Re: [PATCH v7 01/10] dmaengine: Allow drivers to assign static channel IDs

On Thu, Aug 13, 2026 at 03:37:48PM +0900, Koichiro Den wrote:
> The dmaengine core assigns channel IDs in registration order. If a driver
> skips a hardware channel, chan_id can differ from the hardware numbering
> and a client cannot reliably correlate a requested channel with hardware
> resources.

Provide an example, show DMA enginee use chan_id to locate DMA Channel
hardware resource.

other look good

Reviewed-by: Frank Li <[email protected]>

>
> Let a driver request an exact channel ID before device registration.
> Reserve static IDs through the existing IDA so they remain unique, while
> retaining automatic IDA allocation as the default.
>
> Use direction-flattened IDs for dw-edma channels. Unlike the
> direction-local hardware channel number, these IDs are unique within the
> DMA device.
>
> Suggested-by: Frank Li <[email protected]>
> Signed-off-by: Koichiro Den <[email protected]>
> ---
> Changes in v7:
>   - New patch. (Frank)
>     https://lore.kernel.org/r/lm7tadnxsyrypu4mypptlkx5qkytex4qxsijdr5ydud2n3anvf@yakjfjg5ng4u/
>
>  drivers/dma/dmaengine.c            | 13 ++++++++-----
>  drivers/dma/dw-edma/dw-edma-core.c |  1 +
>  include/linux/dmaengine.h          | 20 ++++++++++++++++++++
>  3 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 6ffd8bd82154..cc64a4679e6f 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -1078,6 +1078,7 @@ static int __dma_async_device_channel_register(struct dma_device *device,
>  					       struct dma_chan *chan,
>  					       const char *name)
>  {
> +	unsigned int id;
>  	int rc;
>
>  	chan->local = alloc_percpu(typeof(*chan->local));
> @@ -1089,11 +1090,13 @@ static int __dma_async_device_channel_register(struct dma_device *device,
>  		goto err_free_local;
>  	}
>
> -	/*
> -	 * When the chan_id is a negative value, we are dynamically adding
> -	 * the channel. Otherwise we are static enumerating.
> -	 */
> -	chan->chan_id = ida_alloc(&device->chan_ida, GFP_KERNEL);
> +	if (chan->chan_id & DMA_CHAN_ID_STATIC) {
> +		id = chan->chan_id & ~DMA_CHAN_ID_STATIC;
> +		chan->chan_id = ida_alloc_range(&device->chan_ida, id, id,
> +						GFP_KERNEL);
> +	} else {
> +		chan->chan_id = ida_alloc(&device->chan_ida, GFP_KERNEL);
> +	}
>  	if (chan->chan_id < 0) {
>  		pr_err("%s: unable to alloc ida for chan: %d\n",
>  		       __func__, chan->chan_id);
> diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
> index 1f893dc54c79..d214df55da3c 100644
> --- a/drivers/dma/dw-edma/dw-edma-core.c
> +++ b/drivers/dma/dw-edma/dw-edma-core.c
> @@ -987,6 +987,7 @@ static int dw_edma_channel_setup(struct dw_edma *dw, u32 wr_alloc, u32 rd_alloc)
>  					&dw->chip->dt_region_rd[chan->id];
>
>  		vchan_init(&chan->vc, dma);
> +		dmaengine_set_static_chan_id(&chan->vc.chan, i);
>
>  		dw_edma_core_ch_config(chan);
>  	}
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index fe33a20abc61..f669b79d7731 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -369,6 +369,26 @@ struct dma_chan {
>  	void *private;
>  };
>
> +#define DMA_CHAN_ID_STATIC	BIT(30)
> +
> +/**
> + * dmaengine_set_static_chan_id - request an exact DMA engine channel ID
> + * @chan: DMA channel
> + * @id: channel ID, unique within the DMA device
> + *
> + * Drivers may call this after initializing @chan and before registering its
> + * DMA device. The dmaengine core reserves @id from the device IDA instead of
> + * assigning the next available ID.
> + */
> +static inline void dmaengine_set_static_chan_id(struct dma_chan *chan,
> +						unsigned int id)
> +{
> +	if (WARN_ON_ONCE(id >= DMA_CHAN_ID_STATIC))
> +		return;
> +
> +	chan->chan_id = DMA_CHAN_ID_STATIC | id;
> +}
> +
>  /**
>   * struct dma_chan_dev - relate sysfs device node to backing channel device
>   * @chan: driver channel device
> --
> 2.51.0
>