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authorLinus Torvalds <torvalds@linux-foundation.org>2013-05-02 14:38:53 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-05-02 14:38:53 -0700
commit99bece775f988a4ee21ad3db9fd413caf1704ff6 (patch)
tree5975cdcd92301e54dfe1424ec5d008898b5c9331 /Documentation
parent736a2dd2571ac56b11ed95a7814d838d5311be04 (diff)
parentc39e8e4354ce4daf23336de5daa28a3b01f00aa6 (diff)
Merge branch 'i2c/for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux
Pull i2c changes from Wolfram Sang: - an arbitration driver. While the driver is quite simple, it caused discussion if we need additional arbitration on top of the one specified in the I2C standard. Conclusion is that I accept a few generic mechanisms, but not very specific ones. - the core lost the detach_adapter() call. It has no users anymore and was in the way for other cleanups. attach_adapter() is sadly still there since there are users waiting to be converted. - the core gained a bus recovery infrastructure. I2C defines a way to recover if the data line is stalled. This mechanism is now in the core and drivers can now pass some data to make use of it. - bigger driver cleanups for designware, s3c2410 - removing superfluous refcounting from drivers - removing Ben Dooks as second maintainer due to inactivity. Thanks for all your work so far, Ben! - bugfixes, feature additions, devicetree fixups, simplifications... * 'i2c/for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux: (38 commits) i2c: xiic: must always write 16-bit words to TX_FIFO i2c: octeon: use HZ in timeout value i2c: octeon: Fix i2c fail problem when a process is terminated by a signal i2c: designware-pci: drop superfluous {get|put}_device i2c: designware-plat: drop superfluous {get|put}_device i2c: davinci: drop superfluous {get|put}_device MAINTAINERS: Ben Dooks is inactive regarding I2C i2c: mux: Add i2c-arb-gpio-challenge 'mux' driver i2c: at91: convert to dma_request_slave_channel_compat() i2c: mxs: do error checking and handling in PIO mode i2c: mxs: remove races in PIO code i2c-designware: switch to use runtime PM autosuspend i2c-designware: use usleep_range() in the busy-loop i2c-designware: enable/disable the controller properly i2c-designware: use dynamic adapter numbering on Lynxpoint i2c-designware-pci: use managed functions pcim_* and devm_* i2c-designware-pci: use dev_err() instead of printk() i2c-designware: move to managed functions (devm_*) i2c: remove CONFIG_HOTPLUG ifdefs i2c: s3c2410: Add SMBus emulation for block read ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt80
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+GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
+=================================================================
+This uses GPIO lines and a challenge & response mechanism to arbitrate who is
+the master of an I2C bus in a multimaster situation.
+
+In many cases using GPIOs to arbitrate is not needed and a design can use
+the standard I2C multi-master rules. Using GPIOs is generally useful in
+the case where there is a device on the bus that has errata and/or bugs
+that makes standard multimaster mode not feasible.
+
+
+Algorithm:
+
+All masters on the bus have a 'bus claim' line which is an output that the
+others can see. These are all active low with pull-ups enabled. We'll
+describe these lines as:
+
+- OUR_CLAIM: output from us signaling to other hosts that we want the bus
+- THEIR_CLAIMS: output from others signaling that they want the bus
+
+The basic algorithm is to assert your line when you want the bus, then make
+sure that the other side doesn't want it also. A detailed explanation is best
+done with an example.
+
+Let's say we want to claim the bus. We:
+1. Assert OUR_CLAIM.
+2. Waits a little bit for the other sides to notice (slew time, say 10
+ microseconds).
+3. Check THEIR_CLAIMS. If none are asserted then the we have the bus and we are
+ done.
+4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released.
+5. If not, back off, release the claim and wait for a few more milliseconds.
+6. Go back to 1 (until retry time has expired).
+
+
+Required properties:
+- compatible: i2c-arb-gpio-challenge
+- our-claim-gpio: The GPIO that we use to claim the bus.
+- their-claim-gpios: The GPIOs that the other sides use to claim the bus.
+ Note that some implementations may only support a single other master.
+- Standard I2C mux properties. See mux.txt in this directory.
+- Single I2C child bus node at reg 0. See mux.txt in this directory.
+
+Optional properties:
+- slew-delay-us: microseconds to wait for a GPIO to go high. Default is 10 us.
+- wait-retry-us: we'll attempt another claim after this many microseconds.
+ Default is 3000 us.
+- wait-free-us: we'll give up after this many microseconds. Default is 50000 us.
+
+
+Example:
+ i2c@12CA0000 {
+ compatible = "acme,some-i2c-device";
+ #address-cells = <1>;
+ #size-cells = <0>;
+ };
+
+ i2c-arbitrator {
+ compatible = "i2c-arb-gpio-challenge";
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ i2c-parent = <&{/i2c@12CA0000}>;
+
+ our-claim-gpio = <&gpf0 3 1>;
+ their-claim-gpios = <&gpe0 4 1>;
+ slew-delay-us = <10>;
+ wait-retry-us = <3000>;
+ wait-free-us = <50000>;
+
+ i2c@0 {
+ reg = <0>;
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ i2c@52 {
+ // Normal I2C device
+ };
+ };
+ };