thinkpad x200 X.server not work with gm4500 graphics

[arrow.ebd@gmail.com]

deal,

    My thinkpad x200's X.server can not startup on the kernel 2.6.28,
and dmesg will show as below:
    -->>--
    [drm] Initialized drm 1.1.0 20060810
    pci 0000:00:02.0: power state changed by ACPI to D0
    pci 0000:00:02.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
    pci 0000:00:02.0: setting latency timer to 64
    [drm] Initialized i915 1.6.0 20060119 on minor 0
    pci 0000:00:02.0: pg_start == 0x00001f7f, intel_private.gtt_entries == 0x00001fff
    pci 0000:00:02.0: trying to insert into local/stolen memory
    --<<--

    Then, I change file "drivers/char/agp/intel-agp.c" as this:
    -->>--
    --diff --git a/drivers/char/agp/intel-agp.c b/drivers/char/agp/intel-agp.c
    index 9cf6e9b..e681f75 100644
    --- a/drivers/char/agp/intel-agp.c
    +++ b/drivers/char/agp/intel-agp.c
    @@ -528,7 +528,8 @@ static void intel_i830_init_gtt_entries(void)
                     * stolen, ignore it in stolen gtt entries counting.  However,
                     * 4KB of the stolen memory doesn't get mapped to the GTT.
                     */
    -               size = 4;
    +               /* size = 4; */
    +               size = 516;
            } else {
                    /* On previous hardware, the GTT size was just what was
                     * required to map the aperture.
    --<<--

    And Xorg can start now.
(1) Is it correct ?
    Because older kernel use size as "512 + 4" at here, but now use "size = 4".
(2) How to deal with it if the patch is not right ?
(3) How to get more specification about gm4500 ?