Go - Reading/Writing to PCI config registers via sysfs. (example of controlling CPU boosting for a Ryzen mobile processor)

pci_write_smu_example.go

/*

Example of reading/writing to PCI config registers via sysfs.

In this example we write to the SMU (MP1 mailbox) of a Ryzen mobile processor
to modify a setting that controls CPU's boosting behavior.

There are two modes, "power-saving" (which is usually automatically activated when running on battery power)
and "max-performance" (when connected to AC)

See https://github.com/FlyGoat/RyzenAdj/wiki/Renoir-Tuning-Guide#--power-saving-dc-mode-tune-with-boost-delay
for more information.


*/

package main

import (
	"encoding/binary"
	"fmt"
	"os"
)

const (
	PCI_DEV_CONFIG_PATH    = "/sys/bus/pci/devices/0000:00:00.0/config"
	PCI_REG_OFFSET_ADDRESS = 0xB8
	PCI_REG_OFFSET_DATA    = 0xBC

	MP1_MESSAGE_ADDR  = 0x3B10528
	MP1_RESPONSE_ADDR = 0x3B10564
)

func main() {
	writeMP1(MP1_RESPONSE_ADDR, 0x0) // clear response

	writeMP1(MP1_MESSAGE_ADDR, 0x11) // performance mode
	//writeMP1(MP1_MESSAGE_ADDR, 0x12) // power-saving mode

	resp := uint32(0)

	for resp == 0 {
		resp = readMP1(MP1_RESPONSE_ADDR) // read until we get a response
	}
	fmt.Printf("DONE - Response code: %d\n", resp)
}

func readMP1(addr uint32) uint32 {
	err := writeConfigRegister(PCI_REG_OFFSET_ADDRESS, (addr &^ 0x3))
	if err != nil {
		panic(err)
	}
	val, err := readConfigRegister(PCI_REG_OFFSET_DATA)
	if err != nil {
		panic(err)
	}
	return val
}

func writeMP1(addr, data uint32) {
	err := writeConfigRegister(PCI_REG_OFFSET_ADDRESS, addr)
	if err != nil {
		panic(err)
	}
	err = writeConfigRegister(PCI_REG_OFFSET_DATA, data)
	if err != nil {
		panic(err)
	}
}

type barreg struct {
	offset int64
	*os.File
}

func (r *barreg) Read(b []byte) (int, error) {
	return r.ReadAt(b, r.offset)
}

func (r *barreg) Write(b []byte) (int, error) {
	return r.WriteAt(b, r.offset)
}

func readConfigRegister(offset int64) (uint32, error) {
	f, err := os.Open(PCI_DEV_CONFIG_PATH)
	if err != nil {
		return 0, err
	}
	defer f.Close()

	var val uint32
	err = binary.Read(&barreg{offset: offset, File: f}, binary.LittleEndian, &val)

	return val, err
}

func writeConfigRegister(offset int64, val uint32) error {
	f, err := os.OpenFile(PCI_DEV_CONFIG_PATH, os.O_WRONLY, 0)
	if err != nil {
		return err
	}
	defer f.Close()

	err = binary.Write(&barreg{offset: offset, File: f}, binary.LittleEndian, &val)

	return err
}