linux-kernel - Re: [PATCH v2 08/14] DMA: edma: Use different eventq for cyclic channels

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Date:	Thu, 10 Apr 2014 11:36:30 -0500
From:	Joel Fernandes <joelf@...com>
To:	Peter Ujfalusi <peter.ujfalusi@...com>, <dan.j.williams@...el.com>,
	<vinod.koul@...el.com>, <nsekhar@...com>
CC:	<dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-omap@...r.kernel.org>,
	<davinci-linux-open-source@...ux.davincidsp.com>,
	<mporter@...aro.org>
Subject: Re: [PATCH v2 08/14] DMA: edma: Use different eventq for cyclic channels

On 04/01/2014 08:06 AM, Peter Ujfalusi wrote:
> To improve latency with cyclic DMA operation it is preferred to
> use different eventq/tc than the default which is used by all
> other drivers (mmc, spi, i2c, etc).
> When preparing the cyclic dma ask for non default queue for the
> channel which is going to be used with cyclic mode.
> 
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
> ---
>  drivers/dma/edma.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
> index 1dd9e8806975..10048b40fac8 100644
> --- a/drivers/dma/edma.c
> +++ b/drivers/dma/edma.c
> @@ -628,6 +628,9 @@ static struct dma_async_tx_descriptor *edma_prep_dma_cyclic(
>  		edesc->pset[i].opt |= TCINTEN;
>  	}
>  
> +	/* Use different eventq/tc for cyclic DMA to reduce latency */
> +	edma_request_non_default_queue(echan->ch_num);
> +
>  	return vchan_tx_prep(&echan->vchan, &edesc->vdesc, tx_flags);
>  }
>  
> 

Is there any way to guarantee that the non-default queue is of the
highest priority, or in other words default queue is of lowest priority.
I know you set queue 1 as default because by default 0 is higher
priority. And then assigning non-default queue.

When assigning default to Queue 1, it would be good to also call
assign_priority_to_queue and set QUEPRI to 7 for Queue 1. Since 0, 2 and
4 are all non-defaults.

Thanks,
-Joel
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