drm/ttm: Use dmacoherent allocations on ARM64

Various PCIe controllers on ARM64 platforms don't support cache
snooping, which leads to numerous issues when attempting to use
PCIe graphics cards.

Switching ttm_prot_from_caching to return pgprot_dmacoherent for
ttm_cached pages solves the issue, albeit with a performance hit.
There is a second check in ttm_prot_from_caching that also needs
updating.

Signed-off-by: Yang Bo <bo.yang@smail.nju.edu.cn>
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Author: yanghaku

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -359,7 +359,8 @@ static int ttm_bo_kmap_ttm(struct ttm_buffer_object *bo,
 	if (ret)
 		return ret;
 
-	if (num_pages == 1 && ttm->caching == ttm_cached &&
+	if (!IS_ENABLED(CONFIG_ARM64) &&
+	    num_pages == 1 && ttm->caching == ttm_cached &&
 	    !(man->use_tt && (ttm->page_flags & TTM_TT_FLAG_DECRYPTED))) {
 		/*
 		 * We're mapping a single page, and the desired

drivers/gpu/drm/ttm/ttm_module.c

@@ -63,7 +63,11 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching caching, pgprot_t tmp)
 {
 	/* Cached mappings need no adjustment */
 	if (caching == ttm_cached)
+#ifdef CONFIG_ARM64
+		return pgprot_dmacoherent(tmp);
+#else
 		return tmp;
+#endif
 
 #if defined(__i386__) || defined(__x86_64__)
 	if (caching == ttm_write_combined)