drm: omapdrm: Fix incorrect usage of the term 'cache coherency'

The is_cache_coherent() function currently returns true when the mapping
is not cache-coherent. This isn't a bug as such as the callers interpret
cache-coherent as meaning that the driver has to handle the coherency
manually, but it is nonetheless very confusing. Fix it and add a bit
more documentation to explain how cached buffers are handled.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

1 files changed, 15 insertions, 7 deletions

diff --git a/drivers/gpu/drm/omapdrm/omap_gem.c b/drivers/gpu/drm/omapdrm/omap_gem.c
index 86567e591f4e..eb02a1399a10 100644
--- a/drivers/gpu/drm/omapdrm/omap_gem.c
+++ b/drivers/gpu/drm/omapdrm/omap_gem.c
@@ -719,16 +719,21 @@ fail:
  * Memory Management & DMA Sync
  */
 
-/**
- * shmem buffers that are mapped cached can simulate coherency via using
- * page faulting to keep track of dirty pages
+/*
+ * shmem buffers that are mapped cached are not coherent.
+ *
+ * We keep track of dirty pages using page faulting to perform cache management.
+ * When a page is mapped to the CPU in read/write mode the device can't access
+ * it and omap_obj->dma_addrs[i] is NULL. When a page is mapped to the device
+ * the omap_obj->dma_addrs[i] is set to the DMA address, and the page is
+ * unmapped from the CPU.
  */
 static inline bool is_cached_coherent(struct drm_gem_object *obj)
 {
         struct omap_gem_object *omap_obj = to_omap_bo(obj);
 
-        return (omap_obj->flags & OMAP_BO_MEM_SHMEM) &&
-                ((omap_obj->flags & OMAP_BO_CACHE_MASK) == OMAP_BO_CACHED);
+        return !((omap_obj->flags & OMAP_BO_MEM_SHMEM) &&
+                ((omap_obj->flags & OMAP_BO_CACHE_MASK) == OMAP_BO_CACHED));
 }
 
 /* Sync the buffer for CPU access.. note pages should already be
@@ -739,7 +744,10 @@ void omap_gem_cpu_sync_page(struct drm_gem_object *obj, int pgoff)
         struct drm_device *dev = obj->dev;
         struct omap_gem_object *omap_obj = to_omap_bo(obj);
 
-        if (is_cached_coherent(obj) && omap_obj->dma_addrs[pgoff]) {
+        if (is_cached_coherent(obj))
+                return;
+
+        if (omap_obj->dma_addrs[pgoff]) {
                 dma_unmap_page(dev->dev, omap_obj->dma_addrs[pgoff],
                                 PAGE_SIZE, DMA_BIDIRECTIONAL);
                 omap_obj->dma_addrs[pgoff] = 0;
@@ -756,7 +764,7 @@ void omap_gem_dma_sync_buffer(struct drm_gem_object *obj,
         struct page **pages = omap_obj->pages;
         bool dirty = false;
 
-        if (!is_cached_coherent(obj))
+        if (is_cached_coherent(obj))
                 return;
 
         for (i = 0; i < npages; i++) {