spi: Optionally use GPIO descriptors for CS GPIOs

This augments the SPI core to optionally use GPIO descriptors
for chip select on a per-master-driver opt-in basis.

Drivers using this will rely on the SPI core to look up
GPIO descriptors associated with the device, such as
when using device tree or board files with GPIO descriptor
tables.

When getting descriptors from the device tree, this will in
turn activate the code in gpiolib that was
added in commit 6953c57a
("gpio: of: Handle SPI chipselect legacy bindings")
which means that these descriptors are aware of the active
low semantics that is the default for SPI CS GPIO lines
and we can assume that all of these are "active high" and
thus assign SPI_CS_HIGH to all CS lines on the DT path.

The previously used gpio_set_value() would call down into
gpiod_set_raw_value() and ignore the polarity inversion
semantics.

It seems like many drivers go to great lengths to set up the
CS GPIO line as non-asserted, respecting SPI_CS_HIGH. We pull
this out of the SPI drivers and into the core, and by simply
requesting the line as GPIOD_OUT_LOW when retrieveing it from
the device and relying on the gpiolib to handle any inversion
semantics. This way a lot of code can be simplified and
removed in each converted driver.

The end goal after dealing with each driver in turn, is to
delete the non-descriptor path (of_spi_register_master() for
example) and let the core deal with only descriptors.

The different SPI drivers have complex interactions with the
core so we cannot simply change them all over, we need to use
a stepwise, bisectable approach so that each driver can be
converted and fixed in isolation.

This patch has the intended side effect of adding support for
ACPI GPIOs as it starts relying on gpiod_get_*() to get
the GPIO handle associated with the device.

Cc: Linuxarm <linuxarm@huawei.com>
Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Fangjian (Turing) <f.fangjian@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -19,6 +19,7 @@
 #include <linux/spi/spi.h>
 #include <linux/spi/spi-mem.h>
 #include <linux/of_gpio.h>
+#include <linux/gpio/consumer.h>
 #include <linux/pm_runtime.h>
 #include <linux/pm_domain.h>
 #include <linux/property.h>
@@ -578,7 +579,10 @@ int spi_add_device(struct spi_device *spi)
 		goto done;
 	}
 
-	if (ctlr->cs_gpios)
+	/* Descriptors take precedence */
+	if (ctlr->cs_gpiods)
+		spi->cs_gpiod = ctlr->cs_gpiods[spi->chip_select];
+	else if (ctlr->cs_gpios)
 		spi->cs_gpio = ctlr->cs_gpios[spi->chip_select];
 
 	/* Drivers may modify this initial i/o setup, but will
@@ -772,10 +776,20 @@ static void spi_set_cs(struct spi_device *spi, bool enable)
 	if (spi->mode & SPI_CS_HIGH)
 		enable = !enable;
 
-	if (gpio_is_valid(spi->cs_gpio)) {
-		/* Honour the SPI_NO_CS flag */
-		if (!(spi->mode & SPI_NO_CS))
+	if (spi->cs_gpiod || gpio_is_valid(spi->cs_gpio)) {
+		/*
+		 * Honour the SPI_NO_CS flag and invert the enable line, as
+		 * active low is default for SPI. Execution paths that handle
+		 * polarity inversion in gpiolib (such as device tree) will
+		 * enforce active high using the SPI_CS_HIGH resulting in a
+		 * double inversion through the code above.
+		 */
+		if (!(spi->mode & SPI_NO_CS)) {
+			if (spi->cs_gpiod)
+				gpiod_set_value(spi->cs_gpiod, !enable);
+			else
 				gpio_set_value(spi->cs_gpio, !enable);
+		}
 		/* Some SPI masters need both GPIO CS & slave_select */
 		if ((spi->controller->flags & SPI_MASTER_GPIO_SS) &&
 		    spi->controller->set_cs)
@@ -1615,13 +1629,21 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 		spi->mode |= SPI_CPHA;
 	if (of_property_read_bool(nc, "spi-cpol"))
 		spi->mode |= SPI_CPOL;
-	if (of_property_read_bool(nc, "spi-cs-high"))
-		spi->mode |= SPI_CS_HIGH;
 	if (of_property_read_bool(nc, "spi-3wire"))
 		spi->mode |= SPI_3WIRE;
 	if (of_property_read_bool(nc, "spi-lsb-first"))
 		spi->mode |= SPI_LSB_FIRST;
 
+	/*
+	 * For descriptors associated with the device, polarity inversion is
+	 * handled in the gpiolib, so all chip selects are "active high" in
+	 * the logical sense, the gpiolib will invert the line if need be.
+	 */
+	if (ctlr->use_gpio_descriptors)
+		spi->mode |= SPI_CS_HIGH;
+	else if (of_property_read_bool(nc, "spi-cs-high"))
+		spi->mode |= SPI_CS_HIGH;
+
 	/* Device DUAL/QUAD mode */
 	if (!of_property_read_u32(nc, "spi-tx-bus-width", &value)) {
 		switch (value) {
@@ -2137,6 +2159,60 @@ static int of_spi_register_master(struct spi_controller *ctlr)
 }
 #endif
 
