Set straps bits for 8GB BAR size on GTX 1080

#include <stdint.h>
#include <limits.h>
#include <sys/io.h>

enum BARSizeSelect
{
    _256M = 0,
    _512M = 1,
    _1G = 2,
    _2G = 3,
    _4G = 4,
    _8G = 5,
    _16G = 6,
    _32G = 7
};

#define TARGET_PCI_VENDOR_ID 0x10DEu
#define TARGET_PCI_DEVICE_ID_GTX_1080 0x1B80u

inline uint32_t ReadDWORD(uint32_t addressSelector)
{
    return outl(addressSelector, 0x0cf8U), (uint32_t) inl(0x0cfcU);
}

inline void WriteDWORD(uint32_t addressSelector, uint32_t outputValue)
{
    outl(addressSelector, 0x0cf8U), outl(outputValue, 0x0cfcU);
}

void DebuggerWriteOutputLED(uint8_t ledValue)
{
    outb(ledValue, 0x0080);

    while (int i = (uint32_t)0x8000_0000; --i)
        ;   // could be a delay after the port write, to allow seing the output on the physical LEDs
}

uint32_t configure_bridge_for_GTX_1080_Straps(uint64_t save_area[4u])
{
    uint32_t addressSelector =      // 0x80001804   (PCI vendor ID: 8086, PCI device ID: 6f08)
          1u << 31              // Cmd
        | 0u << 16              // Bus
        | 3u << 11              // Dev
        | 0u <<  8              // Fun
        | 4u <<  0;             // Reg

    // register offset 0x04
    save_area[0u] = ReadDWORD(addressSelector);
    WriteDWORD(addressSelector,  save_area[0u] | 0x0007u);

    // register offset 0x18
    addressSelector +=  0x0014u;
    save_area[1u] = ReadDWORD(addressSelector);
    WriteDWORD(addressSelector,  save_area[1u] | 0x00030300u);

    // register offset 0x1c
    addressSelector += 0x0004u;
    save_area[2u] = ReadDWORD(addressSelector) ;
    WriteDWORD(addressSelector, save_area[2u] & 0xFFFF0000u | 0xe0e0u);

    // register offset 0x20
    addressSelector += 0x0004u;
    save_area[3u] = ReadDWORD(addressSelector);
    WriteDWORD(addressSelector, 0xfb00fa00u);

    return addressSelector;
}

void restore_bridge_config(uint32_t lastAddressSelector, uint64_t const save_area[4])
{
    WriteDWORD(lastAddressSelector, save_area[3u]);

    lastAddressSelector -= 0x0004u;
    WriteDWORD(lastAddressSelector, save_area[2u]);

    lastAddressSelector -= 0x0004u;
    WriteDWORD(lastAddressSelector, save_area[1u]);

    lastAddressSelector -= 0x0014u;
    WriteDWORD(lastAddressSelector, save_area[0u]);
}

uint32_t configure_gtx_1080_for_Straps(uint64_t save_area[2u])
{
    uint32_t addressSelector =      // 80030004h  (PCI vendor ID: 10de, PCI device ID: 1b80)
          1 << 31               // Cmd
        | 3 << 16               // Bus
        | 0 << 11               // Dev
        | 0 <<  8               // Fun
        | 4 <<  0;              // Reg

    // register offset 0x04
    save_area[0u] = ReadDWORD(addressSelector);
    WriteDWORD(addressSelector, save_area[0u] | 0x0003u);

    // register offset 0x10
    addressSelector += 0x000Cu;
                                                    // this call here appears to be missing !!! -- looks like a BUG
    save_area[1u] = save_area[0u] | 0x0003u;        // save_area[1u] = ReadDWORD(addressSelector);
    WriteDWORD(addressSelector, 0xfa000000u);

    return addressSelector;
}

void restore_gtx_1080_config(uint32_t lastAddressSelector, uint64_t const save_area[2])
{
    WriteDWORD(lastAddressSelector, save_area[1u]);

    lastAddressSelector -= 0x000Cu;
    WriteDWORD(lastAddressSelector, save_area[0u]);
}

void set_gtx_1080_straps_to_config_rebar()
{
    uint32_t *straps_address = (uint32_t *)(void *)0xfa10100cu;

    *straps_address = *straps_address & 0xff8fffff | (BARSizeSelect::_8G << 20 | 0x80000000);
}

void configure_GTX_1080_Straps()
{
    if (vid != TARGET_PCI_VENDOR_ID && did != TARGET_PCI_DEVICE_ID_GTX_1080)
        return;

    DebuggerWriteOutputLED(0x77u);

    uint64_t bridge_config_save[4];
    uint64_t gtx_1080_config_save[2];

    uint32_t bridgeAddressSelector = configure_bridge_for_GTX_1080_Straps(bridge_config_save);
    uint32_t gtx1080AddressSelector = configure_gtx_1080_for_Straps(gtx_1080_config_save);

    set_gtx_1080_straps_to_config_rebar();

    restore_gtx_1080_config(gtx1080AddressSelector, gtx_1080_config_save);
    restore_bridge_config(bridgeAddressSelector, bridge_config_save);
}