Date:   Sat, 12 Nov 2022 21:51:36 +0100
From:   Mitja Špes <mitja@...av.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Angelo Compagnucci <angelo.compagnucci@...il.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] iio: adc: mcp3422: allow setting gain and sampling
 per channel

Hi Jonathan,

On Sat, Nov 12, 2022 at 6:15 PM Jonathan Cameron <jic23@...nel.org> wrote:
> Was it possible for these scales to differ before this change?
Yes. The difference is that before this change you could only see and set
available scales that were available for specified sampling rate. Now you're
able to set gain and sampling rate via scale. So before the change you got
these (@240sps):

0.001000000 0.000500000 0.000250000 0.000125000

Now you get the complete set:
/*                 gain x1     gain x2     gain x4     gain x8  */
/* 240 sps    */ 0.001000000 0.000500000 0.000250000 0.000125000
/*  60 sps    */ 0.000250000 0.000125000 0.000062500 0.000031250
/*  15 sps    */ 0.000062500 0.000031250 0.000015625 0.000007812
/*   3.75 sps */ 0.000015625 0.000007812 0.000003906 0.000001953

> If not, then why was the previous patch a fix rather than simply a precursor
> to this change (where it now matters).
I wanted to separate a bug fix from improvements, if these were rejected for
for some reason.

> There are a number of changes in here which are more stylistic cleanup
> than anything to do with the functional change. Please pull those out
> to a precursor patch where we can quickly check they make not functional changes
> rather than having them mixed in here.
Will do.

> I have no particular problem with taking these from hex
> to decimal, though I'm not really seeing the necessity.
>
> However, it is really a style question and should not be in this
> patch where it mostly adds noise making it slightly harder
> to spot the functional changes.
My styling preference. I think indexes should be decimal so they are not
confused with flags.

> >               .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) \
> > -                             | BIT(IIO_CHAN_INFO_SCALE), \
> > -             .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SAMP_FREQ), \
> > +                             | BIT(IIO_CHAN_INFO_SCALE) \
> > +                             | BIT(IIO_CHAN_INFO_SAMP_FREQ), \
>
> Hmm. This is an ABI change.  Hopefully no one will notice however.
Indeed. I've noted this in the cover letter.


> > -static const int mcp3422_read_times[4] = {
> > +static const int mcp3422_read_times[MCP3422_SRATE_COUNT] = {
> Reasonable to make this change, but I think it belongs in a precursor patch.
Will fix.

> > +     for (i = 0; i < MCP3422_SRATE_COUNT; i++) {
> > +             count += sprintf(buf + count, "0.%09u 0.%09u 0.%09u 0.%09u%s",
> > +                     mcp3422_scales[i][0],
> > +                     mcp3422_scales[i][1],
> > +                     mcp3422_scales[i][2],
> > +                     mcp3422_scales[i][3],
> > +                     (i < MCP3422_SRATE_COUNT - 1 ? " " : "\n"));
>
> What does the output of this now look like?
0.001000000 0.000500000 0.000250000 0.000125000
0.000250000 0.000125000 0.000062500 0.000031250
0.000062500 0.000031250 0.000015625 0.000007812
0.000015625 0.000007812 0.000003906 0.000001953
All in one line.

> For available attributes we tend to only show the values available assuming
> just the one thing is changing.  Hence hold sampling frequency static, then
> show what scales are available
> It can get complex if there are nasty interactions so we might have a
> situation where one attribute allows to all the possible values.
> So maybe we have all scales available and on a write try to find
> the nearest frequency to the current at which we can deliver the
> required scale.
That's what's being done here:
+ for (j = 0; j < MCP3422_SRATE_COUNT; j++) {
+ for (i = 0; i < MCP3422_PGA_COUNT; i++) {
+ if (val2 == mcp3422_scales[j][i]) {
+ config &= ~MCP3422_PGA_MASK;
+ config |= MCP3422_PGA_VALUE(i);
+ config &= ~MCP3422_SRATE_MASK;
+ config |= MCP3422_SAMPLE_RATE_VALUE(j);
+ adc->ch_config[req_channel] = config;
+ return 0;
+ }
  }
  }

Before it looked at only one sampling frequency and all gains, now it looks at
all combinations.
Looking at this I agree that it would be better to find nearest instead of
looking for an exact match.

> My gut feeling for this device is make the sampling frequency the dominant
> attribute.  So just list the available sampling frequencies not taking
> scale into account.  For current sampling frequency just list the available
> scales and only accept those to be written to the scale attr.
That way the order in which you set attributes matters. Is that really better?
This device has a settable sampling rate and gain and they are independent.
But we could only set gain via scale which values were sampling rate dependent.

> >       /* meaningful default configuration */
> > +     for (i = 0; i < 4; i++) {
> > +             adc->ch_config[i] = (MCP3422_CONT_SAMPLING
> > +             | MCP3422_CHANNEL_VALUE(i)
> > +             | MCP3422_PGA_VALUE(MCP3422_PGA_1)
> > +             | MCP3422_SAMPLE_RATE_VALUE(MCP3422_SRATE_240));
> > +     }
> > +
> >       config = (MCP3422_CONT_SAMPLING
> >               | MCP3422_CHANNEL_VALUE(0)
> >               | MCP3422_PGA_VALUE(MCP3422_PGA_1)
>
> Perhaps use the first channel configuration for this?
Will fix.

Kind regards,
Mitja

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