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@@ -78,7 +78,8 @@ Identifying GPIOs
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-----------------
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GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
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reserves "negative" numbers for other purposes like marking signals as
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-"not available on this board", or indicating faults.
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+"not available on this board", or indicating faults. Code that doesn't
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+touch the underlying hardware treats these integers as opaque cookies.
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Platforms define how they use those integers, and usually #define symbols
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for the GPIO lines so that board-specific setup code directly corresponds
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@@ -139,10 +140,10 @@ issues including wire-OR and output latencies.
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The get/set calls have no error returns because "invalid GPIO" should have
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been reported earlier in gpio_set_direction(). However, note that not all
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platforms can read the value of output pins; those that can't should always
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-return zero. Also, these calls will be ignored for GPIOs that can't safely
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-be accessed wihtout sleeping (see below).
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+return zero. Also, using these calls for GPIOs that can't safely be accessed
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+without sleeping (see below) is an error.
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-Platform-specific implementations are encouraged to optimise the two
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+Platform-specific implementations are encouraged to optimize the two
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calls to access the GPIO value in cases where the GPIO number (and for
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output, value) are constant. It's normal for them to need only a couple
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of instructions in such cases (reading or writing a hardware register),
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@@ -239,7 +240,8 @@ options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
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system wakeup capabilities.
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Non-error values returned from irq_to_gpio() would most commonly be used
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-with gpio_get_value().
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+with gpio_get_value(), for example to initialize or update driver state
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+when the IRQ is edge-triggered.
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@@ -260,9 +262,10 @@ pullups (or pulldowns) so that the on-chip ones should not be used.
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There are other system-specific mechanisms that are not specified here,
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like the aforementioned options for input de-glitching and wire-OR output.
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Hardware may support reading or writing GPIOs in gangs, but that's usually
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-configuration dependednt: for GPIOs sharing the same bank. (GPIOs are
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+configuration dependent: for GPIOs sharing the same bank. (GPIOs are
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commonly grouped in banks of 16 or 32, with a given SOC having several such
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-banks.) Code relying on such mechanisms will necessarily be nonportable.
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+banks.) Some systems can trigger IRQs from output GPIOs. Code relying on
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+such mechanisms will necessarily be nonportable.
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Dynamic definition of GPIOs is not currently supported; for example, as
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a side effect of configuring an add-on board with some GPIO expanders.
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David Brownell