[Intel-gfx] [PATCH 2/2] drm/i915/dmabuf: Pin pages as we flush for CPU access

[Chris Wilson]

As a precautionary defensive measure against changes to the core (where
we stop changing domains when unbinding), we want to ensure that the pages
are available when we want to start CPU access, otherwise we will not
perform the requisite clflushes to make the contents coherent for the user.

In contrast, we do not have to worry about ensuring the pages are
available when flushing after CPU access, as any subsequent use that
requires the pages in the GTT domain will be perform the flush
themselves.

The tricky part is that we can not force the behaviour within
set-to-cpu-domain as that currently needs to be callable from within the
put_pages cleanup path, thus we place the burden on the caller.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_dmabuf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
index 80bbe43a2e92..6d898a201a46 100644
--- a/drivers/gpu/drm/i915/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
@@ -182,7 +182,15 @@ static int i915_gem_begin_cpu_access(struct dma_buf *dma_buf, enum dma_data_dire
 	if (ret)
 		return ret;
 
+	ret = i915_gem_object_get_pages(obj);
+	if (ret)
+		goto err_unlock;
+
+	i915_gem_object_pin_pages(obj);
 	ret = i915_gem_object_set_to_cpu_domain(obj, write);
+	i915_gem_object_unpin_pages(obj);
+
+err_unlock:
 	mutex_unlock(&dev->struct_mutex);
 	return ret;
 }
-- 
2.8.1