linux-kernel - Re: [RFC PATCH v4 05/38] dmaengine: dw-edma: Add a helper to query linked-list region

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Message-ID: <aXDvkRCZXQ/dPwRd@lizhi-Precision-Tower-5810>
Date: Wed, 21 Jan 2026 10:24:01 -0500
From: Frank Li <Frank.li@....com>
To: Koichiro Den <den@...inux.co.jp>
Cc: dave.jiang@...el.com, cassel@...nel.org, mani@...nel.org,
	kwilczynski@...nel.org, kishon@...nel.org, bhelgaas@...gle.com,
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	linux-renesas-soc@...r.kernel.org, devicetree@...r.kernel.org,
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Subject: Re: [RFC PATCH v4 05/38] dmaengine: dw-edma: Add a helper to query
 linked-list region

On Wed, Jan 21, 2026 at 05:41:11PM +0900, Koichiro Den wrote:
> On Wed, Jan 21, 2026 at 10:38:53AM +0900, Koichiro Den wrote:
> > On Sun, Jan 18, 2026 at 12:05:47PM -0500, Frank Li wrote:
> > > On Sun, Jan 18, 2026 at 10:54:07PM +0900, Koichiro Den wrote:
> > > > A remote eDMA provider may need to expose the linked-list (LL) memory
> > > > region that was configured by platform glue (typically at boot), so the
> > > > peer (host) can map it and operate the remote view of the controller.
> > > >
> > > > Export dw_edma_chan_get_ll_region() to return the LL region associated
> > > > with a given dma_chan.
> > >
> > > This informaiton passed from dwc epc driver. Is it possible to get it from
> > > EPC driver.
> >
> > That makes sense, from an API cleanness perspective, thanks.
> > I'll add a helper function dw_pcie_edma_get_ll_region() in
> > drivers/pci/controller/dwc/pcie-designware.c, instead of the current
> > dw_edma_chan_get_ll_region() in dw-edma-core.c.
>
> Hi Frank,
>
> I looked into exposing LL regions from the EPC driver side, but the key
> issue is channel identification under possibly concurrent dmaengine users.
> In practice, the only stable handle a consumer has is a pointer to struct
> dma_chan, and the only reliable way to map that to the eDMA hardware
> channel is via dw_edma_chan->id.

If possible, I suggest change to one page pre-channel. So there are a fixed
ll mapping.

> I think an EPC-facing API would still need
> that mapping in any case, so keeping the helper in dw-edma seems simpler
> and more robust.
> If you have another idea, I'd appreciate your insights.

I suggest add generally DMA engine API to get such property, some likes
a kind ioctrl \ dma_get_config().

Frank

>
> Regards,
> Koichiro
>
> >
> > Thanks for the review,
> > Koichiro
> >
> > >
> > > Frank
> > > >
> > > > Signed-off-by: Koichiro Den <den@...inux.co.jp>
> > > > ---
> > > >  drivers/dma/dw-edma/dw-edma-core.c | 26 ++++++++++++++++++++++++++
> > > >  include/linux/dma/edma.h           | 14 ++++++++++++++
> > > >  2 files changed, 40 insertions(+)
> > > >
> > > > diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
> > > > index 0eb8fc1dcc34..c4fb66a9b5f5 100644
> > > > --- a/drivers/dma/dw-edma/dw-edma-core.c
> > > > +++ b/drivers/dma/dw-edma/dw-edma-core.c
> > > > @@ -1209,6 +1209,32 @@ int dw_edma_chan_register_notify(struct dma_chan *dchan,
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(dw_edma_chan_register_notify);
> > > >
> > > > +int dw_edma_chan_get_ll_region(struct dma_chan *dchan,
> > > > +			       struct dw_edma_region *region)
> > > > +{
> > > > +	struct dw_edma_chip *chip;
> > > > +	struct dw_edma_chan *chan;
> > > > +
> > > > +	if (!dchan || !region || !dchan->device)
> > > > +		return -ENODEV;
> > > > +
> > > > +	chan = dchan2dw_edma_chan(dchan);
> > > > +	if (!chan)
> > > > +		return -ENODEV;
> > > > +
> > > > +	chip = chan->dw->chip;
> > > > +	if (!(chip->flags & DW_EDMA_CHIP_LOCAL))
> > > > +		return -EINVAL;
> > > > +
> > > > +	if (chan->dir == EDMA_DIR_WRITE)
> > > > +		*region = chip->ll_region_wr[chan->id];
> > > > +	else
> > > > +		*region = chip->ll_region_rd[chan->id];
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(dw_edma_chan_get_ll_region);
> > > > +
> > > >  MODULE_LICENSE("GPL v2");
> > > >  MODULE_DESCRIPTION("Synopsys DesignWare eDMA controller core driver");
> > > >  MODULE_AUTHOR("Gustavo Pimentel <gustavo.pimentel@...opsys.com>");
> > > > diff --git a/include/linux/dma/edma.h b/include/linux/dma/edma.h
> > > > index 3c538246de07..c9ec426e27ec 100644
> > > > --- a/include/linux/dma/edma.h
> > > > +++ b/include/linux/dma/edma.h
> > > > @@ -153,6 +153,14 @@ bool dw_edma_chan_ignore_irq(struct dma_chan *chan);
> > > >  int dw_edma_chan_register_notify(struct dma_chan *chan,
> > > >  				 void (*cb)(struct dma_chan *chan, void *user),
> > > >  				 void *user);
> > > > +
> > > > +/**
> > > > + * dw_edma_chan_get_ll_region - get linked list (LL) memory for a dma_chan
> > > > + * @chan: the target DMA channel
> > > > + * @region: output parameter returning the corresponding LL region
> > > > + */
> > > > +int dw_edma_chan_get_ll_region(struct dma_chan *chan,
> > > > +			       struct dw_edma_region *region);
> > > >  #else
> > > >  static inline int dw_edma_probe(struct dw_edma_chip *chip)
> > > >  {
> > > > @@ -182,6 +190,12 @@ static inline int dw_edma_chan_register_notify(struct dma_chan *chan,
> > > >  {
> > > >  	return -ENODEV;
> > > >  }
> > > > +
> > > > +static inline int dw_edma_chan_get_ll_region(struct dma_chan *chan,
> > > > +					     struct dw_edma_region *region)
> > > > +{
> > > > +	return -EINVAL;
> > > > +}
> > > >  #endif /* CONFIG_DW_EDMA */
> > > >
> > > >  struct pci_epc;
> > > > --
> > > > 2.51.0
> > > >