[PATCH v3 2/7] dmaengine: add a bus for DMA channel devices

["Peng Fan (OSS)" <peng.fan@oss.nxp.com>]

From: Peng Fan <peng.fan@nxp.com>

DMA channel devices are currently created as class devices without a bus.
This works for sysfs, but it prevents per-channel DMA/IOMMU configuration:
of_dma_configure() only probes the IOMMU when dev->bus is set, and the
IOMMU core only installs probe/release notifiers for buses listed in
iommu_buses[].

Drivers that need per-channel IOMMU support currently work around this by
assigning dev->bus after device_register(). That leaves the bus subsystem
reference count unbalanced because bus_add_device() already ran with a NULL
bus, and bus_remove_device() will later drop a reference that was never
taken.

Add a dedicated bus for DMA channel devices and register it during
dmaengine initialisation, before iommu_subsys_init(). The match callback
never succeeds, so no driver can bind to these devices. A follow-up patch
will register the channel devices on this bus.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
---
 drivers/dma/dmaengine.c   | 24 +++++++++++++++++++++---
 include/linux/dmaengine.h |  4 ++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 34be53487a21d..6fea67f088b35 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -1643,17 +1643,35 @@ void dma_run_dependencies(struct dma_async_tx_descriptor *tx)
 }
 EXPORT_SYMBOL_GPL(dma_run_dependencies);
 
+static int dma_chan_bus_match(struct device *dev,
+			      const struct device_driver *drv)
+{
+	return 0;
+}
+
+const struct bus_type dma_chan_bus_type = {
+	.name  = "dma",
+	.match = dma_chan_bus_match,
+};
+
 static int __init dma_bus_init(void)
 {
 	int err = dmaengine_init_unmap_pool();
 
+	if (err)
+		return err;
+
+	err = bus_register(&dma_chan_bus_type);
 	if (err)
 		return err;
 
 	err = class_register(&dma_devclass);
-	if (!err)
-		dmaengine_debugfs_init();
+	if (err) {
+		bus_unregister(&dma_chan_bus_type);
+		return err;
+	}
 
-	return err;
+	dmaengine_debugfs_init();
+	return 0;
 }
 arch_initcall(dma_bus_init);
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 805668fbe4376..14d8f182c7cb5 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -412,6 +412,10 @@ struct dma_chan_dev {
 	bool chan_dma_dev;
 };
 
+#ifdef CONFIG_DMA_ENGINE
+extern const struct bus_type dma_chan_bus_type;
+#endif
+
 /**
  * enum dma_slave_buswidth - defines bus width of the DMA slave
  * device, source or target buses

-- 
2.34.1