This freezes the machine pure and simple !

rockchip-iommu.c

/* Code */

/* ... */

void rk_iommu_flush_tlb(struct device *dev) {
	struct rk_iommu *iommu = rk_iommu_from_dev(dev);
	int i;
	if (!iommu)
		dev_err(dev, "Can't flush nothing !\n");
	else {
		for (i = 0; i < iommu->num_mmu; i++)
			rk_iommu_base_command(iommu->bases[i], RK_MMU_CMD_ZAP_CACHE);
	}
}
EXPORT_SYMBOL(rk_iommu_flush_tlb);

vcodec_iommu_drm.c

static int vcodec_drm_map_iommu(struct vcodec_iommu_session_info *session_info,
				int idx,
				unsigned long *iova,
				unsigned long *size)
{
	struct device *dev = session_info->dev;
	struct vcodec_drm_buffer *drm_buffer;

	/* Force to flush iommu table */
	if (of_machine_is_compatible("rockchip,rk3288"))
		rk_iommu_flush_tlb(session_info->dev);

	mutex_lock(&session_info->list_mutex);
	drm_buffer = vcodec_drm_get_buffer_no_lock(session_info, idx);
	dev_info(dev,
		"( Myy ) vcodec_drm_get_buffer_no_lock(%p, %d) → %p",
		session_info, idx, drm_buffer);
	mutex_unlock(&session_info->list_mutex);

	if (!drm_buffer) {
		dev_err(dev, "can not find %d buffer in list\n", idx);
		return -EINVAL;
	}

	kref_get(&drm_buffer->ref);
	if (iova)
		*iova = drm_buffer->iova;
	if (size)
		*size = drm_buffer->size;
	return 0;
}

static int
vcodec_drm_unmap_iommu(struct vcodec_iommu_session_info *session_info,
		       int idx)
{
	struct device *dev = session_info->dev;
	struct vcodec_drm_buffer *drm_buffer;

	/* Force to flush iommu table */
	if (of_machine_is_compatible("rockchip,rk3288"))
		rk_iommu_flush_tlb(session_info->dev);

	mutex_lock(&session_info->list_mutex);
	drm_buffer = vcodec_drm_get_buffer_no_lock(session_info, idx);
	mutex_unlock(&session_info->list_mutex);

	if (!drm_buffer) {
		dev_err(dev, "can not find %d buffer in list\n", idx);
		return -EINVAL;
	}

	kref_put(&drm_buffer->ref, vcodec_drm_clear_map);

	return 0;
}