gpiolib: Avoid the hotplug performance reduction

The 6.9 kernel introduced a large patchset [1] designed to make gpiochip
usage safe in the presence of potential hotplugging events. The changes
included protecting every gpiochip access with a claim of an interlock.

Running on a Pi 5 these changes reduce GPIO performance from userspace
by around 10%. The penalty would be proportionally higher from kernel,
as seen by SPI speed reductions.

Patch the gpiolib implementation to remove the protection of gpiochip
accesses. By providing alternative implementations of the relevant
macros, the changes are localised and therefore easier to verify.

See: https://github.com/raspberrypi/linux/issues/6854

[1] https://lwn.net/Articles/960024/

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/gpio/gpiolib.c

@@ -53,6 +53,42 @@
 
 #define dont_test_bit(b,d) (0)
 
+#define REJECT_HOTPLUG_TAX 1
+
+#if REJECT_HOTPLUG_TAX
+
+#undef srcu_dereference
+#define srcu_dereference(obj, srcu) obj
+
+#undef srcu_dereference_check
+#define srcu_dereference_check(obj, srcu, held) obj
+
+#undef guard
+#define guard(srcu) (void)
+
+#undef scoped_guard
+#define scoped_guard(x, y)
+#undef list_for_each_entry_srcu
+#define list_for_each_entry_srcu(pos, head, member, srcu) \
+	list_for_each_entry(pos, head, member)
+
+#undef CLASS
+#define CLASS(gcg, gd) \
+	struct gcg gd = gcg ## _init
+
+static struct gpio_chip_guard gpio_chip_guard_init(struct gpio_desc *desc)
+{
+	struct gpio_chip_guard _guard;
+
+	_guard.gdev = desc->gdev;
+	_guard.idx = 0;
+	_guard.gc = _guard.gdev->chip;
+
+	return _guard;
+}
+
+#endif
+
 /* Device and char device-related information */
 static DEFINE_IDA(gpio_ida);
 static dev_t gpio_devt;