Re: [PATCH v2 5/8] iio: dac: ds4424: convert to regmap

Sander Vanheule
2026-02-01T14:52:17Z

Hi Oleksij,

On Tue, 2026-01-27 at 07:09 +0100, Oleksij Rempel wrote:
> Refactor the driver to use the regmap API.
>
> Replace the driver-specific mutex and manual shadow buffers with the
> standard regmap infrastructure for locking and caching.
>
> This ensures the cache is populated from hardware at probe, preventing
> state desynchronization (e.g. across suspend/resume).

[...]

> +static const struct regmap_access_table ds44x4_table = {
> + .yes_ranges = ds44x4_ranges,
> + .n_yes_ranges = ARRAY_SIZE(ds44x4_ranges),
> +};
>
> -static int ds4424_set_value(struct iio_dev *indio_dev,
> -      int val, struct iio_chan_spec const *chan)
> +static const struct regmap_config ds44x2_regmap_config = {
> + .reg_bits = 8,
> + .val_bits = 8,
> + .cache_type = REGCACHE_FLAT,
> + .max_register = DS4424_DAC_ADDR(1),
> + .rd_table = &ds44x2_table,
> + .wr_table = &ds44x2_table,
> +};

Note that REGCACHE_FLAT will allocate 0xF8 unsigned longs you will never use.
REGCACHE_MAPLE will probably be much closer to the size of the original value
cache, for a small look-up performance penalty (but always fast compared to the
I2C bus).

[...]

> @@ -163,49 +184,52 @@ static int ds4424_write_raw(struct iio_dev *indio_dev,
>
>  static int ds4424_verify_chip(struct iio_dev *indio_dev)
>  {
> - int ret, val;
> + struct ds4424_data *data = iio_priv(indio_dev);
> + u8 raw_values[DS4424_MAX_DAC_CHANNELS];
> + int ret;
>
> - ret = ds4424_get_value(indio_dev, &val, 0);
> - if (ret < 0)
> - dev_err(&indio_dev->dev,
> - "%s failed. ret: %d\n", __func__, ret);
> + /* Bulk read all channels starting at 0xf8.
> + * This populates the regmap cache with current HW values.
> + */
> + ret = regmap_bulk_read(data->regmap, DS4424_DAC_ADDR(0),
> +        raw_values, indio_dev->num_channels);

Are you forcing a HW-to-cache sync for performance?

Previously this function would just read a single value to verify a reply was
sent, instead of seeding the cache in data->raw, so that's a change in behavior
(and purpose) of this function, meaning you may want to change the name if you
keep this.

You could (should IMHO) use a sparse cache, which is actually aware of its
content's validity and will transparently access the device on the first access
to initialize itself. I would recommend REGCACHE_MAPLE as above, but
REGCACHE_FLAT_S also works. REGCACHE_FLAT is tricky to properly initialize [1],
and I would not recommend using it in new code.

Using a single regmap_read() here (as ds4424_get_value() before) with a sparse
cache would result in an error if there was a bus error, giving you the
verification behavior again.

[1] https://patch.msgid.link/20251023135032.229511-1-sander@svanheule.net/

Best,
Sander