linux-kernel - Re: [PATCH v3 2/2] iio: adc: ad7173: add AD7173 driver

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Date:   Fri, 6 Oct 2023 17:18:36 +0300
From:   Ceclan Dumitru-Ioan <mitrutzceclan@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     linus.walleij@...aro.org, brgl@...ev.pl, andy@...nel.org,
        linux-gpio@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Michael Walle <michael@...le.cc>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        ChiaEn Wu <chiaen_wu@...htek.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Leonard Göhrs <l.goehrs@...gutronix.de>,
        Mike Looijmans <mike.looijmans@...ic.nl>,
        Haibo Chen <haibo.chen@....com>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        Ceclan Dumitru <dumitru.ceclan@...log.com>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] iio: adc: ad7173: add AD7173 driver

On 10/5/23 20:01, Jonathan Cameron wrote:
> On Thu,  5 Oct 2023 13:59:22 +0300
> Dumitru Ceclan <mitrutzceclan@...il.com> wrote:
> 
>> The AD7173 family offer a complete integrated Sigma-Delta ADC solution
>> which can be used in high precision, low noise single channel
>> applications or higher speed multiplexed applications. The Sigma-Delta
>> ADC is intended primarily for measurement of signals close to DC but also
>> delivers outstanding performance with input bandwidths out to ~10kHz.
>>

...

>> +	case IIO_CHAN_INFO_RAW:
>> +		ret = ad_sigma_delta_single_conversion(indio_dev, chan, val);
> 
> It's fairly usual for it to be safe to grab a single conversion when the
> buffered mode might be enabled.  Do you need an iio_device_claim_direct_mode()
> here?
> 
ad_sigma_delta_single_conversion() calls iio_device_claim_direct_mode()


>> +static int ad7173_update_scan_mode(struct iio_dev *indio_dev,
>> +				   const unsigned long *scan_mask)
>> +{
>> +	struct ad7173_state *st = iio_priv(indio_dev);
>> +	int i, ret = 0;
>> +
>> +	iio_device_claim_direct_mode(indio_dev);
> 
> This looks wrong.
> Firstly iio_device_claim_direct_mode() can fail so you always have
> to check the return value. If it does fail and you then call
> iio_release_direct_mode() it is unbalanced release of a mutex.
> 
> Secondly update_scan_mode is only called as part of buffer setup
> and there should be no races around that (and the mutex this
> tries to grab is already held.
> https://elixir.bootlin.com/linux/latest/source/drivers/iio/industrialio-buffer.c#L1265
> )
> 
> If you are protecting something device specific (rather than
> the mode) then a device specific lock should be taken.
> 

The use of a lock was inspired from ad7124, but from looking at it the only use it has
was to protect concurrent access of the device setup from write_raw (that now uses ...direct_mode())

I think it's best to drop this lock. 


>> +		chan[chan_index].differential = fwnode_property_read_bool(child, "bipolar");
> 
> bipolar doesn't normally == differential. 
> You can have unipolar differential (just that you can't get a negative answer)
> Perhaps just a terminology thing?
>

This device supports only differential channels. Here, the differential flag is used to show
if bipolar coding should be used.


>> +	st->info = device_get_match_data(dev);
>> +	if (!st->info)
>> +		return -ENODEV;
> This works for the cases of DT and ACPI but not for anyone just
> using the spi_device_id table. 
> There is spi_device_get_match_data() to cover all options.
> 
I could not find the spi_device_get_match_data() function in the repo.
It appears however as a suggestion from Andy Shevchenko in a thread:
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2382960.html
 Is this it?