Re: [PATCH v2] hwmon: (sun4i-lradc) Add driver for LRADC found on Allwinner A13/A20 SoC

[Guenter Roeck <linux@roeck-us.net>]

On 4/28/22 17:28, Ruslan Zalata wrote:
> Thank you Guenter for your valuable time.
> 
> I have added update_interval option (it's in ms units, right?) and fixed all other issues you pointed to. Will test it on real hardware and send third version of the patch for review.
> 
> Regarding IRQ. Alternatively the driver would need to sit and poll conversion ready bit in a loop which might cause a much worse load on system, is not it ? Anyway, the real problem with this piece of hardware is that there's no "conversion ready bit" provided, the only way to know data ready status is to receive an interrupt.
> 

Not necessarily. The data does not have to be "current", after all,
if the hardware is able to continuously convert. If not, the question
is how long a conversion takes. If it doesn't take too long, it would
be better to initiate a conversion and then wait for the completion.

> I think it still needs a semaphore/seqlock to synchronize conversions and reads. I.e. two consequent reads should not return same old value. Although it's not an issue in my case, but could be a problem for others.
> 
Why ? That happens for almost all hwmon devices. They will all report
the most recent conversion value. Some of them can take seconds
to complete a new conversion, so the reported value is always "old"
for a given defition of old (ie any time smaller than a conversion
interval).

Sigh. Looks like I'll have to dig up the documentation and read about
the ADC myself.

Guenter