drm/i915/gem: Add a helper to read data from a GEM object page

Add a simple helper to read data with the CPU from the page of a GEM
object. Do the read either via a kmap if the object has struct pages
or an iomap otherwise. This is needed by the next patch, reading a u64
value from the object (w/o requiring the obj to be mapped to the GPU).

Suggested by Chris.

v2 (Chris):
- Sanitize the type and order of func params.
- Avoid consts requiring too many casts.
- Use BUG_ON instead of WARN_ON, simplify the conditions.
- Fix __iomem sparse errors.
- Leave locking/syncing/pinning up to the caller, require only that the
  caller has pinned the object pages.
- Check for iomem backing store before reading via an iomap.
v3:
- Fix offset passed to io_mapping_map_wc() missing a mem.region.start
  delta. (Chris, Matthew)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120213834.1435710-1-imre.deak@intel.com

drivers/gpu/drm/i915/gem/i915_gem_object.c

@@ -32,6 +32,7 @@
 #include "i915_gem_mman.h"
 #include "i915_gem_object.h"
 #include "i915_globals.h"
+#include "i915_memcpy.h"
 #include "i915_trace.h"
 
 static struct i915_global_object {
@@ -383,6 +384,70 @@ void __i915_gem_object_invalidate_frontbuffer(struct drm_i915_gem_object *obj,
 	}
 }
 
+static void
+i915_gem_object_read_from_page_kmap(struct drm_i915_gem_object *obj, u64 offset, void *dst, int size)
+{
+	void *src_map;
+	void *src_ptr;
+
+	src_map = kmap_atomic(i915_gem_object_get_page(obj, offset >> PAGE_SHIFT));
+
+	src_ptr = src_map + offset_in_page(offset);
+	if (!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ))
+		drm_clflush_virt_range(src_ptr, size);
+	memcpy(dst, src_ptr, size);
+
+	kunmap_atomic(src_map);
+}
+
+static void
+i915_gem_object_read_from_page_iomap(struct drm_i915_gem_object *obj, u64 offset, void *dst, int size)
+{
+	void __iomem *src_map;
+	void __iomem *src_ptr;
+	dma_addr_t dma = i915_gem_object_get_dma_address(obj, offset >> PAGE_SHIFT);
+
+	src_map = io_mapping_map_wc(&obj->mm.region->iomap,
+				    dma - obj->mm.region->region.start,
+				    PAGE_SIZE);
+
+	src_ptr = src_map + offset_in_page(offset);
+	if (!i915_memcpy_from_wc(dst, (void __force *)src_ptr, size))
+		memcpy_fromio(dst, src_ptr, size);
+
+	io_mapping_unmap(src_map);
+}
+
+/**
+ * i915_gem_object_read_from_page - read data from the page of a GEM object
+ * @obj: GEM object to read from
+ * @offset: offset within the object
+ * @dst: buffer to store the read data
+ * @size: size to read
+ *
+ * Reads data from @obj at the specified offset. The requested region to read
+ * from can't cross a page boundary. The caller must ensure that @obj pages
+ * are pinned and that @obj is synced wrt. any related writes.
+ *
+ * Returns 0 on success or -ENODEV if the type of @obj's backing store is
+ * unsupported.
+ */
+int i915_gem_object_read_from_page(struct drm_i915_gem_object *obj, u64 offset, void *dst, int size)
+{
+	GEM_BUG_ON(offset >= obj->base.size);
+	GEM_BUG_ON(offset_in_page(offset) > PAGE_SIZE - size);
+	GEM_BUG_ON(!i915_gem_object_has_pinned_pages(obj));
+
+	if (i915_gem_object_has_struct_page(obj))
+		i915_gem_object_read_from_page_kmap(obj, offset, dst, size);
+	else if (i915_gem_object_has_iomem(obj))
+		i915_gem_object_read_from_page_iomap(obj, offset, dst, size);
+	else
+		return -ENODEV;
+
+	return 0;
+}
+
 void i915_gem_init__objects(struct drm_i915_private *i915)
 {
 	INIT_WORK(&i915->mm.free_work, __i915_gem_free_work);

drivers/gpu/drm/i915/gem/i915_gem_object.h

@@ -200,6 +200,12 @@ i915_gem_object_has_struct_page(const struct drm_i915_gem_object *obj)
 	return i915_gem_object_type_has(obj, I915_GEM_OBJECT_HAS_STRUCT_PAGE);
 }
 
+static inline bool
+i915_gem_object_has_iomem(const struct drm_i915_gem_object *obj)
+{
+	return i915_gem_object_type_has(obj, I915_GEM_OBJECT_HAS_IOMEM);
+}
+
 static inline bool
 i915_gem_object_is_shrinkable(const struct drm_i915_gem_object *obj)
 {
@@ -540,4 +546,6 @@ i915_gem_object_invalidate_frontbuffer(struct drm_i915_gem_object *obj,
 		__i915_gem_object_invalidate_frontbuffer(obj, origin);
 }
 
+int i915_gem_object_read_from_page(struct drm_i915_gem_object *obj, u64 offset, void *dst, int size);
+
 #endif