[PATCH 2/2] drm/amdgpu: Redirect imported dma-buf mmap request to exporter

[Xiaogang.Chen <xiaogang.chen@amd.com>]

From: Xiaogang Chen <xiaogang.chen@amd.com>

The imported GEM object with ttm->page_flags TTM_TT_FLAG_EXTERNAL set and
without TTM_TT_FLAG_EXTERNAL_MAPPABLE set,is only a GPU-side handle for memory
that belongs to the exporter. TTM refuses to build a CPU mapping from it
because those page-table entries would point to pages drm driver does not own.

amdgpu_dma_buf_create_obj creates GEM object for foreign exported buf(not
generated by amdgpu driver) belonging to this catagory. The pages, when GPU
needs them, come from dma_buf_map_attachment on the exporter's scatter/gather
table. The exporter can move or revoke that mapping through move_notify.
A CPU page-table entry installed by the importer would keep the old PFN, and
the importer also does not know the exporter's caching mode or whether the
memory is CPU-visible.

This patch redirects foreign imported dma-buf mmap request to exporter. The
CPU mapping is created by dma_buf_mmap, which points the VMA to the exporter's
file and calls dma_buf->ops->mmap. The exporter's fault handler will fill
page table.

Signed-off-by: Xiaogang Chen <xiaogang.chen@amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index 4e4814caf61a..d872b4bcb5fc 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -369,6 +369,26 @@ static void amdgpu_gem_object_close(struct drm_gem_object *obj,
 static int amdgpu_gem_object_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 {
 	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
+	struct dma_buf *dmabuf;
+	int ret;
+
+	/*
+	 * A foreign import (udmabuf, for example) does not own the pages.
+	 * Point the VMA at the exporter so the later CPU fault uses its
+	 * vm_ops. dma_buf_mmap() resets vm_pgoff and holds the dma-buf file.
+	 * An amdgpu dma-buf stays on the TTM path below.
+	 */
+	dmabuf = obj->import_attach ? obj->import_attach->dmabuf : NULL;
+	if (dmabuf && dmabuf->ops != &amdgpu_dmabuf_ops) {
+		ret = dma_buf_mmap(dmabuf, vma, 0);
+		if (ret)
+			return ret;
+
+		/* Exporter vm_ops will not drop the mapping reference. */
+		drm_gem_object_put(obj);
+		return 0;
+	}
+
 
 	if (amdgpu_ttm_tt_get_usermm(bo->tbo.ttm))
 		return -EPERM;
-- 
2.34.1