linux-kernel - Re: [PATCH v2 2/2] iio: accel: Add driver for Murata SCA3300 accelerometer

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Date:   Mon, 19 Apr 2021 15:29:16 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Tomas Melin <tomas.melin@...sala.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] iio: accel: Add driver for Murata SCA3300 accelerometer

On Mon, Apr 19, 2021 at 2:36 PM Tomas Melin <tomas.melin@...sala.com> wrote:
> On 4/19/21 2:14 PM, Andy Shevchenko wrote:
> > On Mon, Apr 19, 2021 at 1:29 PM Tomas Melin <tomas.melin@...sala.com> wrote:
> >> On 4/17/21 3:39 PM, Andy Shevchenko wrote:
> >>> On Fri, Apr 16, 2021 at 5:21 PM Tomas Melin <tomas.melin@...sala.com> wrote:
> >>>> Add initial support for Murata SCA3300 3-axis industrial
> >>>> accelerometer with digital SPI interface. This device also
> >>>> provides a temperature measurement.
> > ...
> >
> >>>> +       ret = spi_sync_transfer(sca_data->spi, xfers, ARRAY_SIZE(xfers));
> >>>> +       if (ret < 0) {
> >>>> +               dev_err(&sca_data->spi->dev,
> >>>> +                       "transfer error, error: %d\n", ret);
> >>>> +               return -EIO;
> >>> Why shadowing error code?
> >> Returning EIO here to have full control over the return value from this
> >> function. As return value of this is used for testing
> > Care to show what kind of testing requires this?
> > Also why can't it be refactored to accept all error codes?
>
> I was referring to this:
>
> +static int sca3300_read_reg(struct sca3300_data *sca_data, u8 reg, int *val)
> +{
> +       int ret;
> +
> +       mutex_lock(&sca_data->lock);
> +       sca_data->txbuf[0] = 0x0 | (reg << 2);
> +       ret = sca3300_transfer(sca_data, val);
> +       mutex_unlock(&sca_data->lock);
> +       if (ret == -EINVAL)
> +               ret  = sca3300_error_handler(sca_data);
> +
> +       return ret;
> +}
> +
> +static int sca3300_write_reg(struct sca3300_data *sca_data, u8 reg, int val)
> +{
> +       int reg_val = 0;
> +       int ret;
> +
> +       mutex_lock(&sca_data->lock);
> +       sca_data->txbuf[0] = BIT(7) | (reg << 2);
> +       put_unaligned_be16(val, &sca_data->txbuf[1]);
> +       ret = sca3300_transfer(sca_data, &reg_val);
> +       mutex_unlock(&sca_data->lock);
> +       if (ret == -EINVAL)
> +               ret  = sca3300_error_handler(sca_data);
> +
> +       return ret;
> +}
>
> So this goes into error handling only when transfer indicates EINVAL
> (which happens when
>
> transfer otherwise is good, but device return status has error flags set
> i message).

In such cases I would recommend introducing your own error space (with
positive numbers) or playing around with the number of transfers (but
this usually works only if you anticipate several of them in a row).

Something like

#define SCA3300_ERROR_FLAGS 1
...

if (ret > 0)
  return error_handler(..., ret); // ret in case if you want to
convert the code to something in Linux error code space.

> >> for possible status error (EINVAL), feels more confident to have it like
> >> this to avoid any confusion. And atleast spi_sync_transfer() return value
> >>
> >> would be visible in error message.
> >>>> +       }

-- 
With Best Regards,
Andy Shevchenko