drivers/intel/usb4: Add driver for USB4 retimer device

The USB4 retimer device needs to declare a _DSM with specific functions
that allow for GPIO control to turn off the power when an external
device is not connected.  This driver allows the mainboard to provide
the GPIO that is connected to the power control.

BUG=b:156957424

Change-Id: Icfb85dc3c0885d828aba3855a66109043250ab86
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44918
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/Documentation/drivers/retimer.md b/Documentation/drivers/retimer.md
new file mode 100644
index 0000000000..d83b50b26f
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+++ b/Documentation/drivers/retimer.md
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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.
+