source: bootcd/isolinux/syslinux-6.03/gnu-efi/gnu-efi-3.0/apps/modelist.c

#include <efi.h>
#include <efilib.h>

extern EFI_GUID GraphicsOutputProtocol;

static int memcmp(const void *s1, const void *s2, UINTN n)
{
        const unsigned char *c1 = s1, *c2 = s2;
        int d = 0;

        if (!s1 && !s2)
                return 0;
        if (s1 && !s2)
                return 1;
        if (!s1 && s2)
                return -1;

        while (n--) {
                d = (int)*c1++ - (int)*c2++;
                if (d)
                        break;
        }
        return d;
}

static void
print_modes(EFI_GRAPHICS_OUTPUT_PROTOCOL *gop)
{
        int i, imax;
        EFI_STATUS rc;

        imax = gop->Mode->MaxMode;

        Print(L"GOP reports MaxMode %d\n", imax);
        for (i = 0; i < imax; i++) {
                EFI_GRAPHICS_OUTPUT_MODE_INFORMATION *info;
                UINTN SizeOfInfo;
                rc = uefi_call_wrapper(gop->QueryMode, 4, gop, i, &SizeOfInfo,
                                        &info);
                if (EFI_ERROR(rc) && rc == EFI_NOT_STARTED) {
                        rc = uefi_call_wrapper(gop->SetMode, 2, gop,
                                gop->Mode->Mode);
                        rc = uefi_call_wrapper(gop->QueryMode, 4, gop, i,
                                &SizeOfInfo, &info);
                }

                if (EFI_ERROR(rc)) {
                        CHAR16 Buffer[64];
                        StatusToString(Buffer, rc);
                        Print(L"%d: Bad response from QueryMode: %s (%d)\n",
                                i, Buffer, rc);
                        continue;
                }
                Print(L"%c%d: %dx%d ", memcmp(info,gop->Mode->Info,sizeof(*info)) == 0 ? '*' : ' ', i,
                        info->HorizontalResolution,
                        info->VerticalResolution);
                switch(info->PixelFormat) {
                        case PixelRedGreenBlueReserved8BitPerColor:
                                Print(L"RGBR");
                                break;
                        case PixelBlueGreenRedReserved8BitPerColor:
                                Print(L"BGRR");
                                break;
                        case PixelBitMask:
                                Print(L"R:%08x G:%08x B:%08x X:%08x",
                                        info->PixelInformation.RedMask,
                                        info->PixelInformation.GreenMask,
                                        info->PixelInformation.BlueMask,
                                        info->PixelInformation.ReservedMask);
                                break;
                        case PixelBltOnly:
                                Print(L"(blt only)");
                                break;
                        default:
                                Print(L"(Invalid pixel format)");
                                break;
                }
                Print(L" pitch %d\n", info->PixelsPerScanLine);
        }
}

static EFI_STATUS
SetWatchdog(UINTN seconds)
{
        EFI_STATUS rc;
        rc = uefi_call_wrapper(BS->SetWatchdogTimer, 4, seconds, 0x1ffff,
                                0, NULL);
        if (EFI_ERROR(rc)) {
                CHAR16 Buffer[64];
                StatusToString(Buffer, rc);
                Print(L"Bad response from QueryMode: %s (%d)\n", Buffer, rc);
        }
        return rc;
}

EFI_STATUS
efi_main (EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *systab)
{
        EFI_STATUS rc;
        EFI_GRAPHICS_OUTPUT_PROTOCOL *gop;

        InitializeLib(image_handle, systab);

        SetWatchdog(10);

        rc = LibLocateProtocol(&GraphicsOutputProtocol, (void **)&gop);
        if (EFI_ERROR(rc))
                return rc;

        print_modes(gop);

        SetWatchdog(0);
        return EFI_SUCCESS;
}