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Subject[PATCH v2 3/4] iio: adc: ti-ads7950: use SPI_CS_WORD to reduce CPU usage
Date
This changes how the SPI message for the triggered buffer is setup in
the TI ADS7950 A/DC driver. By using the SPI_CS_WORD flag, we can read
multiple samples in a single SPI transfer. If the SPI controller
supports DMA transfers, we can see a significant reduction in CPU usage.

For example, on an ARM9 system running at 456MHz reading just 4 channels
at 100Hz: before this change, top shows the CPU usage of the IRQ thread
of this driver to be ~7.7%. After this change, the CPU usage drops to
~3.8%.

Signed-off-by: David Lechner <david@lechnology.com>
---

It was brought up in v1 that changing the endianness *could* possible break
users who are taking shortcuts by making assumptions on the data format instead
of using the full ABI to determine the format. Since this only *might* be a
problem I would like to make this change anyway to avoid a bunch of byte
swapping. If it turns out that it really is a problem instead of *might be*
a problem, then we can fix it later.

drivers/iio/adc/ti-ads7950.c | 53 +++++++++++++++++++++---------------
1 file changed, 31 insertions(+), 22 deletions(-)

diff --git a/drivers/iio/adc/ti-ads7950.c b/drivers/iio/adc/ti-ads7950.c
index a5bd5944bc66..0ad63592cc3c 100644
--- a/drivers/iio/adc/ti-ads7950.c
+++ b/drivers/iio/adc/ti-ads7950.c
@@ -51,7 +51,7 @@

struct ti_ads7950_state {
struct spi_device *spi;
- struct spi_transfer ring_xfer[TI_ADS7950_MAX_CHAN + 2];
+ struct spi_transfer ring_xfer;
struct spi_transfer scan_single_xfer[3];
struct spi_message ring_msg;
struct spi_message scan_single_msg;
@@ -65,11 +65,11 @@ struct ti_ads7950_state {
* DMA (thus cache coherency maintenance) requires the
* transfer buffers to live in their own cache lines.
*/
- __be16 rx_buf[TI_ADS7950_MAX_CHAN + TI_ADS7950_TIMESTAMP_SIZE]
+ u16 rx_buf[TI_ADS7950_MAX_CHAN + 2 + TI_ADS7950_TIMESTAMP_SIZE]
____cacheline_aligned;
- __be16 tx_buf[TI_ADS7950_MAX_CHAN];
- __be16 single_tx;
- __be16 single_rx;
+ u16 tx_buf[TI_ADS7950_MAX_CHAN + 2];
+ u16 single_tx;
+ u16 single_rx;

};

@@ -108,7 +108,7 @@ enum ti_ads7950_id {
.realbits = bits, \
.storagebits = 16, \
.shift = 12 - (bits), \
- .endianness = IIO_BE, \
+ .endianness = IIO_CPU, \
}, \
}

@@ -249,23 +249,14 @@ static int ti_ads7950_update_scan_mode(struct iio_dev *indio_dev,
len = 0;
for_each_set_bit(i, active_scan_mask, indio_dev->num_channels) {
cmd = TI_ADS7950_CR_WRITE | TI_ADS7950_CR_CHAN(i) | st->settings;
- st->tx_buf[len++] = cpu_to_be16(cmd);
+ st->tx_buf[len++] = cmd;
}

/* Data for the 1st channel is not returned until the 3rd transfer */
- len += 2;
- for (i = 0; i < len; i++) {
- if ((i + 2) < len)
- st->ring_xfer[i].tx_buf = &st->tx_buf[i];
- if (i >= 2)
- st->ring_xfer[i].rx_buf = &st->rx_buf[i - 2];
- st->ring_xfer[i].len = 2;
- st->ring_xfer[i].cs_change = 1;
- }
- /* make sure last transfer's cs_change is not set */
- st->ring_xfer[len - 1].cs_change = 0;
+ st->tx_buf[len++] = 0;
+ st->tx_buf[len++] = 0;

- spi_message_init_with_transfers(&st->ring_msg, st->ring_xfer, len);
+ st->ring_xfer.len = len * 2;

return 0;
}
@@ -281,7 +272,7 @@ static irqreturn_t ti_ads7950_trigger_handler(int irq, void *p)
if (ret < 0)
goto out;

- iio_push_to_buffers_with_timestamp(indio_dev, st->rx_buf,
+ iio_push_to_buffers_with_timestamp(indio_dev, &st->rx_buf[2],
iio_get_time_ns(indio_dev));

out:
@@ -298,13 +289,13 @@ static int ti_ads7950_scan_direct(struct iio_dev *indio_dev, unsigned int ch)
mutex_lock(&indio_dev->mlock);

cmd = TI_ADS7950_CR_WRITE | TI_ADS7950_CR_CHAN(ch) | st->settings;
- st->single_tx = cpu_to_be16(cmd);
+ st->single_tx = cmd;

ret = spi_sync(st->spi, &st->scan_single_msg);
if (ret)
goto out;

- ret = be16_to_cpu(st->single_rx);
+ ret = st->single_rx;

out:
mutex_unlock(&indio_dev->mlock);
@@ -378,6 +369,14 @@ static int ti_ads7950_probe(struct spi_device *spi)
const struct ti_ads7950_chip_info *info;
int ret;

+ spi->bits_per_word = 16;
+ spi->mode |= SPI_CS_WORD;
+ ret = spi_setup(spi);
+ if (ret < 0) {
+ dev_err(&spi->dev, "Error in spi setup\n");
+ return ret;
+ }
+
indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
if (!indio_dev)
return -ENOMEM;
@@ -398,6 +397,16 @@ static int ti_ads7950_probe(struct spi_device *spi)
indio_dev->num_channels = info->num_channels;
indio_dev->info = &ti_ads7950_info;

+ /* build spi ring message */
+ spi_message_init(&st->ring_msg);
+
+ st->ring_xfer.tx_buf = &st->tx_buf[0];
+ st->ring_xfer.rx_buf = &st->rx_buf[0];
+ /* len will be set later */
+ st->ring_xfer.cs_change = true;
+
+ spi_message_add_tail(&st->ring_xfer, &st->ring_msg);
+
/*
* Setup default message. The sample is read at the end of the first
* transfer, then it takes one full cycle to convert the sample and one
--
2.17.1
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