[PATCH v3 4/7] dmaengine: move channel devices to the DMA channel bus

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

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

Register each dma_chan_dev on dma_chan_bus_type before device_register()
and make it a bus device instead of a class device. Setting the bus
before registration lets bus_add_device() take the bus subsystem
reference, so the matching bus_remove_device() calls drop references that
were actually acquired. Attach dma_dev_groups to the bus and set the
device release callback directly.

Keep the existing /sys/class/dma interface by registering a class_compat
named "dma" and creating a link for each channel device. The channel
devices now live on the DMA bus, while /sys/class/dma/dmaNchanM remains
available to userspace. The uevent subsystem name stays "dma" because
the bus has the same name as the old class.

Remove the post-registration dev->bus assignment from the Tegra GPC DMA
driver. Leaving it in place would overwrite the bus selected by the DMA
engine core and reintroduce the reference imbalance.

The DMA channel bus match callback never matches, so no driver can bind
to these channel devices.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
---
 drivers/dma/dmaengine.c        | 27 ++++++++++++++++-----------
 drivers/dma/tegra186-gpc-dma.c |  1 -
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 6fea67f088b35..0e2eea339f1ce 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -239,11 +239,7 @@ static void chan_dev_release(struct device *dev)
 	kfree(chan_dev);
 }
 
-static struct class dma_devclass = {
-	.name		= "dma",
-	.dev_groups	= dma_dev_groups,
-	.dev_release	= chan_dev_release,
-};
+static struct class_compat *dma_chan_class;
 
 /* --- client and device registration --- */
 
@@ -1115,8 +1111,9 @@ static int __dma_async_device_channel_register(struct dma_device *device,
 		goto err_free_dev;
 	}
 
-	dmaengine_chan_dev(chan)->class = &dma_devclass;
 	dmaengine_chan_dev(chan)->parent = device->dev;
+	dmaengine_chan_dev(chan)->bus = &dma_chan_bus_type;
+	dmaengine_chan_dev(chan)->release = chan_dev_release;
 	chan->chan_dev->chan = chan;
 	chan->chan_dev->dev_id = device->dev_id;
 	spin_lock_init(&chan->lock);
@@ -1128,6 +1125,11 @@ static int __dma_async_device_channel_register(struct dma_device *device,
 	rc = device_register(dmaengine_chan_dev(chan));
 	if (rc)
 		goto err_out_ida;
+
+	rc = class_compat_create_link(dma_chan_class, dmaengine_chan_dev(chan));
+	if (rc)
+		dev_warn(device->dev, "Failed to create compatibility class link\n");
+
 	chan->client_count = 0;
 	device->chancnt++;
 
@@ -1172,6 +1174,7 @@ static void __dma_async_device_channel_unregister(struct dma_device *device,
 	chan->chan_dev->chan = NULL;
 	mutex_unlock(&dma_list_mutex);
 	ida_free(&device->chan_ida, chan->chan_id);
+	class_compat_remove_link(dma_chan_class, dmaengine_chan_dev(chan));
 	device_unregister(dmaengine_chan_dev(chan));
 	free_percpu(chan->local);
 }
@@ -1306,6 +1309,7 @@ int dma_async_device_register(struct dma_device *device)
 		mutex_lock(&dma_list_mutex);
 		chan->chan_dev->chan = NULL;
 		mutex_unlock(&dma_list_mutex);
+		class_compat_remove_link(dma_chan_class, dmaengine_chan_dev(chan));
 		device_unregister(dmaengine_chan_dev(chan));
 		free_percpu(chan->local);
 	}
@@ -1650,8 +1654,9 @@ static int dma_chan_bus_match(struct device *dev,
 }
 
 const struct bus_type dma_chan_bus_type = {
-	.name  = "dma",
-	.match = dma_chan_bus_match,
+	.name		= "dma",
+	.match		= dma_chan_bus_match,
+	.dev_groups	= dma_dev_groups,
 };
 
 static int __init dma_bus_init(void)
@@ -1665,10 +1670,10 @@ static int __init dma_bus_init(void)
 	if (err)
 		return err;
 
-	err = class_register(&dma_devclass);
-	if (err) {
+	dma_chan_class = class_compat_register("dma");
+	if (!dma_chan_class) {
 		bus_unregister(&dma_chan_bus_type);
-		return err;
+		return -ENOMEM;
 	}
 
 	dmaengine_debugfs_init();
diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-dma.c
index e76c58f213d84..71a935d873868 100644
--- a/drivers/dma/tegra186-gpc-dma.c
+++ b/drivers/dma/tegra186-gpc-dma.c
@@ -1541,7 +1541,6 @@ static int tegra_dma_probe(struct platform_device *pdev)
 		tdc = to_tegra_dma_chan(chan);
 
 		if (use_iommu_map) {
-			chdev->bus = pdev->dev.bus;
 			dma_coerce_mask_and_coherent(chdev, DMA_BIT_MASK(cdata->addr_bits));
 
 			ret = of_dma_configure_id(chdev, pdev->dev.of_node,

-- 
2.34.1