Re: [RFC net-next 2/3] net: airoha: npu: attach a coherent pool in the NPU window

The EN7581 NPU can only address host DRAM at 0x80000000-0xbfffffff;
higher addresses alias the lower 1 GiB. The coherent buffers allocated
on the NPU device are all shared with the NPU: since mt76 uses the NPU
device as dma_dev, these are the NPU TX/RX descriptor rings, the HW-RRO
ba_bitmap/addr_elem/msdu_pg/session buffers and the NPU TXD buffers.
On a 2 GiB board nothing keeps them below 0xc0000000; they normally
land there only because of early-boot allocation order. With low DRAM
reserved away on W1700K the SoC hangs right after mt7996 firmware load.

If the NPU node has an optional "dma" reserved-memory region, attach it
as the device coherent pool so all of these come from the window. The
next patch moves the mailbox buffers to coherent memory as well.

Unlike a restricted-dma-pool this leaves streaming mappings alone.
Packet buffers are passed through to the WiFi DMA engine: on W1700K
with 96% of TX skbs and ~20% of RX page_pool buffers above 0xc0000000,
host-path traffic and sha256 file round-trips stay intact, at the same
throughput as without a pool, whereas a restricted-dma-pool (swiotlb
bouncing) costs ~22% of host-path TX.

A device coherent pool has no fallback once full; mt76 and the mailbox
use ~10.7 MiB with MT7996, so the DT region should be 16 MiB.

This patch was written with the help of Claude (Anthropic, claude-
opus-5-5).

Link: https://lore.kernel.org/r/atCRsquzA1UxSpCc@mediatek.com (local)
Tested-by: Daniel Pawlik <redacted>
Signed-off-by: Chris Scott <redacted>
---
 drivers/net/ethernet/airoha/airoha_npu.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/net/ethernet/airoha/airoha_npu.c b/drivers/net/ethernet/airoha/airoha_npu.c
index 4d3195eb00..069fd60680 100644
--- a/drivers/net/ethernet/airoha/airoha_npu.c
+++ b/drivers/net/ethernet/airoha/airoha_npu.c

@@ -693,6 +693,11 @@ static const struct regmap_config regmap_config = {
 	.disable_locking	= true,
 };
 
+static void airoha_npu_rmem_release(void *data)
+{
+	of_reserved_mem_device_release(data);
+}
+
 static int airoha_npu_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;

@@ -776,6 +781,19 @@ static int airoha_npu_probe(struct platform_device *pdev)
 	if (err)
 		return err;
 
+	if (of_property_match_string(dev->of_node, "memory-region-names",
+				     "dma") >= 0) {
+		err = of_reserved_mem_device_init_by_name(dev, dev->of_node,
+							  "dma");
+		if (err)
+			return err;
+
+		err = devm_add_action_or_reset(dev, airoha_npu_rmem_release,
+					       dev);
+		if (err)
+			return err;
+	}