48 lines
2.1 KiB
ReStructuredText
48 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 tegra HTE provider also known as GTE (Generic Timestamping Engine)
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driver implements two GTE instances: 1) GPIO GTE and 2) LIC
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(Legacy Interrupt Controller) IRQ GTE. Both GTE instances get the timestamp
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from the system counter TSC which has 31.25MHz clock rate, and the driver
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converts clock tick rate to nanoseconds before storing it as timestamp 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. Only the always on (AON) GPIO controller
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instance supports timestamping GPIOs in real time as it is tightly coupled with
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the GPIO GTE. To support this, GPIOLIB adds two optional APIs as mentioned
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below. The GPIO GTE code supports both kernel and userspace consumers. The
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kernel space consumers can directly talk to HTE subsystem while userspace
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consumers timestamp requests go through GPIOLIB CDEV framework to HTE
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subsystem. The hte devicetree binding described at
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``Documentation/devicetree/bindings/timestamp`` provides an example of how a
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consumer can request an GPIO line.
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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. The hte devicetree
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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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