41 lines
1.7 KiB
Markdown
41 lines
1.7 KiB
Markdown
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# USB4 Retimers
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# Introduction
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As USB speeds continue to increase (up to 5G, 10G, and even 20G or higher in
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newer revisions of the spec), it becomes more difficult to maintain signal
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integrity for longer traces. Devices such as retimers and redrivers can be used
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to help signals maintain their integrity over long distances.
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A redriver is a device that boosts the high-frequency content of a signal in
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order to compensate for the attenuation typically caused by travelling through
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various circuit components (PCB, connectors, CPU, etc.). Redrivers are not
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protocol-aware, which makes them relatively simple. However, their effectiveness
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is limited, and may not work at all in some scenarios.
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A retimer is a device that retransmits a fresh copy of the signal it receives,
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by doing CDR and retransmitting the data (i.e., it is protocol-aware). Since
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this is a digital component, it may have firmware.
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# Driver Usage
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Some operating systems may have the ability to update firmware on USB4 retimers,
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and ultimately will need some way to power the device on and off so that its new
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firmware can be loaded. This is achieved by providing a GPIO signal that can be
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used for this purpose; its active state must be the one in which power is
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applied to the retimer. This driver will generate the required ACPI AML code
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which will toggle the GPIO in response to the kernel's request (through the
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`_DSM` ACPI method). Simply put something like the following in your devicetree:
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```
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device pci 0.0 on
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chip drivers/intel/usb4/retimer
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register "power_gpio" = "ACPI_GPIO_OUTPUT_ACTIVE_HIGH(GPP_A0)"
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device generic 0 on end
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end
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end
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```
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replacing the GPIO with the appropriate pin and polarity.
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