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

Frank Li
2026-01-22T01:54:49Z

On Thu, Jan 22, 2026 at 10:19:24AM +0900, Koichiro Den wrote:
> On Wed, Jan 21, 2026 at 10:24:01AM -0500, Frank Li wrote:
> > 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 agree that this would make the LL layout more deterministic and would
> indeed simplify locating the region for a given dw_edma_chan ID. That said,
> my concern was that even with a fixed per-channel layout, we still need a
> reliable way to map a struct dma_chan obtained by a consumer to the
> corresponding dw_edma_chan ID, especially in the presence of potentially
> concurrent dmaengine users.
>
> >
> > > 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().
>
> I think such a helper, combined with your one page per-channel idea, would
> resolve the issue cleanly. For example, a helper like dma_get_hw_info()
> returning struct dma_hw_info, whose first field is a hw_id, could work
> well. Consumers could then use this helper, and if they know they are
> dealing with a dw-edma channel, they can derive the LL location
> straightforwardly as {hw_id * fixed_stride (e.g. PAGE_SIZE)}. Adding hw_id
> to struct dma_slave_caps would make the necessary diff smaller, but I think
> it would not semantically fit in the structure.

It is worth to try.

Frank
>
> Thanks,
> Koichiro
>
> >
> > Frank
> >
> > >
> > > Regards,
> > > Koichiro
> > >
> > > >
> > > > Thanks for the review,
> > > > Koichiro
> > > >
> > > > >
> > > > > Frank
> > > > > >
> > > > > > Signed-off-by: Koichiro Den <den@valinux.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@synopsys.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
> > > > > >