netdev - Re: [PATCH 1/9] ioatdma: Push pending transactions to hardware more frequently

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Date:	Fri, 02 Mar 2007 20:27:06 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jeff@...zik.org
Cc:	christopher.leech@...el.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/9] ioatdma: Push pending transactions to hardware
 more frequently

From: Jeff Garzik <jeff@...zik.org>
Date: Fri, 02 Mar 2007 22:14:39 -0500

> Chris Leech wrote:
> > Every 20 descriptors turns out to be to few append commands with
> > newer/faster CPUs.  Pushing every 4 still cuts down on MMIO writes to an
> > acceptable level without letting the DMA engine run out of work.
> > 
> > Signed-off-by: Chris Leech <christopher.leech@...el.com>
> > ---
> > 
> >  drivers/dma/ioatdma.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/dma/ioatdma.c b/drivers/dma/ioatdma.c
> > index 8e87261..0f77a9d 100644
> > --- a/drivers/dma/ioatdma.c
> > +++ b/drivers/dma/ioatdma.c
> > @@ -310,7 +310,7 @@ static dma_cookie_t do_ioat_dma_memcpy(struct ioat_dma_chan *ioat_chan,
> >  	list_splice_init(&new_chain, ioat_chan->used_desc.prev);
> >  
> >  	ioat_chan->pending += desc_count;
> > -	if (ioat_chan->pending >= 20) {
> > +	if (ioat_chan->pending >= 4) {
> 
> This sounds like something that will always be wrong -- or in other 
> words, always be right for only the latest CPUs.  Can this be made 
> dynamic, based on some timing factor?

In fact I think this has been tweaked twice in the vanilla tree
already.
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