iio: addac: stx104: Add 8254 Counter/Timer support

The STX104 features an 8254 Counter/Timer chip providing three
counter/timers which can be used for frequency measurement, frequency
output, pulse width modulation, pulse width measurement, event count,
etc. The STX104 provides a register bank selection to bank select
between the 8254 Bank and the Indexed Register Array Bank; the Indexed
Register Array is not utilized by this driver, so the 8254 Bank is
selected unconditionally.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/20230916112031.3634-1-william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Use regmap_read_poll_timeout() for conversion poll

ADC sample captures take a certain amount of time to complete after
initiated; this conversion time range can be anywhere from 5
microseconds to 53.68 seconds depending on the configuration of the
Analog Input Frame Timer register. When the conversion is in progress,
the ADC Status register CNV bit is high. Call regmap_read_poll_timeout()
to poll until the ADC conversion is completed (or timeout if more than
53.68 seconds passes).

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/9ef433f107afd1d4dcd2d97ef0e932d7045c2bbd.1680790580.git.william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Migrate to the regmap API

The regmap API supports IO port accessors so we can take advantage of
regmap abstractions rather than handling access to the device registers
directly in the driver.

In addition, to improve code organization in stx104_probe(), the
devm_iio_device_register() call is moved above GPIO configuration in
order to keep relevant code closer together.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/0bcdfc4738cc019fb2ff83f61eb46a3488bc166d.1680790580.git.william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Improve indentation in stx104_write_raw()

By bailing out early if chan->output is false for the IIO_CHAN_INFO_RAW,
indentation can be decreased by a tab and code readability improved.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/487d17da9e2612f3e6b2bd1c3def2fa1b955db9b.1680790580.git.william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Use define rather than hardcoded limit for write val

The DAC register is 16 bits wide, so the value passed by write_raw()
should be checked against that limit. Rather than hardcoding the 16-bit
maximum value limit, use a define to improve readability and make the
intention of the code clearer. The explicit cast is also avoided by
instead explicitly checking for negative values.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/4c9f4f1b4a270d133be70c82a091351b531b5e3e.1680790580.git.william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Fix race condition when converting analog-to-digital

The ADC conversion procedure requires several device I/O operations
performed in a particular sequence. If stx104_read_raw() is called
concurrently, the ADC conversion procedure could be clobbered. Prevent
such a race condition by utilizing a mutex.

Fixes: 4075a283ae83 ("iio: stx104: Add IIO support for the ADC channels")
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/2ae5e40eed5006ca735e4c12181a9ff5ced65547.1680790580.git.william.gray@linaro.org
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Fix race condition for stx104_write_raw()

The priv->chan_out_states array and actual DAC value can become
mismatched if stx104_write_raw() is called concurrently. Prevent such a
race condition by utilizing a mutex.

Fixes: 97a445dad37a ("iio: Add IIO support for the DAC on the Apex Embedded Systems STX104")
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/c95c9a77fcef36b2a052282146950f23bbc1ebdc.1680790580.git.william.gray@linaro.org
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: addac: stx104: Replace bitops.h header inclusion with bits.h

The <linux/bitops.h> header is included in the stx104 driver so that we
can use the BIT() macro. This macro is actually defined in the
<linux/bits.h> header, so replace the <linux/bitops.h> header inclusion
with <linux/bits.h>.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/20230318185503.341914-1-william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: stx104: Move to addac subdirectory

The stx104 driver supports both ADC and DAC functionality.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/20220815222921.138945-1-william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>