[PATCH v4] spi: bcm2835: Convert to use CS GPIO descriptors

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

This converts the BCM2835 SPI master driver to use GPIO
descriptors for chip select handling.

The BCM2835 driver was relying on the core to drive the
CS high/low so very small changes were needed for this
part. If it managed to request the CS from the device tree
node, all is pretty straight forward.

However for native GPIOs this driver has a quite unorthodox
loopback to request some GPIOs from the SoC GPIO chip by
looking it up from the device tree using gpiochip_find()
and then offseting hard into its numberspace. This has
been augmented a bit by using gpiochip_request_own_desc()
but this code really needs to be verified. If "native CS"
is actually an SoC GPIO, why is it even done this way?
Should this GPIO not just be defined in the device tree
like any other CS GPIO? I'm confused.

Cc: Lukas Wunner <lukas@xxxxxxxxx>
Cc: Stefan Wahren <stefan.wahren@xxxxxxxx>
Cc: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
Cc: Chris Boot <bootc@xxxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
ChangeLog v3->v4:
- Fix the offset of the chipselect line to be 8 - CS
  as in the original code.
- Use the modified gpiochip_request_own_desc() to set up
  line inversion semantics if need be. Look at the OF
  node of the SPI device for flags.
ChangeLog v2->v3:
- Fix unused variable "err" compile-time message.
ChangeLog RFT->v2:
- Rebased on v5.1-rc1

I would very much appreciate if someone took this for
a ride on top of linux-next (there are some fixes in
the -rcs you need) and see if all still works as expected.
---
 drivers/spi/spi-bcm2835.c | 58 +++++++++++++++++++++++++++------------
 1 file changed, 40 insertions(+), 18 deletions(-)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 6f243a90c844..a40c09c553d3 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -25,7 +25,9 @@
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_device.h>
-#include <linux/of_gpio.h>
+#include <linux/gpio/consumer.h>
+#include <linux/gpio/machine.h> /* FIXME: using chip internals */
+#include <linux/gpio/driver.h> /* FIXME: using chip internals */
 #include <linux/of_irq.h>
 #include <linux/spi/spi.h>
 
@@ -930,14 +932,19 @@ static int chip_match_name(struct gpio_chip *chip, void *data)
 
 static int bcm2835_spi_setup(struct spi_device *spi)
 {
-	int err;
 	struct gpio_chip *chip;
+	enum gpio_lookup_flags lflags;
+
 	/*
 	 * sanity checking the native-chipselects
 	 */
 	if (spi->mode & SPI_NO_CS)
 		return 0;
-	if (gpio_is_valid(spi->cs_gpio))
+	/*
+	 * The SPI core has successfully requested the CS GPIO line from the
+	 * device tree, so we are done.
+	 */
+	if (spi->cs_gpiod)
 		return 0;
 	if (spi->chip_select > 1) {
 		/* error in the case of native CS requested with CS > 1
@@ -948,29 +955,43 @@ static int bcm2835_spi_setup(struct spi_device *spi)
 			"setup: only two native chip-selects are supported\n");
 		return -EINVAL;
 	}
-	/* now translate native cs to GPIO */
+
+	/*
+	 * Translate native CS to GPIO
+	 *
+	 * FIXME: poking around in the gpiolib internals like this is
+	 * not very good practice. Find a way to locate the real problem
+	 * and fix it. Why is the GPIO descriptor in spi->cs_gpiod
+	 * sometimes not assigned correctly? Erroneous device trees?
+	 */
 
 	/* get the gpio chip for the base */
 	chip = gpiochip_find("pinctrl-bcm2835", chip_match_name);
 	if (!chip)
 		return 0;
 
-	/* and calculate the real CS */
-	spi->cs_gpio = chip->base + 8 - spi->chip_select;
+	/*
+	 * Retrieve the corresponding GPIO line used for CS.
+	 * The inversion semantics will be handled by the GPIO core
+	 * code, so we pass GPIOS_OUT_LOW for "unasserted" and
+	 * the correct flag for inversion semantics. The SPI_CS_HIGH
+	 * on spi->mode cannot be checked for polarity in this case
+	 * as the flag use_gpio_descriptors enforces SPI_CS_HIGH.
+	 */
+	if (of_property_read_bool(spi->dev.of_node, "spi-cs-high"))
+		lflags = GPIO_ACTIVE_HIGH;
+	else
+		lflags = GPIO_ACTIVE_LOW;
+	spi->cs_gpiod = gpiochip_request_own_desc(chip, 8 - spi->chip_select,
+						  DRV_NAME,
+						  lflags,
+						  GPIOD_OUT_LOW);
+	if (IS_ERR(spi->cs_gpiod))
+		return PTR_ERR(spi->cs_gpiod);
 
 	/* and set up the "mode" and level */
-	dev_info(&spi->dev, "setting up native-CS%i as GPIO %i\n",
-		 spi->chip_select, spi->cs_gpio);
-
-	/* set up GPIO as output and pull to the correct level */
-	err = gpio_direction_output(spi->cs_gpio,
-				    (spi->mode & SPI_CS_HIGH) ? 0 : 1);
-	if (err) {
-		dev_err(&spi->dev,
-			"could not set CS%i gpio %i as output: %i",
-			spi->chip_select, spi->cs_gpio, err);
-		return err;
-	}
+	dev_info(&spi->dev, "setting up native-CS%i to use GPIO\n",
+		 spi->chip_select);
 
 	return 0;
 }
@@ -988,6 +1009,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, ctlr);
 
+	ctlr->use_gpio_descriptors = true;
 	ctlr->mode_bits = BCM2835_SPI_MODE_BITS;
 	ctlr->bits_per_word_mask = SPI_BPW_MASK(8);
 	ctlr->num_chipselect = 3;
-- 
2.21.0