linux-kernel - Re: [PATCH] iio: adc: ad7173: fix compiling without gpiolib

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Message-ID: <f5de85b7-489c-4a81-a111-fbe7a893694c@baylibre.com>
Date: Sat, 26 Apr 2025 17:45:02 -0500
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>,
 Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Guillaume Ranquet <granquet@...libre.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] iio: adc: ad7173: fix compiling without gpiolib

On 4/26/25 10:18 AM, Jonathan Cameron wrote:
> On Wed, 23 Apr 2025 00:03:38 +0300
> Andy Shevchenko <andy.shevchenko@...il.com> wrote:
> 
>> On Tue, Apr 22, 2025 at 11:12 PM David Lechner <dlechner@...libre.com> wrote:
>>>

...

>>> Not related to the fix, but I also question the use of the regmap here.
>>> This is one of the ad_sigma_delta drivers that does funny things with
>>> the SPI bus, like keeping it locked during the entire time a buffer is
>>> enabled. So, if someone tried to use a GPIO during a buffered read, the
>>> GPIO call could block (waiting for the SPI bus mutex) until the buffer
>>> is disabled, which could be an indefinitely long time. And to make it
>>> even worse, this is not an interruptible wait, so the GPIO consumer
>>> would effectively be deadlocked.  
>>
>> I would say either the entire buffer mode is broken (in software), or
>> hardware is broken and GPIO shouldn't be supported at all if the
>> buffer mode is enabled. I think the best solution here is to remove
>> the GPIO chip before enabling buffered mode. If GPIO is in use, fail
>> the buffer mode.
> I'd kind of assume that anyone using these GPIOs is doing it in a fashion
> related closely to the ADC itself.
> 
> Can we make any other use fail more cleanly? 
> 
> J
>>
> 

My inclination would be to implement it like [1] where we use iio_claim_direct()
to return -EBUSY during buffered reads to avoid the deadlock-like possibility
instead of using the gpio regmap.

[1]: https://lore.kernel.org/linux-iio/2a789531fda5031c135fc207a547f2c3f00a13ea.1744325346.git.Jonathan.Santos@analog.com/