diff --git a/linux-core/drm_compat.h b/linux-core/drm_compat.h
index c7f72e33..e09be479 100644
--- a/linux-core/drm_compat.h
+++ b/linux-core/drm_compat.h
@@ -333,9 +333,24 @@ typedef _Bool                   bool;
 
 
 #if (defined(CONFIG_X86) && defined(CONFIG_X86_32) && defined(CONFIG_HIGHMEM))
+/*
+ * pgd_offset_k() is a macro that uses the symbol init_mm,
+ * check that it is available.
+ */
+#  if ((LINUX_VERSION_CODE < KERNEL_VERSION(2,6,25)) || \
+	defined(CONFIG_UNUSED_SYMBOLS))
 #define DRM_KMAP_ATOMIC_PROT_PFN
 extern void *kmap_atomic_prot_pfn(unsigned long pfn, enum km_type type,
 				  pgprot_t protection);
+#  else
+#warning "init_mm is not available on this kernel!"
+static inline void *kmap_atomic_prot_pfn(unsigned long pfn, enum km_type type,
+					 pgprot_t protection)
+{
+	/* stub */
+	return NULL;
+}
+#  endif /* no init_mm */
 #endif
 
 #if !defined(flush_agp_mappings)