Re: [PATCH v3 2/2] dmaengine: fsl-edma: Support dynamic scatter/gather chaining

[Frank Li]

On Mon, May 11, 2026 at 03:57:20PM +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 done when the transaction is submitted by fsl_edma_tx_submit().
> To do so, the .tx_submit() callback is overridden for non-cyclic
> transactions prepared by fsl_edma_prep_peripheral_dma_vec() and
> fsl_edma_prep_slave_sg(). This ensures that transactions are linked
> in the order they are submitted.
>
> 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 | 90 +++++++++++++++++++++++++++++++++++++++----
>  drivers/dma/fsl-edma-trace.h  |  5 +++
>  2 files changed, 88 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/dma/fsl-edma-common.c b/drivers/dma/fsl-edma-common.c
> index c10190164926..b83d1b91dca2 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;

Does fsl_edma_desc_residue() needs to update?

> +		}
>  	} else {
>  		vchan_cyclic_callback(&fsl_chan->edesc->vdesc);
>  	}
> @@ -673,6 +676,68 @@ 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 fsl_edma_hw_tcd *last_tcd;
> +	struct fsl_edma_desc *prev_desc;
> +	struct virt_dma_desc *vdesc;
> +	u16 csr;
> +
> +	lockdep_assert_held(&fsl_chan->vchan.lock);
> +
> +	if (!(flags & FSL_EDMA_DRV_SPLIT_REG))
> +		return;
> +
> +	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;

Suppose you only check submit queue,

issue transfer will move submit queue to issue queue.

Frank
> --
> 2.54.0
>