[PATCH v3 08/12] iio: adc: ad_sigma_delta: use spi_optimize_message()

David Lechner
2025-07-01T21:39:39Z

Use spi_optimize_message() to improve the performance of buffered reads.

By setting up the SPI message and pre-optimizing it in the buffer
postenable callback, we can reduce overhead during each sample read.

A rough estimate shows that this reduced the CPU usage of the interrupt
handler thread from 22% to 16% using an EVAL-AD4112ARDZ board on a
DE10-Nano (measuring a single channel at the default 6.2 kHz sample
rate).

Signed-off-by: David Lechner <dlechner@baylibre.com>
---
drivers/iio/adc/ad_sigma_delta.c | 72 ++++++++++++++++------------------
include/linux/iio/adc/ad_sigma_delta.h | 3 ++
2 files changed, 37 insertions(+), 38 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index ce549775ac3d68fd11e660b6b993ac661301756a..124c42e19f2e25723b67ea38f7d016f00ff91342 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -466,8 +466,9 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
{
struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
const struct iio_scan_type *scan_type = &indio_dev->channels[0].scan_type;
+ struct spi_transfer *xfer = sigma_delta->sample_xfer;
unsigned int i, slot, samples_buf_size;
- unsigned int channel;
+ unsigned int channel, scan_size;
u8 *samples_buf;
int ret;

@@ -510,6 +511,28 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
return -ENOMEM;

sigma_delta->samples_buf = samples_buf;
+ scan_size = BITS_TO_BYTES(scan_type->realbits + scan_type->shift);
+ /* For 24-bit data, there is an extra byte of padding. */
+ xfer[1].rx_buf = &sigma_delta->rx_buf[scan_size == 3 ? 1 : 0];
+ xfer[1].len = scan_size + (sigma_delta->status_appended ? 1 : 0);
+ xfer[1].cs_change = 1;
+
+ if (sigma_delta->info->has_registers) {
+ xfer[0].tx_buf = &sigma_delta->sample_addr;
+ xfer[0].len = 1;
+
+ ad_sd_set_read_reg_addr(sigma_delta,
+ sigma_delta->info->data_reg ?: AD_SD_REG_DATA,
+ &sigma_delta->sample_addr);
+ spi_message_init_with_transfers(&sigma_delta->sample_msg, xfer, 2);
+ } else {
+ spi_message_init_with_transfers(&sigma_delta->sample_msg,
+ &xfer[1], 1);
+ }
+
+ ret = spi_optimize_message(sigma_delta->spi, &sigma_delta->sample_msg);
+ if (ret)
+ return ret;

spi_bus_lock(sigma_delta->spi->controller);
sigma_delta->bus_locked = true;
@@ -529,6 +552,7 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)

err_unlock:
spi_bus_unlock(sigma_delta->spi->controller);
+ spi_unoptimize_message(&sigma_delta->sample_msg);

return ret;
}
@@ -550,7 +574,10 @@ static int ad_sd_buffer_postdisable(struct iio_dev *indio_dev)

ad_sigma_delta_disable_all(sigma_delta);
sigma_delta->bus_locked = false;
- return spi_bus_unlock(sigma_delta->spi->controller);
+ spi_bus_unlock(sigma_delta->spi->controller);
+ spi_unoptimize_message(&sigma_delta->sample_msg);
+
+ return 0;
}

static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
@@ -560,50 +587,19 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
const struct iio_scan_type *scan_type = &indio_dev->channels[0].scan_type;
struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
u8 *data = sigma_delta->rx_buf;
- unsigned int transfer_size;
unsigned int sample_size;
unsigned int sample_pos;
unsigned int status_pos;
unsigned int reg_size;
- unsigned int data_reg;
+ int ret;

reg_size = BITS_TO_BYTES(scan_type->realbits + scan_type->shift);
+ /* For 24-bit data, there is an extra byte of padding. */
+ status_pos = reg_size + (reg_size == 3 ? 1 : 0);

- if (sigma_delta->info->data_reg != 0)
- data_reg = sigma_delta->info->data_reg;
- else
- data_reg = AD_SD_REG_DATA;
-
- /* Status word will be appended to the sample during transfer */
- if (sigma_delta->status_appended)
- transfer_size = reg_size + 1;
- else
- transfer_size = reg_size;
-
- switch (reg_size) {
- case 4:
- case 2:
- case 1:
- status_pos = reg_size;
- ad_sd_read_reg_raw(sigma_delta, data_reg, transfer_size, &data[0]);
- break;
- case 3:
- /*
- * Data array after transfer will look like (if status is appended):
- * data[] = { [0][sample][sample][sample][status] }
- * Keeping the first byte 0 shifts the status position by 1 byte to the right.
- */
- status_pos = reg_size + 1;
-
- /* We store 24 bit samples in a 32 bit word. Keep the upper
- * byte set to zero. */
- ad_sd_read_reg_raw(sigma_delta, data_reg, transfer_size, &data[1]);
- break;
-
- default:
- dev_err_ratelimited(&indio_dev->dev, "Unsupported reg_size: %u\n", reg_size);
+ ret = spi_sync_locked(sigma_delta->spi, &sigma_delta->sample_msg);
+ if (ret)
goto irq_handled;
- }

/*
* For devices sampling only one channel at
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index 5056677c9941afadc2383febbcafeb02e23a4f44..2037bb68b44115681ff48f66b580b63f50c2ea9e 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -105,6 +105,8 @@ struct ad_sigma_delta {
bool status_appended;
/* map slots to channels in order to know what to expect from devices */
unsigned int *slots;
+ struct spi_message sample_msg;
+ struct spi_transfer sample_xfer[2];
u8 *samples_buf;

/*
@@ -116,6 +118,7 @@ struct ad_sigma_delta {
*/
u8 tx_buf[4] __aligned(IIO_DMA_MINALIGN);
u8 rx_buf[16] __aligned(8);
+ u8 sample_addr;
};

static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,

--
2.43.0