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

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Message-ID: <87wpn1th2q.wl%kuninori.morimoto.gx@renesas.com>
Date:	Wed, 11 May 2016 03:28:04 +0000
From:	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:	Vinod Koul <vinod.koul@...el.com>,
	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()


Hi Laurent, Vinod,

> > > 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
> 
> 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.

Laurent, thank you for your help
Vinod, does above clear for you ?