Re: [PATCH v11 4/6] iio: adc: ad4691: add SPI offload support

[David Lechner <dlechner@baylibre.com>]

On 5/18/26 10:14 AM, Sabau, Radu bogdan wrote:
>> -----Original Message-----
>> From: David Lechner <dlechner@baylibre.com>
>> Sent: Saturday, May 16, 2026 8:53 PM
> 
> ...
> 
>>>  static ssize_t sampling_frequency_show(struct device *dev,
>>>  				       struct device_attribute *attr,
>>>  				       char *buf)
>>> @@ -880,6 +1229,9 @@ static ssize_t sampling_frequency_show(struct
>> device *dev,
>>>  	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
>>>  	struct ad4691_state *st = iio_priv(indio_dev);
>>>
>>> +	if (st->manual_mode && st->offload)
>>> +		return sysfs_emit(buf, "%llu\n", READ_ONCE(st->offload-
>>> trigger_hz));
>>
>> Why do we need READ_ONCE?
>>
> 
> trigger_hz is u64 and if the target is 32-bit, a 64-bit access compiles to two 32-bit
> instructions, so show() reading it without a lock and store() writing it concurrently
> can produce a torn value at the compiler level. READ_ONCE/WRITE_ONCE suppress
> the compiler transformations that would allow that splitting or caching. We could
> have st->lock in show() instead, but that felt heavier than necessary for a single
> scalar where a transiently stale-but-whole read is fine.
> 

I would go with the mutex. It will be easier for people to understand.