[RFT PATCH v2] spi: bcm2835: reduce the abuse of the GPIO API

* [RFT PATCH v2] spi: bcm2835: reduce the abuse of the GPIO API
@ 2023-09-01 11:15 Bartosz Golaszewski
  2023-09-01 12:15 ` Linus Walleij
  0 siblings, 1 reply; 8+ messages in thread
From: Bartosz Golaszewski @ 2023-09-01 11:15 UTC (permalink / raw)
  To: Mark Brown, Florian Fainelli, Andy Shevchenko, Linus Walleij,
	Ray Jui, Scott Branden
  Cc: Broadcom internal kernel review list, linux-spi,
	linux-rpi-kernel, linux-arm-kernel, linux-kernel, linux-gpio,
	Bartosz Golaszewski

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Currently the bcm2835 SPI driver uses functions that are available
exclusively to GPIO providers as a way to handle a platform quirk. Let's
use a slightly better alternative that avoids poking around in GPIOLIB's
internals and use GPIO lookup tables.

Link: https://www.spinics.net/lists/linux-gpio/msg36218.html
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
This is only build-tested. It should work, but it would be great if
someone from broadcom could test this.

v1 -> v2:
- don't use devres for managing the GPIO but put it manually in .cleanup()
- add a mailing list link explaining the background of the bug
- fix kerneldoc

 drivers/spi/spi-bcm2835.c | 59 +++++++++++++++++++++++----------------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index e7bb2714678a..e06738705075 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -11,6 +11,7 @@
  * spi-atmel.c, Copyright (C) 2006 Atmel Corporation
  */
 
+#include <linux/cleanup.h>
 #include <linux/clk.h>
 #include <linux/completion.h>
 #include <linux/debugfs.h>
@@ -26,9 +27,10 @@
 #include <linux/of_address.h>
 #include <linux/platform_device.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/gpio/machine.h> /* FIXME: using GPIO lookup tables */
 #include <linux/of_irq.h>
+#include <linux/overflow.h>
+#include <linux/slab.h>
 #include <linux/spi/spi.h>
 
 /* SPI register offsets */
@@ -83,6 +85,7 @@ MODULE_PARM_DESC(polling_limit_us,
  * struct bcm2835_spi - BCM2835 SPI controller
  * @regs: base address of register map
  * @clk: core clock, divided to calculate serial clock
+ * @cs_gpio: chip-select GPIO descriptor
  * @clk_hz: core clock cached speed
  * @irq: interrupt, signals TX FIFO empty or RX FIFO ¾ full
  * @tfr: SPI transfer currently processed
@@ -117,6 +120,7 @@ MODULE_PARM_DESC(polling_limit_us,
 struct bcm2835_spi {
 	void __iomem *regs;
 	struct clk *clk;
+	struct gpio_desc *cs_gpio;
 	unsigned long clk_hz;
 	int irq;
 	struct spi_transfer *tfr;
@@ -1156,15 +1160,11 @@ static void bcm2835_spi_handle_err(struct spi_controller *ctlr,
 	bcm2835_spi_reset_hw(bs);
 }
 
-static int chip_match_name(struct gpio_chip *chip, void *data)
-{
-	return !strcmp(chip->label, data);
-}
-
 static void bcm2835_spi_cleanup(struct spi_device *spi)
 {
 	struct bcm2835_spidev *target = spi_get_ctldata(spi);
 	struct spi_controller *ctlr = spi->controller;
+	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 
 	if (target->clear_rx_desc)
 		dmaengine_desc_free(target->clear_rx_desc);
@@ -1175,6 +1175,9 @@ static void bcm2835_spi_cleanup(struct spi_device *spi)
 				 sizeof(u32),
 				 DMA_TO_DEVICE);
 
+	gpiod_put(bs->cs_gpio);
+	spi_set_csgpiod(spi, 0, NULL);
+
 	kfree(target);
 }
 
@@ -1221,7 +1224,7 @@ static int bcm2835_spi_setup(struct spi_device *spi)
 	struct spi_controller *ctlr = spi->controller;
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 	struct bcm2835_spidev *target = spi_get_ctldata(spi);
-	struct gpio_chip *chip;
+	struct gpiod_lookup_table *lookup __free(kfree) = NULL;
 	int ret;
 	u32 cs;
 
@@ -1288,29 +1291,37 @@ static int bcm2835_spi_setup(struct spi_device *spi)
 	}
 
 	/*
-	 * Translate native CS to GPIO
+	 * TODO: The code below is a slightly better alternative to the utter
+	 * abuse of the GPIO API that I found here before. It creates a
+	 * temporary lookup table, assigns it to the SPI device, gets the GPIO
+	 * descriptor and then releases the lookup table.
 	 *
-	 * 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?
+	 * More on the problem that it addresses:
+	 *   https://www.spinics.net/lists/linux-gpio/msg36218.html
 	 */
+	lookup = kzalloc(struct_size(lookup, table, 1), GFP_KERNEL);
+	if (!lookup) {
+		ret = -ENOMEM;
+		goto err_cleanup;
+	}
 
-	/* get the gpio chip for the base */
-	chip = gpiochip_find("pinctrl-bcm2835", chip_match_name);
-	if (!chip)
-		return 0;
+	lookup->dev_id = dev_name(&spi->dev);
+	lookup->table[0].key = "pinctrl-bcm2835";
+	lookup->table[0].chip_hwnum = (8 - (spi_get_chipselect(spi, 0)));
+	lookup->table[0].con_id = "cs";
+	lookup->table[0].flags = GPIO_LOOKUP_FLAGS_DEFAULT;
 
-	spi_set_csgpiod(spi, 0, gpiochip_request_own_desc(chip,
-							  8 - (spi_get_chipselect(spi, 0)),
-							  DRV_NAME,
-							  GPIO_LOOKUP_FLAGS_DEFAULT,
-							  GPIOD_OUT_LOW));
-	if (IS_ERR(spi_get_csgpiod(spi, 0))) {
-		ret = PTR_ERR(spi_get_csgpiod(spi, 0));
+	gpiod_add_lookup_table(lookup);
+
+	bs->cs_gpio = gpiod_get(&spi->dev, "cs", GPIOD_OUT_LOW);
+	gpiod_remove_lookup_table(lookup);
+	if (IS_ERR(bs->cs_gpio)) {
+		ret = PTR_ERR(bs->cs_gpio);
 		goto err_cleanup;
 	}
 
+	spi_set_csgpiod(spi, 0, bs->cs_gpio);
+
 	/* and set up the "mode" and level */
 	dev_info(&spi->dev, "setting up native-CS%i to use GPIO\n",
 		 spi_get_chipselect(spi, 0));
-- 
2.39.2


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