From: Linus Walleij <linus.wall...@linaro.org> This elaborates a bit on the pinctrl vs GPIO arangements in the hardware.
Inspired by some drawings in a mail from Christian Ruppert. Cc: Rob Landley <r...@landley.net> Cc: Christian Ruppert <christian.rupp...@abilis.com> Signed-off-by: Linus Walleij <linus.wall...@linaro.org> --- Documentation/pinctrl.txt | 37 ++++++++++++++++++++++++++++++++----- 1 file changed, 32 insertions(+), 5 deletions(-) diff --git a/Documentation/pinctrl.txt b/Documentation/pinctrl.txt index 447fd4c..41ecad0 100644 --- a/Documentation/pinctrl.txt +++ b/Documentation/pinctrl.txt @@ -784,11 +784,38 @@ 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 +The GPIO portions of a pin and its relation to a certain pin controller +logic can be constructed in several ways. Here are three examples: + +(A) + + +- SPI + Physical pins --- GPIO --- pinctrl -+- I2C + +- mmc + +(B) + +- GPIO + Physical pins -+ +- SPI + +- pinctrl -+- I2C + +- mmc + +(C) + +- SPI + Physical pins --- pinctrl -+- I2C + +- mmc + +- GPIO + +In (A) the GPIO-like functionality of the pin is *always* available. +For example it is possible to read the GPIO input register to "spy" on +the SPI, I2C or MMC line while it is being used by the peripheral. +In (B) the GPIO functionality is orthogonal to any device using the +pin, and in (C) the GPIO case is the same as "some peripheral". + +For this reason 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 -- 1.7.11.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
