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([101.0.62.236]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82f0c4e3d41sm19077474b3a.48.2026.04.14.10.03.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Apr 2026 10:03:43 -0700 (PDT) From: Nikhil Gautam To: linux-iio@vger.kernel.org Cc: jic23@kernel.org, dlechner@baylibre.com, Nikhil Gautam Subject: [PATCH v6 3/3] iio: dac: mcp4821: add configurable gain support Date: Tue, 14 Apr 2026 22:33:08 +0530 Message-ID: <20260414170309.6151-4-nikhilgtr@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260414170309.6151-1-nikhilgtr@gmail.com> References: <20260414170309.6151-1-nikhilgtr@gmail.com> Precedence: bulk X-Mailing-List: linux-iio@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Add support for configuring the DAC gain using the GA bit The MCP4821 supports two gain settings: - 1x gain → 2.048V full-scale - 2x gain → 4.096V full-scale Scale write support is added in the IIO interface. Only scale values advertised via the scale_available attribute are accepted, ensuring consistency between the configured gain and exposed scale values. Signed-off-by: Nikhil Gautam --- drivers/iio/dac/mcp4821.c | 128 ++++++++++++++++++++++++++++++++------ 1 file changed, 108 insertions(+), 20 deletions(-) diff --git a/drivers/iio/dac/mcp4821.c b/drivers/iio/dac/mcp4821.c index 102ae3718514..18b5934fb8a2 100644 --- a/drivers/iio/dac/mcp4821.c +++ b/drivers/iio/dac/mcp4821.c @@ -12,13 +12,13 @@ * MCP48x2: https://ww1.microchip.com/downloads/en/DeviceDoc/20002249B.pdf * * TODO: - * - Configurable gain * - Regulator control */ #include #include #include +#include #include #include @@ -26,10 +26,30 @@ #include #define MCP4821_ACTIVE_MODE BIT(12) +#define MCP4821_GAIN_ENABLE BIT(13) #define MCP4802_SECOND_CHAN BIT(15) -/* DAC uses an internal Voltage reference of 4.096V at a gain of 2x */ -#define MCP4821_2X_GAIN_VREF_MV 4096 +/* DAC uses an internal Voltage reference of 2.048V */ +#define MCP4821_VREF_MV 2048 + +/* + * MCP48xx DAC output: + * + * Vout = (Vref * D / 2^N) * G + * + * where: + * - Vref = 2.048V (internal reference) + * - N = DAC resolution (12 bits for MCP4821) + * - G = gain selection: + * 1x when GA bit = 1 + * 2x when GA bit = 0 (default) + * + * Therefore full-scale voltage is: + * - 1x gain: 2.048V + * - 2x gain: 4.096V + * + * Scale = Vfull-scale / 2^N + */ enum mcp4821_supported_device_ids { ID_MCP4801, @@ -43,6 +63,8 @@ enum mcp4821_supported_device_ids { struct mcp4821_state { struct spi_device *spi; u16 dac_value[2]; + int gain; + int scale_avail[4]; }; struct mcp4821_chip_info { @@ -57,6 +79,7 @@ struct mcp4821_chip_info { .channel = (channel_id), \ .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \ .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE), \ + .info_mask_shared_by_type_available = BIT(IIO_CHAN_INFO_SCALE), \ .scan_type = { \ .realbits = (resolution), \ .shift = 12 - (resolution), \ @@ -121,8 +144,9 @@ static int mcp4821_read_raw(struct iio_dev *indio_dev, case IIO_CHAN_INFO_RAW: *val = state->dac_value[chan->channel]; return IIO_VAL_INT; + case IIO_CHAN_INFO_SCALE: - *val = MCP4821_2X_GAIN_VREF_MV; + *val = MCP4821_VREF_MV * state->gain; *val2 = chan->scan_type.realbits; return IIO_VAL_FRACTIONAL_LOG2; default: @@ -130,6 +154,17 @@ static int mcp4821_read_raw(struct iio_dev *indio_dev, } } +static void mcp4821_calc_scale(int