LoongArch: Add writecombine support for drm

LoongArch maintains cache coherency in hardware, but its WUC attribute
(Weak-ordered UnCached, which is similar to WC) is out of the scope of
cache coherency machanism. This means WUC can only used for write-only
memory regions.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

drivers/gpu/drm/drm_vm.c

@@ -69,7 +69,7 @@ static pgprot_t drm_io_prot(struct drm_local_map *map,
 	pgprot_t tmp = vm_get_page_prot(vma->vm_flags);
 
 #if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__) || \
-    defined(__mips__)
+    defined(__mips__) || defined(__loongarch__)
 	if (map->type == _DRM_REGISTERS && !(map->flags & _DRM_WRITE_COMBINING))
 		tmp = pgprot_noncached(tmp);
 	else

drivers/gpu/drm/ttm/ttm_module.c

@@ -74,7 +74,7 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching caching, pgprot_t tmp)
 #endif /* CONFIG_UML */
 #endif /* __i386__ || __x86_64__ */
 #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
-	defined(__powerpc__) || defined(__mips__)
+	defined(__powerpc__) || defined(__mips__) || defined(__loongarch__)
 	if (caching == ttm_write_combined)
 		tmp = pgprot_writecombine(tmp);
 	else

include/drm/drm_cache.h

@@ -67,6 +67,14 @@ static inline bool drm_arch_can_wc_memory(void)
 	 * optimization entirely for ARM and arm64.
 	 */
 	return false;
+#elif defined(CONFIG_LOONGARCH)
+	/*
+	 * LoongArch maintains cache coherency in hardware, but its WUC attribute
+	 * (Weak-ordered UnCached, which is similar to WC) is out of the scope of
+	 * cache coherency machanism. This means WUC can only used for write-only
+	 * memory regions.
+	 */
+	return false;
 #else
 	return true;
 #endif