drm/xe/stolen: Exclude reserved lmem portion

The address set by firmware in GEN12_DSMBASE in driver initialization
doesn't mean "anything above that and until end of lmem is part of DSM".
In fact, there may be a few KB that is not part of DSM on the end of
lmem. How large is that space is platform-dependent, but since it's
always less than the DSM granularity, it can be simplified by simply
aligning the size down.
Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_ttm_stolen_mgr.c

@@ -88,7 +88,13 @@ static s64 detect_bar2_dgfx(struct xe_device *xe, struct xe_ttm_stolen_mgr *mgr)
 	if (mgr->stolen_base + stolen_size <= pci_resource_len(pdev, 2))
 		mgr->io_base = pci_resource_start(pdev, 2) + mgr->stolen_base;
 
-	return stolen_size;
+	/*
+	 * There may be few KB of platform dependent reserved memory at the end
+	 * of lmem which is not part of the DSM. Such reserved memory portion is
+	 * always less then DSM granularity so align down the stolen_size to DSM
+	 * granularity to accommodate such reserve lmem portion.
+	 */
+	return ALIGN_DOWN(stolen_size, SZ_1M);
 }
 
 static u32 detect_bar2_integrated(struct xe_device *xe, struct xe_ttm_stolen_mgr *mgr)