[RFC PATCH 15/15] spi: implement SW control for CS times

[Alexandru Ardelean <alexandru.ardelean@analog.com>]

This change implements CS control for setup, hold & inactive delays.

The `cs_setup` delay is completely new, and can help with cases where
asserting the CS, also brings the device out of power-sleep, where there
needs to be a longer (than usual), before transferring data.

The `cs_hold` time can overlap with the `delay` (or `delay_usecs`) from an
SPI transfer. The main difference is that `cs_hold` implies that CS will be
de-asserted.

The `cs_inactive` delay does not have a clear use-case yet. It has been
implemented mostly because the `spi_set_cs_timing()` function implements
it. To some degree, this could overlap or replace `cs_change_delay`, but
this will require more consideration/investigation in the future.

All these delays have been added to the `spi_controller` struct, as they
would typically be configured by calling `spi_set_cs_timing()` after an
`spi_setup()` call.

Software-mode for CS control, implies that the `set_cs_timing()` hook has
not been provided for the `spi_controller` object.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---
 drivers/spi/spi.c       | 50 ++++++++++++++++++++++++++++++++++++++++-
 include/linux/spi/spi.h |  5 +++++
 2 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 54d00c0a26d2..acbbfee837ed 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -775,6 +775,15 @@ int spi_register_board_info(struct spi_board_info const *info, unsigned n)
 
 static void spi_set_cs(struct spi_device *spi, bool enable)
 {
+	bool enable1 = enable;
+
+	if (!spi->controller->set_cs_timing) {
+		if (enable1)
+			spi_delay_exec(&spi->controller->cs_setup, NULL);
+		else
+			spi_delay_exec(&spi->controller->cs_hold, NULL);
+	}
+
 	if (spi->mode & SPI_CS_HIGH)
 		enable = !enable;
 
@@ -800,6 +809,11 @@ static void spi_set_cs(struct spi_device *spi, bool enable)
 	} else if (spi->controller->set_cs) {
 		spi->controller->set_cs(spi, !enable);
 	}
+
+	if (!spi->controller->set_cs_timing) {
+		if (!enable1)
+			spi_delay_exec(&spi->controller->cs_inactive, NULL);
+	}
 }
 
 #ifdef CONFIG_HAS_DMA
@@ -3144,6 +3158,11 @@ int spi_setup(struct spi_device *spi)
 }
 EXPORT_SYMBOL_GPL(spi_setup);
 
+static inline bool _spi_delay_clock_cycles(struct spi_delay *d)
+{
+	return d && d->unit == SPI_DELAY_UNIT_SCK;
+}
+
 /**
  * spi_set_cs_timing - configure CS setup, hold, and inactive delays
  * @spi: the device that requires specific CS timing configuration
@@ -3156,10 +3175,39 @@ EXPORT_SYMBOL_GPL(spi_setup);
 int spi_set_cs_timing(struct spi_device *spi, struct spi_delay *setup,
 		      struct spi_delay *hold, struct spi_delay *inactive)
 {
+	size_t len;
+
 	if (spi->controller->set_cs_timing)
 		return spi->controller->set_cs_timing(spi, setup, hold,
 						      inactive);
-	return -ENOTSUPP;
+
+	if (_spi_delay_clock_cycles(setup) ||
+	    _spi_delay_clock_cycles(hold) ||
+	    _spi_delay_clock_cycles(inactive)) {
+		dev_err(&spi->dev,
+			"Clock-cycle delays for CS not supported in SW mode\n");
+		return -ENOTSUPP;
+	}
+
+	len = sizeof(struct spi_delay);
+
+	/* copy delays to controller */
+	if (setup)
+		memcpy(&spi->controller->cs_setup, setup, len);
+	else
+		memset(&spi->controller->cs_setup, 0, len);
+
+	if (hold)
+		memcpy(&spi->controller->cs_hold, hold, len);
+	else
+		memset(&spi->controller->cs_hold, 0, len);
+
+	if (inactive)
+		memcpy(&spi->controller->cs_inactive, inactive, len);
+	else
+		memset(&spi->controller->cs_inactive, 0, len);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(spi_set_cs_timing);
 
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 292e752ce34a..def49a76299f 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -602,6 +602,11 @@ struct spi_controller {
 	/* Optimized handlers for SPI memory-like operations. */
 	const struct spi_controller_mem_ops *mem_ops;
 
+	/* CS delays */
+	struct spi_delay	cs_setup;
+	struct spi_delay	cs_hold;
+	struct spi_delay	cs_inactive;
+
 	/* gpio chip select */
 	int			*cs_gpios;
 	struct gpio_desc	**cs_gpiods;
-- 
2.20.1