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

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Date:	Fri, 02 Mar 2007 22:14:39 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Chris Leech <christopher.leech@...el.com>
CC:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/9] ioatdma: Push pending transactions to hardware more
 frequently

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?

	Jeff



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