[PATCH 1/2] drm/i915: Use CPU mapping for userspace dma-buf mmap()

[Tiago Vignatti <tiago.vignatti@intel.com>]

For now we're opting out devices that don't have the LLC CPU cache (mostly
"Atom" devices). Alternatively, we could build up a path to mmap them through
GTT WC (and ignore the fact that will be dead-slow for reading). Or, an even
more complex work I believe, would involve on setting up dma-buf ioctls to
allow userspace flush, controlling manually the synchronization via
begin{,end}_cpu_access.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
---
 drivers/gpu/drm/i915/i915_gem_dmabuf.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
index e9c2bfd..e6cb402 100644
--- a/drivers/gpu/drm/i915/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
@@ -193,7 +193,26 @@ static void i915_gem_dmabuf_kunmap(struct dma_buf *dma_buf, unsigned long page_n
 
 static int i915_gem_dmabuf_mmap(struct dma_buf *dma_buf, struct vm_area_struct *vma)
 {
-	return -EINVAL;
+	struct drm_i915_gem_object *obj = dma_buf_to_obj(dma_buf);
+	struct drm_device *dev = obj->base.dev;
+	int ret;
+
+	if (obj->base.size < vma->vm_end - vma->vm_start)
+		return -EINVAL;
+
+	/* On non-LLC machines we'd need to be careful cause CPU and GPU don't
+	 * share the CPU's L3 cache and coherency may hurt when CPU mapping. */
+	if (!HAS_LLC(dev))
+		return -EINVAL;
+
+	if (!obj->base.filp)
+		return -EINVAL;
+
+	ret = obj->base.filp->f_op->mmap(obj->base.filp, vma);
+	fput(vma->vm_file);
+	vma->vm_file = get_file(obj->base.filp);
+
+	return ret;
 }
 
 static int i915_gem_begin_cpu_access(struct dma_buf *dma_buf, size_t start, size_t length, enum dma_data_direction direction)
-- 
2.1.0

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