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

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Message-ID: <e4d1a6c8-1afd-671e-76bf-b5bde9dc282f@roeck-us.net>
Date:   Thu, 28 Apr 2022 22:32:15 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Ruslan Zalata <rz@...micro.ru>
Cc:     Jean Delvare <jdelvare@...e.com>, Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH v2] hwmon: (sun4i-lradc) Add driver for LRADC found on
 Allwinner A13/A20 SoC

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