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