIO_CHAN_INFO_SAMP_FREQ),
> >>>> +	.info_mask_shared_by_all_available =
> >>>> +				BIT(IIO_CHAN_INFO_HARDWAREGAIN) |  
> >>> Hardware gain is normally only used for cases where the gain is not
> >>> directly related to scale.  Things like the gain on a signal that is
> >>> being used for measuring time of flight.  Here you should probably
> >>> just be using SCALE.  
> >> In this case, SCALE depends on SAMP_FREQ as well as GAIN. Which will be
> >> very confusing to userspace.
> >>
> >> Hoping for some leeway here...  
> > Sorry no. Though I think applying the shift as mentioned above deals
> > with your data rate dependent scaling and makes this all a lot easier.  
> 
> True.
> 
> ...
> >>>> +	iio_device_unregister(indio_dev);
> >>>> +
> >>>> +	ads1100_set_conv_mode(data, ADS1100_SINGLESHOT);
> >>>> +
> >>>> +	pm_runtime_disable(&client->dev);  
> >>> I'd expect runtime pm to be disable before messing with the conv mode.
> >>> With a little care you can use devm_runtime_pm_enable()  
> >> Setting the conv mode involves I2C traffic. After runtime_disable, the
> >> power supply to the chip may have been disabled, leading to
> >> communication errors on the I2C bus. Hence I thought it appropriate to
> >> write the config register before turning off power.  
> > pm_runtime_disable() is disabling the runtime management of the power
> > not the power itself.  That you need to do after turning off the
> > management (thus avoiding any races)  
> 
> Ah. But does that mean I'd need to disable the Vdd power supply here as 
> well?
> 

Yes. I'd missed that.

Jonathan