The patch titled
doc: gpio.txt describes open-drain emulation
has been added to the -mm tree. Its filename is
doc-gpiotxt-describes-open-drain-emulation.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: doc: gpio.txt describes open-drain emulation
From: David Brownell <david-b@xxxxxxxxxxx>
Update the GPIO docs to describe the idiom whereby open drain signals are
emulated by toggling the GPIO direction.
Signed-off-by: David Brownell <dbrownell@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/gpio.txt | 31 ++++++++++++++++++++++++++++++-
1 files changed, 30 insertions(+), 1 deletion(-)
diff -puN Documentation/gpio.txt~doc-gpiotxt-describes-open-drain-emulation Documentation/gpio.txt
--- a/Documentation/gpio.txt~doc-gpiotxt-describes-open-drain-emulation
+++ a/Documentation/gpio.txt
@@ -27,7 +27,7 @@ The exact capabilities of GPIOs vary bet
- Output values are writable (high=1, low=0). Some chips also have
options about how that value is driven, so that for example only one
value might be driven ... supporting "wire-OR" and similar schemes
- for the other value.
+ for the other value (notably, "open drain" signaling).
- Input values are likewise readable (1, 0). Some chips support readback
of pins configured as "output", which is very useful in such "wire-OR"
@@ -247,6 +247,35 @@ with gpio_get_value(), for example to in
when the IRQ is edge-triggered.
+Emulating Open Drain Signals
+----------------------------
+Sometimes shared signals need to use "open drain" signaling, where only the
+low signal level is actually driven. (That term applies to CMOS transistors;
+"open collector" is used for TTL.) A pullup resistor causes the high signal
+level. This is sometimes called a "wire-AND"; or more practically, from the
+negative logic (low=true) perspective this is a "wire-OR".
+
+One common example of an open drain signal is a shared active-low IRQ line.
+Also, bidirectional data bus signals sometimes use open drain signals.
+
+Some GPIO controllers directly support open drain outputs; many don't. When
+you need open drain signaling but your hardware doesn't directly support it,
+there's a common idiom you can use to emulate it with any GPIO pin that can
+be used as either an input or an output:
+
+ LOW: gpio_direction_output(gpio, 0) ... this drives the signal
+ and overrides the pullup.
+
+ HIGH: gpio_direction_input(gpio) ... this turns off the output,
+ so the pullup (or some other device) controls the signal.
+
+If you are "driving" the signal high but gpio_get_value(gpio) reports a low
+value (after the appropriate rise time passes), you know some other component
+is driving the shared signal low. That's not necessarily an error. As one
+common example, that's how I2C clocks are stretched: a slave that needs a
+slower clock delays the rising edge of SCK, and the I2C master adjusts its
+signaling rate accordingly.
+
What do these conventions omit?
===============================
_
Patches currently in -mm which might be from david-b@xxxxxxxxxxx are
omap_cf-oops-on-suspend-fix.patch
update-documentation-driver-model-platformtxt.patch
doc-gpiotxt-describes-open-drain-emulation.patch
make-proc-acpi-wakeup-more-useful.patch
at91_cf-minor-fix.patch
scsi-newstyle-hotplug-coldplug-support.patch
usb-gadget-rndis-fix-struct-rndis_packet_msg_type.patch
fix-hotplug-for-legacy-platform-drivers.patch
fix-hotplug-for-legacy-platform-drivers-update.patch
blackfin-on-chip-rtc-controller-driver.patch
blackfin-blackfin-on-chip-spi-controller-driver.patch
rework-pm_ops-pm_disk_mode-kill-misuse.patch
power-management-remove-firmware-disk-mode.patch
power-management-implement-pm_opsvalid-for-everybody.patch
documentation-ask-driver-writers-to-provide-pm-support.patch
init-dma-masks-in-pnp_dev.patch
rtc-add-rtc-class-driver-for-the-maxim-max6900.patch
char-cs5535_gpio-add-module_device_table.patch
parport-dev-driver-model-support.patch
parport-dev-driver-model-support-powerpc-fix.patch
layered-parport-code-uses-parport-dev.patch
pnpacpi-sets-pnpdev-devarchdata.patch
minor-spi_butterfly-cleanup.patch
rtc-remove-sys-class-rtc-dev.patch
rtc-rtc-interfaces-dont-use-class_device.patch
rtc-simplified-rtc-sysfs-attribute-handling.patch
rtc-simplified-proc-driver-rtc-handling.patch
rtc-remove-rest-of-class_device.patch
rtc-suspend-resume-restores-system-clock.patch
rtc-simplified-rtc-sysfs-attribute-handling-tidy.patch
rtc-kconfig-cleanup.patch
rtc-cmos-wakeup-interface.patch
acpi-wakeup-hooks-for-rtc-cmos.patch
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