pinctrl: document the "GPIO mode" pitfall

Recently as adoption of the pinctrl framework is reaching
niches where the pins are reconfigured during system sleep
and datasheets often talk about something called "GPIO mode",
some engineers become confused by this, thinking that since
it is named "GPIO (something something)" it must be modeled
in the kernel using <linux/gpio.h>.

To clarify things, let's put in this piece of documentation,
or just start off the discussion here.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Pankaj Dev <pankaj.dev@st.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/pinctrl.txt

@@ -736,6 +736,13 @@ All the above functions are mandatory to implement for a pinmux driver.
 Pin control interaction with the GPIO subsystem
 ===============================================
 
+Note that the following implies that the use case is to use a certain pin
+from the Linux kernel using the API in <linux/gpio.h> with gpio_request()
+and similar functions. There are cases where you may be using something
+that your datasheet calls "GPIO mode" but actually is just an electrical
+configuration for a certain device. See the section below named
+"GPIO mode pitfalls" for more details on this scenario.
+
 The public pinmux API contains two functions named pinctrl_request_gpio()
 and pinctrl_free_gpio(). These two functions shall *ONLY* be called from
 gpiolib-based drivers as part of their gpio_request() and
@@ -774,6 +781,111 @@ obtain the function "gpioN" where "N" is the global GPIO pin number if no
 special GPIO-handler is registered.
 
 
+GPIO mode pitfalls
+==================
+
+Sometime the developer may be confused by a datasheet talking about a pin
+being possible to set into "GPIO mode". It appears that what hardware
+engineers mean with "GPIO mode" is not necessarily the use case that is
+implied in the kernel interface <linux/gpio.h>: a pin that you grab from
+kernel code and then either listen for input or drive high/low to
+assert/deassert some external line.
+
+Rather hardware engineers think that "GPIO mode" means that you can
+software-control a few electrical properties of the pin that you would
+not be able to control if the pin was in some other mode, such as muxed in
+for a device.
+
+Example: a pin is usually muxed in to be used as a UART TX line. But during
+system sleep, we need to put this pin into "GPIO mode" and ground it.
+
+If you make a 1-to-1 map to the GPIO subsystem for this pin, you may start
+to think that you need to come up with something real complex, that the
+pin shall be used for UART TX and GPIO at the same time, that you will grab
+a pin control handle and set it to a certain state to enable UART TX to be
+muxed in, then twist it over to GPIO mode and use gpio_direction_output()
+to drive it low during sleep, then mux it over to UART TX again when you
+wake up and maybe even gpio_request/gpio_free as part of this cycle. This
+all gets very complicated.
+
+The solution is to not think that what the datasheet calls "GPIO mode"
+has to be handled by the <linux/gpio.h> interface. Instead view this as
+a certain pin config setting. Look in e.g. <linux/pinctrl/pinconf-generic.h>
+and you find this in the documentation:
+
+  PIN_CONFIG_OUTPUT: this will configure the pin in output, use argument
+     1 to indicate high level, argument 0 to indicate low level.
+
+So it is perfectly possible to push a pin into "GPIO mode" and drive the
+line low as part of the usual pin control map. So for example your UART
+driver may look like this:
+
+#include <linux/pinctrl/consumer.h>
+
+struct pinctrl          *pinctrl;
+struct pinctrl_state    *pins_default;
+struct pinctrl_state    *pins_sleep;
+
+pins_default = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_DEFAULT);
+pins_sleep = pinctrl_lookup_state(uap->pinctrl, PINCTRL_STATE_SLEEP);
+
+/* Normal mode */
+retval = pinctrl_select_state(pinctrl, pins_default);
+/* Sleep mode */
+retval = pinctrl_select_state(pinctrl, pins_sleep);
+
+And your machine configuration may look like this:
+--------------------------------------------------
+
+static unsigned long uart_default_mode[] = {
+    PIN_CONF_PACKED(PIN_CONFIG_DRIVE_PUSH_PULL, 0),
+};
+
+static unsigned long uart_sleep_mode[] = {
+    PIN_CONF_PACKED(PIN_CONFIG_OUTPUT, 0),
+};
+
+static struct pinctrl_map __initdata pinmap[] = {
+    PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo",
+                      "u0_group", "u0"),
+    PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_DEFAULT, "pinctrl-foo",
+                        "UART_TX_PIN", uart_default_mode),
+    PIN_MAP_MUX_GROUP("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo",
+                      "u0_group", "gpio-mode"),
+    PIN_MAP_CONFIGS_PIN("uart", PINCTRL_STATE_SLEEP, "pinctrl-foo",
+                        "UART_TX_PIN", uart_sleep_mode),
+};
+
+foo_init(void) {
+    pinctrl_register_mappings(pinmap, ARRAY_SIZE(pinmap));
+}
+
+Here the pins we want to control are in the "u0_group" and there is some
+function called "u0" that can be enabled on this group of pins, and then
+everything is UART business as usual. But there is also some function
+named "gpio-mode" that can be mapped onto the same pins to move them into
+GPIO mode.
+
+This will give the desired effect without any bogus interaction with the
+GPIO subsystem. It is just an electrical configuration used by that device
+when going to sleep, it might imply that the pin is set into something the
+datasheet calls "GPIO mode" but that is not the point: it is still used
+by that UART device to control the pins that pertain to that very UART
+driver, putting them into modes needed by the UART. GPIO in the Linux
+kernel sense are just some 1-bit line, and is a different use case.
+
+How the registers are poked to attain the push/pull and output low
+configuration and the muxing of the "u0" or "gpio-mode" group onto these
+pins is a question for the driver.
+
+Some datasheets will be more helpful and refer to the "GPIO mode" as
+"low power mode" rather than anything to do with GPIO. This often means
+the same thing electrically speaking, but in this latter case the
+software engineers will usually quickly identify that this is some
+specific muxing/configuration rather than anything related to the GPIO
+API.
+
+
 Board/machine configuration
 ==================================