linux-kernel - Re: [PATCH v2] meson saradc: add iio device attrib to switch channel 7 mux

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Date:   Mon, 29 May 2023 22:04:18 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     George Stark <GNStark@...rdevices.ru>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        "lars@...afoo.de" <lars@...afoo.de>,
        "neil.armstrong@...aro.org" <neil.armstrong@...aro.org>,
        "khilman@...libre.com" <khilman@...libre.com>,
        "jbrunet@...libre.com" <jbrunet@...libre.com>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "nuno.sa@...log.com" <nuno.sa@...log.com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-amlogic@...ts.infradead.org" 
        <linux-amlogic@...ts.infradead.org>, kernel <kernel@...rdevices.ru>
Subject: Re: [PATCH v2] meson saradc: add iio device attrib to switch channel
 7 mux

Hi George,

On Sun, May 28, 2023 at 11:52 PM George Stark <GNStark@...rdevices.ru> wrote:
[...]
> > Based on the limited description we have here, I'm not understanding why
> > you don't just express this as a set of channels. One channel per mux
> > setting, with the in_voltageX_label providing the information on what the
> > channel is connected to.
> >
> > This is an interesting facility, so good to enable for high precision calibration
> > but we still want to map it to standards signals.  Userspace doesn't
> > care that these are all being measured via the same input 7 - which
> > is itself probably an input to a MUX.
> >
> > Jonathan
>
> Hello Jonathan
>
> Thanks for the review.
>
> Your idea of exposing the mux setting as iio channels is very
> interesting and at least worth trying.
> The sysfs approach was chosen because of the code changes are simple and
> neat (compare to channels approach).
> Also calibration by using those mux inputs are already supported in the
> driver (performed at probe stage) so I expect very special usecases for
> those mux settings like debug or device production stage tests. In those
> usescases hardware specific knowledge is required anyway.
Another downside to the debugfs approach is user support:
If someone reports odd values on ADC channel 7 then we need to make
sure to double check if the mux has been altered from userspace (the
calibration during initialization ensures to leave channel 7 in a
consistent state, while a user may change the mux, forget about it and
then complain that values are wrong).


Best regards,
Martin