spi: SPI_CS_WORD followup

This is a followup to the series "spi: introduce SPI_CS_WORD mode flag" [1] to address some review comments.

- The first two patches handle the case for GPIO CS
- The last patch is the IIO patch from the previous series with an expanded commit message.

[1]: https://lore.kernel.org/lkml/[email protected]/

David Lechner (3):
spi: always use software fallback for SPI_CS_WORD when using cs_gio
spi: spi-davinci: Don't error when SPI_CS_WORD and cs_gpio
iio: adc: ti-ads7950: use SPI_CS_WORD to reduce CPU usage

 drivers/iio/adc/ti-ads7950.c | 53 +++++++++++++++++++++---------------
 drivers/spi/spi-davinci.c    |  3 --
 drivers/spi/spi.c            |  6 ++--
 3 files changed, 35 insertions(+), 27 deletions(-)

2.17.1

This modifies the condition for using the software fallback implementation for SPI_CS_WORD when the SPI controller is using a GPIO for the CS line. When using a GPIO for CS, the hardware implementation won't work, so we just enable the software fallback globally in this case.

---
 drivers/spi/spi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index e4fb87b304a0..03833924ca6c 100644
@@ -2841,11 +2841,13 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 		return -EINVAL;
 
 	/* If an SPI controller does not support toggling the CS line on each
-	 * transfer (indicated by the SPI_CS_WORD flag), we can emulate it by
+	 * transfer (indicated by the SPI_CS_WORD flag) or we are using a GPIO
+	 * for the CS line, we can emulate the CS-per-word hardware function by
 	 * splitting transfers into one-word transfers and ensuring that
 	 * cs_change is set for each transfer.
 	 */
-	if ((spi->mode & SPI_CS_WORD) && !(ctlr->mode_bits & SPI_CS_WORD)) {
+	if ((spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
+					  gpio_is_valid(spi->cs_gpio))) {
 		size_t maxsize;
 		int ret;
 

2.17.1

This remove the check and subsequent return of error for the case when a SPI device requires SPI_CS_WORD and is also configured to use a GPIO for the CS line.

Commit a134cc414e86 ("spi: always use software fallback for SPI_CS_WORD when using cs_gio") handles this case now, so this check is no longer necessary.

---
 drivers/spi/spi-davinci.c | 3 ---
 1 file changed, 3 deletions(-)

index 205f763c7383..56adec83f8fc 100644
@@ -441,9 +441,6 @@ static int davinci_spi_setup(struct spi_device *spi)
 
 		if (internal_cs) {
 			set_io_bits(dspi->base + SPIPC0, 1 << spi->chip_select);
-		} else if (spi->mode & SPI_CS_WORD) {
-			dev_err(&spi->dev, "SPI_CS_WORD can't be use with GPIO CS\n");
-			return -EINVAL;
 		}
 	}
 

2.17.1

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%.

The use of big-endian for the raw data was cargo culted from another driver when this driver was originally written. It used an SPI word size of 8 bits and big-endian byte ordering to effectively emulate 16 bit words.

Now, in order to inject a CS toggle between each word, we need to use the correct word size, otherwise we would get a CS toggle half way through each word 16-bit. The SPI subsystem uses CPU byte ordering for multi-byte words. So, the data we get back from the SPI is going to be CPU endian now no matter what. Converting that to big endian will just add overhead on little endian systems so we opt to change the raw data format from big endian to CPU endian.

There is a small risk that this could break some lazy userspace programs that use the raw data without checking the data format. We can address this if/when it actually comes up.

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

index a5bd5944bc66..0ad63592cc3c 100644
@@ -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

And I just realized I forgot to pick up...

The patch

spi: spi-davinci: Don't error when SPI_CS_WORD and cs_gpio

has been applied to the spi tree at

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git

All being well this means that it will be integrated into the linux-next tree (usually sometime in the next 24 hours) and sent to Linus during the next merge window (or sooner if it is a bug fix), however if problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing and review of the tree, please engage with people reporting problems and send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they should be sent as incremental updates against current git, existing patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying to this mail.

Thanks,
Mark

From: David Lechner <[email protected]>
Date: Tue, 18 Sep 2018 12:08:49 -0500
Subject: [PATCH] spi: spi-davinci: Don't error when SPI_CS_WORD and cs_gpio

This remove the check and subsequent return of error for the case when a SPI device requires SPI_CS_WORD and is also configured to use a GPIO for the CS line.

Commit a134cc414e86 ("spi: always use software fallback for SPI_CS_WORD when using cs_gio") handles this case now, so this check is no longer necessary.

---
 drivers/spi/spi-davinci.c | 3 ---
 1 file changed, 3 deletions(-)

index 205f763c7383..56adec83f8fc 100644
@@ -441,9 +441,6 @@ static int davinci_spi_setup(struct spi_device *spi)
 
 		if (internal_cs) {
 			set_io_bits(dspi->base + SPIPC0, 1 << spi->chip_select);
-		} else if (spi->mode & SPI_CS_WORD) {
-			dev_err(&spi->dev, "SPI_CS_WORD can't be use with GPIO CS\n");
-			return -EINVAL;
 		}
 	}
 

2.19.0

The patch

spi: always use software fallback for SPI_CS_WORD when using cs_gio

has been applied to the spi tree at

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git

All being well this means that it will be integrated into the linux-next tree (usually sometime in the next 24 hours) and sent to Linus during the next merge window (or sooner if it is a bug fix), however if problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing and review of the tree, please engage with people reporting problems and send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they should be sent as incremental updates against current git, existing patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying to this mail.

Thanks,
Mark

From: David Lechner <[email protected]>
Date: Tue, 18 Sep 2018 12:08:48 -0500
Subject: [PATCH] spi: always use software fallback for SPI_CS_WORD when using
cs_gio

This modifies the condition for using the software fallback implementation for SPI_CS_WORD when the SPI controller is using a GPIO for the CS line. When using a GPIO for CS, the hardware implementation won't work, so we just enable the software fallback globally in this case.

---
 drivers/spi/spi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index be73d236919f..a358acdd98d3 100644
@@ -2832,11 +2832,13 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 		return -EINVAL;
 
 	/* If an SPI controller does not support toggling the CS line on each
-	 * transfer (indicated by the SPI_CS_WORD flag), we can emulate it by
+	 * transfer (indicated by the SPI_CS_WORD flag) or we are using a GPIO
+	 * for the CS line, we can emulate the CS-per-word hardware function by
 	 * splitting transfers into one-word transfers and ensuring that
 	 * cs_change is set for each transfer.
 	 */
-	if ((spi->mode & SPI_CS_WORD) && !(ctlr->mode_bits & SPI_CS_WORD)) {
+	if ((spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
+					  gpio_is_valid(spi->cs_gpio))) {
 		size_t maxsize;
 		int ret;
 

2.19.0

I've applied this on top of the original series on the basis I doubt anyone will be testing this particular driver in such a way as to hit the changes you made in the last two patches + I don't think Mark has tagged those for me to easily pick up. If it's a problem I can apply them to the IIO tree as well and git will sort it out come merge window time. However, that's inelegant so I won't do it unless needed.

Applied to the togreg branch of iio.git and pushed out as testing for the autobuilders to play with it

Thanks,

Jonathan