Re: [PATCH v2 2/2] dmaengine: fsl-edma: Support dynamic scatter/gather chaining

[Frank Li]

On Wed, May 06, 2026 at 04:10:36PM +0200, Benoît Monin wrote:
> Implement dynamic linking of scatter/gather transfers to enable
> chaining multiple DMA descriptors without stopping the channel.
> This avoids waiting for the channel to go idle if there is another
> transaction already issued.
>
> Add fsl_edma_link_sg() to dynamically link the last TCD of a previously
> submitted descriptor to the first TCD of a new descriptor by setting
> the scatter/gather address and the E_SG flag, and keeping the channel
> active by clearing the DREQ bit.
>
> Linking is only done if the last TCD was set to disable the DMA channel,
> to prevent corrupting cyclic transaction.
>
> Update fsl_edma_xfer_desc() to avoid re-initializing the hardware when a
> transfer is already in progress, allowing seamless chaining of descriptors.
>
> Modify the transfer completion handler to check the DONE flag in the
> channel CSR before marking the transfer complete. Since this flag is
> only available on SoC with the split registers layout, we only link
> transactions for DMA controllers flagged with FSL_EDMA_DRV_SPLIT_REG.
>
> Add trace event for scatter/gather linking operations.
>
> Signed-off-by: Benoît Monin <benoit.monin@xxxxxxxxxxx>
> ---
>  drivers/dma/fsl-edma-common.c | 64 ++++++++++++++++++++++++++++++++++++++++---
>  drivers/dma/fsl-edma-trace.h  |  5 ++++
>  2 files changed, 65 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma/fsl-edma-common.c b/drivers/dma/fsl-edma-common.c
> index 26a5ecf493b9..7094c747defa 100644
> --- a/drivers/dma/fsl-edma-common.c
> +++ b/drivers/dma/fsl-edma-common.c
> @@ -58,7 +58,10 @@ void fsl_edma_tx_chan_handler(struct fsl_edma_chan *fsl_chan)
>  		list_del(&fsl_chan->edesc->vdesc.node);
>  		vchan_cookie_complete(&fsl_chan->edesc->vdesc);
>  		fsl_chan->edesc = NULL;
> -		fsl_chan->status = DMA_COMPLETE;
> +		if (!(fsl_edma_drvflags(fsl_chan) & FSL_EDMA_DRV_SPLIT_REG) ||
> +		    (edma_readl_chreg(fsl_chan, ch_csr) & EDMA_V3_CH_CSR_DONE)) {
> +			fsl_chan->status = DMA_COMPLETE;
> +		}
>  	} else {
>  		vchan_cyclic_callback(&fsl_chan->edesc->vdesc);
>  	}
> @@ -673,6 +676,51 @@ struct dma_async_tx_descriptor *fsl_edma_prep_dma_cyclic(
>  	return vchan_tx_prep(&fsl_chan->vchan, &fsl_desc->vdesc, flags);
>  }
>
> +static void fsl_edma_link_sg(struct fsl_edma_chan *fsl_chan, struct fsl_edma_desc *fsl_desc)
> +{
> +	u32 flags = fsl_edma_drvflags(fsl_chan);
> +	struct virt_dma_desc *vdesc;
> +	struct fsl_edma_desc *prev_desc;
> +	struct fsl_edma_hw_tcd *last_tcd;

keep revise christmas tree order.

> +	u16 csr;
> +
> +	if (!(flags & FSL_EDMA_DRV_SPLIT_REG))
> +		return;
> +
> +	guard(spinlock_irqsave)(&fsl_chan->vchan.lock);
> +
> +	vdesc = list_last_entry_or_null(&fsl_chan->vchan.desc_issued,
> +					struct virt_dma_desc, node);
> +	if (!vdesc)
> +		vdesc = list_last_entry_or_null(&fsl_chan->vchan.desc_submitted,
> +						struct virt_dma_desc, node);
> +	if (!vdesc)
> +		return;
> +
> +	prev_desc = to_fsl_edma_desc(vdesc);
> +	last_tcd = prev_desc->tcd[prev_desc->n_tcds - 1].vtcd;
> +
> +	csr = fsl_edma_get_tcd_to_cpu(fsl_chan, last_tcd, csr);
> +	if (!(csr & EDMA_TCD_CSR_D_REQ))
> +		return;
> +
> +	fsl_edma_set_tcd_to_le(fsl_chan, last_tcd, fsl_desc->tcd[0].ptcd, dlast_sga);
> +
> +	csr &= ~EDMA_TCD_CSR_D_REQ;
> +	csr |= EDMA_TCD_CSR_E_SG;
> +	fsl_edma_set_tcd_to_le(fsl_chan, last_tcd, csr, csr);
> +
> +	if (prev_desc == fsl_chan->edesc && prev_desc->n_tcds == 1) {
> +		if (flags & FSL_EDMA_DRV_CLEAR_DONE_E_SG)
> +			edma_writel_chreg(fsl_chan, edma_readl_chreg(fsl_chan, ch_csr), ch_csr);
> +
> +		edma_cp_tcd_to_reg(fsl_chan, last_tcd, dlast_sga);
> +		edma_cp_tcd_to_reg(fsl_chan, last_tcd, csr);
> +	}
> +
> +	trace_edma_link_sg(fsl_chan, last_tcd);
> +}
> +
>  struct dma_async_tx_descriptor *fsl_edma_prep_peripheral_dma_vec(
>  		struct dma_chan *chan, const struct dma_vec *vecs,
>  		size_t nb, enum dma_transfer_direction direction,
> @@ -780,6 +828,9 @@ struct dma_async_tx_descriptor *fsl_edma_prep_peripheral_dma_vec(
>  		}
>  	}
>
> +	if (!fsl_desc->iscyclic)
> +		fsl_edma_link_sg(fsl_chan, fsl_desc);
> +
>  	return vchan_tx_prep(&fsl_chan->vchan, &fsl_desc->vdesc, flags);
>  }
>
> @@ -883,6 +934,8 @@ struct dma_async_tx_descriptor *fsl_edma_prep_slave_sg(
>  		}
>  	}
>
> +	fsl_edma_link_sg(fsl_chan, fsl_desc);
> +

I think link sg should be in submit callback. enhence vchan_tx_submit().

Caller is not neccessary call prep then submit(),

tx1 = prep();
tx2 = prep();

submit(tx2);
submit(tx1);

is allowed.

Overall:
	prep() callback link itself for sg
	submit() call back link current to last of submit queue
	issue_transfer() call back, link header or submit queue to issue queue

Frank