drm/radeon: Fix sideport problems on certain RS690 boards

Some RS690 boards with 64MB of sideport memory show up as
having 128MB sideport + 256MB of UMA.  In this case,
just skip the sideport memory and use UMA.  This fixes
rendering corruption and should improve performance.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=35457

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/rs690.c

@@ -162,6 +162,16 @@ static void rs690_mc_init(struct radeon_device *rdev)
 	base = RREG32_MC(R_000100_MCCFG_FB_LOCATION);
 	base = G_000100_MC_FB_START(base) << 16;
 	rdev->mc.igp_sideport_enabled = radeon_atombios_sideport_present(rdev);
+	/* Some boards seem to be configured for 128MB of sideport memory,
+	 * but really only have 64MB.  Just skip the sideport and use
+	 * UMA memory.
+	 */
+	if (rdev->mc.igp_sideport_enabled &&
+	    (rdev->mc.real_vram_size == (384 * 1024 * 1024))) {
+		base += 128 * 1024 * 1024;
+		rdev->mc.real_vram_size -= 128 * 1024 * 1024;
+		rdev->mc.mc_vram_size = rdev->mc.real_vram_size;
+	}
 
 	/* Use K8 direct mapping for fast fb access. */ 
 	rdev->fastfb_working = false;