linux-kernel - Re: [PATCH] dma: rcar-dmac: use list_add() on rcar_dmac_desc

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Date:	Mon, 2 May 2016 15:07:12 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	sakato <ryusuke.sakato.bx@...esas.com>,
	OSD2 ML <osd2@...renesas.com>, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma: rcar-dmac: use list_add() on rcar_dmac_desc_put()

On Fri, Apr 22, 2016 at 01:50:04AM +0000, Kuninori Morimoto wrote:
> 
> From: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
> 
> Current rcar_dmac_desc_put() is using list_add_tail() in order to
> push used descriptor to list of free descriptors, and next DMA transfer
> try to reuse it from this list. But because it is using *_tail(),
> this reuse effect can't be obtained without using all of them.
> For a longer-term solution, we should allocate hardware descriptors
> using GFP_KERNEL instead of GFP_NOWAIT, but it is difficult today.
> This patch uses list_add() instead of list_add_tail() for short-term
> solution.

So how does reuse case help by not moving the descriptor to tail.

Also you are not reusing descriptor, you are reusing a descriptor memory,
these are two different things.

Lastly how does this help? Something doesn't seem right

> 
> Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
> ---
>  drivers/dma/sh/rcar-dmac.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/sh/rcar-dmac.c b/drivers/dma/sh/rcar-dmac.c
> index 02b86c6..616c63a 100644
> --- a/drivers/dma/sh/rcar-dmac.c
> +++ b/drivers/dma/sh/rcar-dmac.c
> @@ -519,7 +519,7 @@ static void rcar_dmac_desc_put(struct rcar_dmac_chan *chan,
>  
>  	spin_lock_irqsave(&chan->lock, flags);
>  	list_splice_tail_init(&desc->chunks, &chan->desc.chunks_free);
> -	list_add_tail(&desc->node, &chan->desc.free);
> +	list_add(&desc->node, &chan->desc.free);
>  	spin_unlock_irqrestore(&chan->lock, flags);
>  }
>  

-- 
~Vinod