+/**
+ * spi_get_gpio_descs() - grab chip select GPIOs for the master
+ * @ctlr: The SPI master to grab GPIO descriptors for
+ */
+static int spi_get_gpio_descs(struct spi_controller *ctlr)
+{
+	int nb, i;
+	struct gpio_desc **cs;
+	struct device *dev = &ctlr->dev;
+
+	nb = gpiod_count(dev, "cs");
+	ctlr->num_chipselect = max_t(int, nb, ctlr->num_chipselect);
+
+	/* No GPIOs at all is fine, else return the error */
+	if (nb == 0 || nb == -ENOENT)
+		return 0;
+	else if (nb < 0)
+		return nb;
+
+	cs = devm_kcalloc(dev, ctlr->num_chipselect, sizeof(*cs),
+			  GFP_KERNEL);
+	if (!cs)
+		return -ENOMEM;
+	ctlr->cs_gpiods = cs;
+
+	for (i = 0; i < nb; i++) {
+		/*
+		 * Most chipselects are active low, the inverted
+		 * semantics are handled by special quirks in gpiolib,
+		 * so initializing them GPIOD_OUT_LOW here means
+		 * "unasserted", in most cases this will drive the physical
+		 * line high.
+		 */
+		cs[i] = devm_gpiod_get_index_optional(dev, "cs", i,
+						      GPIOD_OUT_LOW);
+
+		if (cs[i]) {
+			/*
+			 * If we find a CS GPIO, name it after the device and
+			 * chip select line.
+			 */
+			char *gpioname;
+
+			gpioname = devm_kasprintf(dev, GFP_KERNEL, "%s CS%d",
+						  dev_name(dev), i);
+			if (!gpioname)
+				return -ENOMEM;
+			gpiod_set_consumer_name(cs[i], gpioname);
+		}
+	}
+
+	return 0;
+}
+
 static int spi_controller_check_ops(struct spi_controller *ctlr)
 {
 	/*
@@ -2199,10 +2275,17 @@ int spi_register_controller(struct spi_controller *ctlr)
 		return status;
 
 	if (!spi_controller_is_slave(ctlr)) {
+		if (ctlr->use_gpio_descriptors) {
+			status = spi_get_gpio_descs(ctlr);
+			if (status)
+				return status;
+		} else {
+			/* Legacy code path for GPIOs from DT */
 			status = of_spi_register_master(ctlr);
 			if (status)
 				return status;
 		}
+	}
 
 	/* even if it's just one always-selected device, there must
 	 * be at least one chipselect
@@ -2915,6 +2998,7 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 	 * cs_change is set for each transfer.
 	 */
 	if ((spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
+					  spi->cs_gpiod ||
 					  gpio_is_valid(spi->cs_gpio))) {
 		size_t maxsize;
 		int ret;

include/linux/spi/spi.h

@@ -12,6 +12,7 @@
 #include <linux/kthread.h>
 #include <linux/completion.h>
 #include <linux/scatterlist.h>
+#include <linux/gpio/consumer.h>
 
 struct dma_chan;
 struct property_entry;
@@ -116,7 +117,10 @@ void spi_statistics_add_transfer_stats(struct spi_statistics *stats,
  * @modalias: Name of the driver to use with this device, or an alias
  *	for that name.  This appears in the sysfs "modalias" attribute
  *	for driver coldplugging, and in uevents used for hotplugging
- * @cs_gpio: gpio number of the chipselect line (optional, -ENOENT when
+ * @cs_gpio: LEGACY: gpio number of the chipselect line (optional, -ENOENT when
+ *	not using a GPIO line) use cs_gpiod in new drivers by opting in on
+ *	the spi_master.
+ * @cs_gpiod: gpio descriptor of the chipselect line (optional, NULL when
  *	not using a GPIO line)
  *
  * @statistics: statistics for the spi_device
@@ -163,7 +167,8 @@ struct spi_device {
 	void			*controller_data;
 	char			modalias[SPI_NAME_SIZE];
 	const char		*driver_override;
-	int			cs_gpio;	/* chip select gpio */
+	int			cs_gpio;	/* LEGACY: chip select gpio */
+	struct gpio_desc	*cs_gpiod;	/* chip select gpio desc */
 
 	/* the statistics */
 	struct spi_statistics	statistics;
@@ -376,9 +381,17 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *	     controller has native support for memory like operations.
  * @unprepare_message: undo any work done by prepare_message().
  * @slave_abort: abort the ongoing transfer request on an SPI slave controller
- * @cs_gpios: Array of GPIOs to use as chip select lines; one per CS
- *	number. Any individual value may be -ENOENT for CS lines that
+ * @cs_gpios: LEGACY: array of GPIO descs to use as chip select lines; one per
+ *	CS number. Any individual value may be -ENOENT for CS lines that
+ *	are not GPIOs (driven by the SPI controller itself). Use the cs_gpiods
+ *	in new drivers.
+ * @cs_gpiods: Array of GPIO descs to use as chip select lines; one per CS
+ *	number. Any individual value may be NULL for CS lines that
  *	are not GPIOs (driven by the SPI controller itself).
+ * @use_gpio_descriptors: Turns on the code in the SPI core to parse and grab
+ *	GPIO descriptors rather than using global GPIO numbers grabbed by the
+ *	driver. This will fill in @cs_gpiods and @cs_gpios should not be used,
+ *	and SPI devices will have the cs_gpiod assigned rather than cs_gpio.
  * @statistics: statistics for the spi_controller
  * @dma_tx: DMA transmit channel
  * @dma_rx: DMA receive channel
@@ -557,6 +570,8 @@ struct spi_controller {
 
 	/* gpio chip select */
 	int			*cs_gpios;
+	struct gpio_desc	**cs_gpiods;
+	bool			use_gpio_descriptors;
 
 	/* statistics */
 	struct spi_statistics	statistics;