driver-api/gpio/legacy.rst

50     through a serial bus normally can't.  Some systems support both types.
60 Note that this is called a "convention" because you don't need to do it this
61 way, and it's no crime if you don't.  There **are** cases where portability
63 glue logic that may even change between board revisions, and can't ever be
68 Plus, this doesn't require any implementation framework, just an interface.
78 is strictly required.  Drivers that can't work without
89 use them; drivers don't need to add them explicitly.
96 "not available on this board", or indicating faults.  Code that doesn't
141 be checked, since the get/set calls don't have error returns and since
155 that particular GPIO can't be used in that mode.  It's generally a bad
157 it probably wasn't validated to do more than boot Linux.  (Similarly,
165 Those don't need to sleep, and can safely be done from inside hard
178 pin ... that won't always match the specified output value, because of
183 platforms can read the value of output pins; those that can't should always
184 return zero.  Also, using these calls for GPIOs that can't safely be accessed
201 This requires sleeping, which can't be done from inside IRQ handlers.
215 on GPIOs that can't be accessed from hardIRQ handlers, these calls act
361 else a negative errno code if the mapping can't be done.  (For example,
362 some GPIOs can't be used as IRQs.)  It is an unchecked error to use a GPIO
363 number that wasn't set up as an input using gpio_direction_input(), or
364 to use an IRQ number that didn't originally come from gpio_to_irq().
387 Some GPIO controllers directly support open drain outputs; many don't.  When
388 you need open drain signaling but your hardware doesn't directly support it,
461 (When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
513 Any debugfs dump method should normally ignore signals which haven't been
589 standard kernels won't know about.  And for some tasks, simple userspace
637 		doesn't support changing the direction of a GPIO, or
638 		it was exported by kernel code that didn't explicitly