[PATCH v4 07/17] dmaengine: sh: rz-dmac: Save the start LM descriptor

Claudiu
2026-04-11T11:43:21Z

Save the start LM descriptor to avoid looping through the entire
channel's LM descriptor list when computing the residue. This avoids
unnecessary iterations.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
---

Changes in v4:
- none

Changes in v3:
- none, this patch is new

drivers/dma/sh/rz-dmac.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/dma/sh/rz-dmac.c b/drivers/dma/sh/rz-dmac.c
index 6bea7c8c7053..0f871c0a28bd 100644
--- a/drivers/dma/sh/rz-dmac.c
+++ b/drivers/dma/sh/rz-dmac.c
@@ -58,6 +58,7 @@ struct rz_dmac_desc {
/* For slave sg */
struct scatterlist *sg;
unsigned int sgcount;
+ struct rz_lmdesc *start_lmdesc;
};

#define to_rz_dmac_desc(d) container_of(d, struct rz_dmac_desc, vd)
@@ -343,6 +344,8 @@ static void rz_dmac_prepare_desc_for_memcpy(struct rz_dmac_chan *channel)
struct rz_dmac_desc *d = channel->desc;
u32 chcfg = CHCFG_MEM_COPY;

+ d->start_lmdesc = lmdesc;
+
/* prepare descriptor */
lmdesc->sa = d->src;
lmdesc->da = d->dest;
@@ -377,6 +380,7 @@ static void rz_dmac_prepare_descs_for_slave_sg(struct rz_dmac_chan *channel)
}

lmdesc = channel->lmdesc.tail;
+ d->start_lmdesc = lmdesc;

for (i = 0, sg = sgl; i < sg_len; i++, sg = sg_next(sg)) {
if (d->direction == DMA_DEV_TO_MEM) {
@@ -693,9 +697,10 @@ rz_dmac_get_next_lmdesc(struct rz_lmdesc *base, struct rz_lmdesc *lmdesc)
return next;
}

-static u32 rz_dmac_calculate_residue_bytes_in_vd(struct rz_dmac_chan *channel, u32 crla)
+static u32 rz_dmac_calculate_residue_bytes_in_vd(struct rz_dmac_chan *channel,
+ struct rz_dmac_desc *desc, u32 crla)
{
- struct rz_lmdesc *lmdesc = channel->lmdesc.head;
+ struct rz_lmdesc *lmdesc = desc->start_lmdesc;
struct dma_chan *chan = &channel->vc.chan;
struct rz_dmac *dmac = to_rz_dmac(chan->device);
u32 residue = 0, i = 0;
@@ -794,7 +799,7 @@ static u32 rz_dmac_chan_get_residue(struct rz_dmac_chan *channel,
* Calculate number of bytes transferred in processing virtual descriptor.
* One virtual descriptor can have many lmdesc.
*/
- return crtb + rz_dmac_calculate_residue_bytes_in_vd(channel, crla);
+ return crtb + rz_dmac_calculate_residue_bytes_in_vd(channel, current_desc, crla);
}

static enum dma_status rz_dmac_tx_status(struct dma_chan *chan,
--
2.43.0