1.. SPDX-License-Identifier: GPL-2.0+
2
3HTE Kernel provider driver
4==========================
5
6Description
7-----------
8The Nvidia tegra194 HTE provider driver implements two GTE
9(Generic Timestamping Engine) instances: 1) GPIO GTE and 2) LIC
10(Legacy Interrupt Controller) IRQ GTE. Both GTE instances get the
11timestamp from the system counter TSC which has 31.25MHz clock rate, and the
12driver converts clock tick rate to nanoseconds before storing it as timestamp
13value.
14
15GPIO GTE
16--------
17
18This GTE instance timestamps GPIO in real time. For that to happen GPIO
19needs to be configured as input. The always on (AON) GPIO controller instance
20supports timestamping GPIOs in real time and it has 39 GPIO lines. The GPIO GTE
21and AON GPIO controller are tightly coupled as it requires very specific bits
22to be set in GPIO config register before GPIO GTE can be used, for that GPIOLIB
23adds two optional APIs as below. The GPIO GTE code supports both kernel
24and userspace consumers. The kernel space consumers can directly talk to HTE
25subsystem while userspace consumers timestamp requests go through GPIOLIB CDEV
26framework to HTE subsystem.
27
28See gpiod_enable_hw_timestamp_ns() and gpiod_disable_hw_timestamp_ns().
29
30For userspace consumers, GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE flag must be
31specified during IOCTL calls. Refer to ``tools/gpio/gpio-event-mon.c``, which
32returns the timestamp in nanoseconds.
33
34LIC (Legacy Interrupt Controller) IRQ GTE
35-----------------------------------------
36
37This GTE instance timestamps LIC IRQ lines in real time. There are 352 IRQ
38lines which this instance can add timestamps to in real time. The hte
39devicetree binding described at ``Documentation/devicetree/bindings/timestamp``
40provides an example of how a consumer can request an IRQ line. Since it is a
41one-to-one mapping with IRQ GTE provider, consumers can simply specify the IRQ
42number that they are interested in. There is no userspace consumer support for
43this GTE instance in the HTE framework.
44
45The provider source code of both IRQ and GPIO GTE instances is located at
46``drivers/hte/hte-tegra194.c``. The test driver
47``drivers/hte/hte-tegra194-test.c`` demonstrates HTE API usage for both IRQ
48and GPIO GTE.
49