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

[Biju Das]

Hi Claudiu,

> -----Original Message-----
> From: Claudiu <claudiu.beznea@xxxxxxxxx>
> Sent: 11 April 2026 12:43
> Subject: [PATCH v4 07/17] dmaengine: sh: rz-dmac: Save the start LM descriptor
> 
> From: Claudiu Beznea <claudiu.beznea.uj@xxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxx>
> ---
> 
> 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)

U32 normally used with register read/writes hardware related.

Here it is just computation which returns number of bytes. Unsigned int will be
appropriate instead of u32.

Cheers,
Biju

>  {
> -	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