linux-kernel - [PATCH v2] dma: rcar-dmac: use list_add() on rcar_dmac_desc

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Date:	Mon, 30 May 2016 00:41:48 +0000
From:	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
To:	Vinod Koul <vinod.koul@...el.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
CC:	<dmaengine@...r.kernel.org>,
	sakato <ryusuke.sakato.bx@...esas.com>,
	<linux-kernel@...r.kernel.org>, OSD2 ML <osd2@...renesas.com>
Subject: [PATCH v2] dma: rcar-dmac: use list_add() on rcar_dmac_desc_put()

From: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>

For each descriptor, in addition to the memory used by the descriptors
structure itself, the driver allocates a list of chunks as well as a
buffer for hardware descriptors. Descriptors themselves are preallocated,
and allocation of the chunks and buffer is performed the first time the
descriptor is used. The memory isn't freed when the transfer is completed,
as the chunks and buffer will be needed again when the descriptor is
reused internally, so the driver keeps the memory around.

If only a few descriptors are used concurrently, the current
list_add_tail() implementation will result in all preallocated descriptors
being used before going back to the first one, and will thus allocate
chunks and a buffer for all preallocated descriptors. Using list_add()
will put the complete descriptor at the head of the list of available
descriptors, so the next transfer will be more likely to reuse a
descriptor that already has associated memory instead of one that has
never been used before.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
---
v1 -> v2

 - used Laurent's explain on log

 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);
 }

-- 
1.9.1