[RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available

[Icenowy Zheng]

As we can now acquire the presence of the full DMA coherency (snooping
capability) from ttm_device, we can now map the CPU side memory as
write-combined when cached is requested and snooping is not avilable.

Signed-off-by: Icenowy Zheng <uwu at icenowy.me>
---
 drivers/gpu/drm/ttm/ttm_bo_util.c | 4 ++++
 drivers/gpu/drm/ttm/ttm_tt.c      | 4 ++++
 include/drm/ttm/ttm_caching.h     | 3 ++-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 0b3f4267130c4..6519ce047787d 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -302,6 +302,10 @@ pgprot_t ttm_io_prot(struct ttm_buffer_object *bo, struct ttm_resource *res,
 		caching = res->bus.caching;
 	}
 
+	/* Downgrade cached mapping for non-snooping devices */
+	if (!bo->bdev->dma_coherent && caching == ttm_cached)
+		caching = ttm_write_combined;
+
 	return ttm_prot_from_caching(caching, tmp);
 }
 EXPORT_SYMBOL(ttm_io_prot);
diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
index 7b00ddf0ce49f..3335df45fba5e 100644
--- a/drivers/gpu/drm/ttm/ttm_tt.c
+++ b/drivers/gpu/drm/ttm/ttm_tt.c
@@ -152,6 +152,10 @@ static void ttm_tt_init_fields(struct ttm_tt *ttm,
 			       enum ttm_caching caching,
 			       unsigned long extra_pages)
 {
+	/* Downgrade cached mapping for non-snooping devices */
+	if (!bo->bdev->dma_coherent && caching == ttm_cached)
+		caching = ttm_write_combined;
+
 	ttm->num_pages = (PAGE_ALIGN(bo->base.size) >> PAGE_SHIFT) + extra_pages;
 	ttm->page_flags = page_flags;
 	ttm->dma_address = NULL;
diff --git a/include/drm/ttm/ttm_caching.h b/include/drm/ttm/ttm_caching.h
index a18f43e93abab..f92d7911f50e4 100644
--- a/include/drm/ttm/ttm_caching.h
+++ b/include/drm/ttm/ttm_caching.h
@@ -47,7 +47,8 @@ enum ttm_caching {
 
 	/**
 	 * @ttm_cached: Fully cached like normal system memory, requires that
-	 * devices snoop the CPU cache on accesses.
+	 * devices snoop the CPU cache on accesses. Downgraded to
+	 * ttm_write_combined when the snooping capaiblity is missing.
 	 */
 	ttm_cached
 };
-- 
2.45.2