Re: [PATCH 1/2] dmaengine: add device_link support

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

On Fri, Sep 12, 2025 at 12:00:41AM +0200, Marco Felsch wrote:
> Shift the device dependency handling to the driver core by adding
> support for devlinks.
>
> The link between the consumer device and the dmaengine device is
> established by the consumer via the dma_request_chan() automatically if
> the dmaengine driver supports it (create_devlink flag set).
>
> By adding the devlink support it is ensured that the supplier can't be
> removed while the consumer still uses the dmaengine. Furthermore it
> ensures that the supplier driver is present and actual bound before the
> consumer is uses the supplier.
>
> Additional PM and runtime-PM dependency handling can be added easily too
> by setting the required flags (not implemented by this commit).
>
> The new create_devlink flag controlls the devlink creation to not cause
> any regressions with existing dmaengine drivers. This flag can be
> removed once all drivers are successfully tested to support devlinks.
>
> Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
> ---

Add previous discussion link:
https://lore.kernel.org/all/aLhUv+mtr1uZTCth@lizhi-Precision-Tower-5810/

Another thread
https://lore.kernel.org/dmaengine/20250801120007.GB4906@pendragon.ideasonboard.com/

Add Laurent Pinchart, who may instest this topic also.

Add Rob Herring, who may know why dma engine can't create dev_link default
like other provider (clk, phy, gpio ...)


>  drivers/dma/dmaengine.c   | 15 +++++++++++++++
>  include/linux/dmaengine.h |  3 +++
>  2 files changed, 18 insertions(+)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 758fcd0546d8bde8e8dddc6039848feeb1e24475..e81985ab806ae87ff3aa4739fe6f6328b2587f2e 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -858,6 +858,21 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>  	/* No functional issue if it fails, users are supposed to test before use */
>  #endif
>
> +	/*
> +	 * Devlinks between the dmaengine device and the consumer device
> +	 * are optional till all dmaengine drivers are converted/tested.
> +	 */
> +	if (chan->device->create_devlink) {
> +		struct device_link *dl;
> +
> +		dl = device_link_add(dev, chan->device->dev, DL_FLAG_AUTOREMOVE_CONSUMER);

I suggest link to per channel device, instead dma engine devices.
chan->dev->device like phy drivers because some dma-engine have per channel
resources, like power domain and clocks.

Frank


> +		if (!dl) {
> +			dev_err(dev, "failed to create device link to %s\n",
> +					dev_name(chan->device->dev));
> +			return ERR_PTR(-EINVAL);
> +		}
> +	}
> +
>  	chan->name = kasprintf(GFP_KERNEL, "dma:%s", name);
>  	if (!chan->name)
>  		return chan;
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index bb146c5ac3e4ccd7bc0afbf3b28e5b3d659ad62f..c67737a358df659f2bf050a9ccb8d23b17ceb357 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -817,6 +817,8 @@ struct dma_filter {
>   *	DMA tansaction with no software intervention for reinitialization.
>   *	Zero value means unlimited number of entries.
>   * @descriptor_reuse: a submitted transfer can be resubmitted after completion
> + * @create_devlink: create a devlink between a dma_chan_dev supplier and
> + *	dma-channel consumer device
>   * @residue_granularity: granularity of the transfer residue reported
>   *	by tx_status
>   * @device_alloc_chan_resources: allocate resources and return the
> @@ -894,6 +896,7 @@ struct dma_device {
>  	u32 max_burst;
>  	u32 max_sg_burst;
>  	bool descriptor_reuse;
> +	bool create_devlink;
>  	enum dma_residue_granularity residue_granularity;
>
>  	int (*device_alloc_chan_resources)(struct dma_chan *chan);
>
> --
> 2.47.3
>