[PATCH v1 2/3] Documentation: gpio: Add a section on what to return in ->get() callback

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

The ->get() callback depending on other settings and hardware support
may return different values, while the line outside the chip is kept
in the same state. Let's discuss that in the documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 Documentation/driver-api/gpio/driver.rst | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/driver-api/gpio/driver.rst b/Documentation/driver-api/gpio/driver.rst
index bf6319cc531b..3d2f36001a7a 100644
--- a/Documentation/driver-api/gpio/driver.rst
+++ b/Documentation/driver-api/gpio/driver.rst
@@ -251,6 +251,30 @@ supports more versatile control over electrical properties and can handle
 different pull-up or pull-down resistance values.
 
 
+Considerations of the ->get() returned value
+--------------------------------------------
+
+Due to different possible electrical configurations and software applications
+the value that ->get() callback returns may vary depending on the other settings.
+This will allow to use pins in the I2C emulation mode or other not so standard
+uses.
+
+The below table gathered the most used cases.
+
+==========  ==========  ===============  =======================
+  Input       Output         State        What value to return?
+==========  ==========  ===============  =======================
+ Disabled    Disabled    Hi-Z             input buffer
+ Disabled    OS/OD/etc   Single ended     [cached] output buffer
+    x        Push-Pull   Out              [cached] output buffer
+ Enabled     Disabled    In               input buffer
+ Enabled     OS/OD/etc   Bidirectional    input buffer
+==========  ==========  ===============  =======================
+
+The [cached] here is used in a broader sense: either pure software cache, or
+read back value from the GPIO output buffer (not all hardware support that).
+
+
 GPIO drivers providing IRQs
 ===========================
 
-- 
2.35.1