linux-kernel - [PATCH RFC 09/12] dmaengine: ll-dma: support multi-descriptor memcpy for large transfers

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Message-Id: <20260128-dma_ll_comlib-v1-9-1b1fa2c671f9@nxp.com>
Date: Wed, 28 Jan 2026 13:05:28 -0500
From: Frank Li <Frank.Li@....com>
To: Vinod Koul <vkoul@...nel.org>
Cc: linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org, 
 imx@...ts.linux.dev, joy.zou@....com, Frank Li <Frank.Li@....com>
Subject: [PATCH RFC 09/12] dmaengine: ll-dma: support multi-descriptor
 memcpy for large transfers

Each DMA descriptor has a maximum transfer length limitation.
When a memcpy request exceeds this limit, split it into multiple
linked DMA descriptors to complete the transfer.

Signed-off-by: Frank Li <Frank.Li@....com>
---
 drivers/dma/ll-dma.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/drivers/dma/ll-dma.c b/drivers/dma/ll-dma.c
index da13ba4dcdfe403af0ad3678bf4c0ff60f715a63..313ca274df945081fc569ddb6a172298c25bc11c 100644
--- a/drivers/dma/ll-dma.c
+++ b/drivers/dma/ll-dma.c
@@ -17,6 +17,7 @@
  */
 #include <linux/cleanup.h>
 #include <linux/device.h>
+#include <linux/dma-mapping.h>
 #include <linux/dmapool.h>
 #include <linux/dmaengine.h>
 #include <linux/module.h>
@@ -76,6 +77,15 @@ struct dma_ll_desc *vchan_dma_ll_alloc_desc(struct dma_chan *chan, u32 n,
 			goto err;
 
 		memset(desc->its[i].vaddr, 0, vchan->ll.size);
+
+		if (!i)
+			continue;
+
+		if (vchan->ll.ops->set_ll_next(desc, i - 1,
+					       desc->its[i].paddr)) {
+			i++; /* Need free current one */
+			goto err;
+		}
 	}
 
 	return desc;
@@ -107,21 +117,32 @@ vchan_dma_ll_prep_memcpy(struct dma_chan *chan,
 			 dma_addr_t dma_dst, dma_addr_t dma_src, size_t len,
 			 unsigned long flags)
 {
+	unsigned int max_seg = dma_get_max_seg_size(chan->device->dev);
 	struct virt_dma_chan *vchan = to_virt_chan(chan);
 	struct dma_ll_desc *desc;
-	int ret;
+	unsigned int ndesc;
+	int i, ret;
 
-	desc = vchan_dma_ll_alloc_desc(chan, 1, flags);
+	ndesc = (len + max_seg - 1) / max_seg;
+
+	desc = vchan_dma_ll_alloc_desc(chan, ndesc, flags);
 	if (!desc)
 		return NULL;
 
 	desc->iscyclic = false;
 
-	ret = vchan->ll.ops->set_lli(desc, 0, dma_src, dma_dst,
-				     len, true, NULL);
+	for (i = 0; i < ndesc; i++) {
+		ret = vchan->ll.ops->set_lli(desc, i,
+					     dma_src, dma_dst,
+					     min(len, max_seg),
+					     i == ndesc - 1, NULL);
+		if (ret)
+			goto err;
 
-	if (ret)
-		goto err;
+		dma_src += max_seg;
+		dma_dst += max_seg;
+		len -= max_seg;
+	}
 
 	return __vchan_tx_prep(vchan, &desc->vdesc);
 

-- 
2.34.1