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| -rw-r--r-- | Documentation/driver-api/hte/tegra194-hte.rst | 48 |
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diff --git a/Documentation/driver-api/hte/tegra194-hte.rst b/Documentation/driver-api/hte/tegra194-hte.rst new file mode 100644 index 000000000..f2d617265 --- /dev/null +++ b/Documentation/driver-api/hte/tegra194-hte.rst @@ -0,0 +1,48 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +HTE Kernel provider driver +========================== + +Description +----------- +The Nvidia tegra194 HTE provider driver implements two GTE +(Generic Timestamping Engine) instances: 1) GPIO GTE and 2) LIC +(Legacy Interrupt Controller) IRQ GTE. Both GTE instances get the +timestamp from the system counter TSC which has 31.25MHz clock rate, and the +driver converts clock tick rate to nanoseconds before storing it as timestamp +value. + +GPIO GTE +-------- + +This GTE instance timestamps GPIO in real time. For that to happen GPIO +needs to be configured as input. The always on (AON) GPIO controller instance +supports timestamping GPIOs in real time and it has 39 GPIO lines. The GPIO GTE +and AON GPIO controller are tightly coupled as it requires very specific bits +to be set in GPIO config register before GPIO GTE can be used, for that GPIOLIB +adds two optional APIs as below. The GPIO GTE code supports both kernel +and userspace consumers. The kernel space consumers can directly talk to HTE +subsystem while userspace consumers timestamp requests go through GPIOLIB CDEV +framework to HTE subsystem. + +See gpiod_enable_hw_timestamp_ns() and gpiod_disable_hw_timestamp_ns(). + +For userspace consumers, GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE flag must be +specified during IOCTL calls. Refer to ``tools/gpio/gpio-event-mon.c``, which +returns the timestamp in nanoseconds. + +LIC (Legacy Interrupt Controller) IRQ GTE +----------------------------------------- + +This GTE instance timestamps LIC IRQ lines in real time. There are 352 IRQ +lines which this instance can add timestamps to in real time. The hte +devicetree binding described at ``Documentation/devicetree/bindings/timestamp`` +provides an example of how a consumer can request an IRQ line. Since it is a +one-to-one mapping with IRQ GTE provider, consumers can simply specify the IRQ +number that they are interested in. There is no userspace consumer support for +this GTE instance in the HTE framework. + +The provider source code of both IRQ and GPIO GTE instances is located at +``drivers/hte/hte-tegra194.c``. The test driver +``drivers/hte/hte-tegra194-test.c`` demonstrates HTE API usage for both IRQ +and GPIO GTE. |