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