Re: [PATCH v6 4/9] dmaengine: move channel devices to the DMA channel bus

[Frank Li <Frank.li@oss.nxp.com>]

On Thu, Nov 12, 2026 at 03:53:07PM +0800, Peng Fan (OSS) wrote:
> From: Peng Fan <peng.fan@nxp.com>
>
> Register each dma_chan_dev on dmaengine_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 bus is named "dmaengine-chan" rather than "dma" to avoid colliding
> with the SuperH DMA bus, which is also named "dma" and is registered later
> from a postcore_initcall. As a result the device's uevent SUBSYSTEM and
> "subsystem" link use "dmaengine-chan", while /sys/class/dma is unchanged.
>
> 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>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  drivers/dma/dmaengine.c        | 23 +++++++++++++++--------
>  drivers/dma/tegra186-gpc-dma.c |  1 -
>  2 files changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index e1022d297674..45e328d28689 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 = &dmaengine_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);
>  	}
> @@ -1652,6 +1656,7 @@ static int dma_chan_bus_match(struct device *dev,
>  const struct bus_type dmaengine_chan_bus_type = {
>  	.name  = "dmaengine-chan",
>  	.match = dma_chan_bus_match,
> +	.dev_groups = dma_dev_groups,
>  };
>
>  static int __init dma_bus_init(void)
> @@ -1665,9 +1670,11 @@ static int __init dma_bus_init(void)
>  	if (err)
>  		goto err_destroy_unmap_pool;
>
> -	err = class_register(&dma_devclass);
> -	if (err)
> +	dma_chan_class = class_compat_register("dma");
> +	if (!dma_chan_class) {
> +		err = -ENOMEM;
>  		goto err_unregister_bus;
> +	}
>
>  	dmaengine_debugfs_init();
>  	return 0;
> diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-dma.c
> index ec12b7288b43..93b136f875d4 100644
> --- a/drivers/dma/tegra186-gpc-dma.c
> +++ b/drivers/dma/tegra186-gpc-dma.c
> @@ -1540,7 +1540,6 @@ 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
>