rockchip: translate DMA for the media IOMMU masters

The video codecs sit behind their own IOMMUs, but with the default
domain set to passthrough their buffers have to be physically
contiguous and come out of the 16 MiB CMA pool. A 4K frame needs
15.8 MiB of it, so hardware decoding of 4K video fails allocation and
falls back to software: 10 seconds of 4K HEVC then cost 22 seconds of
CPU, where the decoder does the same work with 0.15.

Have rockchip-iommu ask for a translated domain for the masters it
serves, so the display, codec, ISP and RGA blocks get scatter-gather
while everything behind another IOMMU, PCIe included, keeps the system
default and its throughput.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/rockchip/patches-6.18/142-iommu-rockchip-translate-DMA-for-IOMMU-masters.patch

@@ -0,0 +1,47 @@
+From f69d2a3b86e90f4141d45be7d234f9a5ab464fe8 Mon Sep 17 00:00:00 2001
+From: Daniel Golle <daniel@makrotopia.org>
+Date: Thu, 24 Sep 2026 17:39:45 +0100
+Subject: [PATCH] iommu/rockchip: use a translated domain for IOMMU masters
+
+The masters behind rockchip-iommu are media blocks: the display
+controller, the video codecs, the ISP and the 2D engine. They are wired
+to the IOMMU so that their buffers do not have to be physically
+contiguous. With a system-wide passthrough default, which is what
+IOMMU_DEFAULT_PASSTHROUGH gives, those masters are handed an identity
+domain instead, and every buffer has to come from CMA. A 4K frame needs
+15.8 MiB of it, so hardware decoding fails the allocation and falls back
+to software.
+
+Return IOMMU_DOMAIN_DMA from def_domain_type so these masters get a
+translated domain whatever the system default is, the way apple-dart
+does for its no-bypass configurations. Masters behind other IOMMUs are
+untouched, so a PCIe root port behind an SMMU keeps the system default
+and its performance.
+
+Signed-off-by: Daniel Golle <daniel@makrotopia.org>
+---
+ drivers/iommu/rockchip-iommu.c | 7 +++++++
+ 1 file changed, 7 insertions(+)
+
+--- a/drivers/iommu/rockchip-iommu.c
++++ b/drivers/iommu/rockchip-iommu.c
+@@ -1174,12 +1174,19 @@ static int rk_iommu_of_xlate(struct devi
+ 	return 0;
+ }
+ 
++/* An identity domain would put these media masters back onto CMA. */
++static int rk_iommu_def_domain_type(struct device *dev)
++{
++	return IOMMU_DOMAIN_DMA;
++}
++
+ static const struct iommu_ops rk_iommu_ops = {
+ 	.identity_domain = &rk_identity_domain,
+ 	.domain_alloc_paging = rk_iommu_domain_alloc_paging,
+ 	.probe_device = rk_iommu_probe_device,
+ 	.release_device = rk_iommu_release_device,
+ 	.device_group = generic_single_device_group,
++	.def_domain_type = rk_iommu_def_domain_type,
+ 	.of_xlate = rk_iommu_of_xlate,
+ 	.default_domain_ops = &(const struct iommu_domain_ops) {
+ 		.attach_dev	= rk_iommu_attach_device,