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

[Koichiro Den]

On Thu, Aug 13, 2026 at 01:56:31PM -0500, Frank Li wrote:
> 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.

Got it, I'll add something like:

  For example, idma32 uses chan_id to select DMA_CTL_CH() and DMA_XBAR_SEL(),
  and therefore implicitly relies on its fixed ascending registraion order to
  keep chan_id aligned with the hardware channel number.

If you meant something different, please let me know.

Best regards,
Koichiro

> 
> other look good
> 
> Reviewed-by: Frank Li <Frank.Li@xxxxxxx>
> 
> >
> > 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 <Frank.Li@xxxxxxx>
> > Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> > ---
> > 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
> >