vref_mv, int resolution, + int *val, int *val2) +{ + s64 tmp; + int micro; + + tmp = (s64)vref_mv * MICRO >> resolution; + *val = div_s64_rem(tmp, MICRO, µ); + *val2 = micro; +} + static int mcp4821_write_raw(struct iio_dev *indio_dev, struct iio_chan_spec const *chan, int val, int val2, long mask) @@ -138,35 +173,85 @@ static int mcp4821_write_raw(struct iio_dev *indio_dev, u16 write_val; __be16 write_buffer; int ret; + int v, v2; - if (val2 != 0) - return -EINVAL; + switch (mask) { + case IIO_CHAN_INFO_RAW: - if (val < 0 || val >= BIT(chan->scan_type.realbits)) - return -EINVAL; + if (val2 != 0) + return -EINVAL; - if (mask != IIO_CHAN_INFO_RAW) - return -EINVAL; + if (val < 0 || val >= BIT(chan->scan_type.realbits)) + return -EINVAL; + + write_val = MCP4821_ACTIVE_MODE | val << chan->scan_type.shift; + if (chan->channel) + write_val |= MCP4802_SECOND_CHAN; - write_val = MCP4821_ACTIVE_MODE | val << chan->scan_type.shift; - if (chan->channel) - write_val |= MCP4802_SECOND_CHAN; + /* GA bit = 1 -> 1x gain */ + if (state->gain == 1) + write_val |= MCP4821_GAIN_ENABLE; - write_buffer = cpu_to_be16(write_val); - ret = spi_write(state->spi, &write_buffer, sizeof(write_buffer)); - if (ret) { - dev_err(&state->spi->dev, "Failed to write to device: %d", ret); - return ret; + write_buffer = cpu_to_be16(write_val); + ret = spi_write(state->spi, &write_buffer, sizeof(write_buffer)); + if (ret) { + dev_err(&state->spi->dev, "Failed to write to device: %d", ret); + return ret; + } + + state->dac_value[chan->channel] = val; + return 0; + + case IIO_CHAN_INFO_SCALE: + mcp4821_calc_scale(MCP4821_VREF_MV, chan->scan_type.realbits, &v, &v2); + if (val == v && val2 == v2) { + state->gain = 1; + return 0; + } + + mcp4821_calc_scale(MCP4821_VREF_MV * 2, + chan->scan_type.realbits, &v, &v2); + if (val == v && val2 == v2) { + state->gain = 2; + return 0; + } + return -EINVAL; + default: + return -EINVAL; } +} - state->dac_value[chan->channel] = val; +static inline void mcp4821_init_avail_gain(struct mcp4821_state *state, + int resolution) +{ + state->scale_avail[0] = MCP4821_VREF_MV; + state->scale_avail[1] = resolution; + state->scale_avail[2] = MCP4821_VREF_MV * 2; + state->scale_avail[3] = resolution; +} + +static int mcp4821_read_avail(struct iio_dev *indio_dev, + struct iio_chan_spec const *chan, + const int **vals, int *type, int *length, + long info) +{ + struct mcp4821_state *state = iio_priv(indio_dev); - return 0; + switch (info) { + case IIO_CHAN_INFO_SCALE: + *vals = state->scale_avail; + *type = IIO_VAL_FRACTIONAL_LOG2; + *length = ARRAY_SIZE(state->scale_avail); + return IIO_AVAIL_LIST; + default: + return -EINVAL; + } } static const struct iio_info mcp4821_info = { .read_raw = &mcp4821_read_raw, .write_raw = &mcp4821_write_raw, + .read_avail = &mcp4821_read_avail, }; static int mcp4821_probe(struct spi_device *spi) @@ -182,12 +267,15 @@ static int mcp4821_probe(struct spi_device *spi) state = iio_priv(indio_dev); state->spi = spi; + /* default gain is 2x */ + state->gain = 2; info = spi_get_device_match_data(spi); indio_dev->name = info->name; indio_dev->info = &mcp4821_info; indio_dev->modes = INDIO_DIRECT_MODE; indio_dev->channels = info->channels; indio_dev->num_channels = info->num_channels; + mcp4821_init_avail_gain(state, info->channels[0].scan_type.realbits); return devm_iio_device_register(&spi->dev, indio_dev); } -- 2.43.0