linux-kernel - Re: [PATCH] Allow to use DMA_CTRL_REUSE flag for all channel types

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Date:   Mon, 1 May 2017 11:21:26 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH] Allow to use DMA_CTRL_REUSE flag for all channel types

On Fri, Apr 28, 2017 at 04:37:46PM +0300, Eugeniy Paltsev wrote:
> In the current implementation dma_get_slave_caps is used to check
> state of descriptor_reuse option. But dma_get_slave_caps includes
> check if the channel supports slave transactions.
> So DMA_CTRL_REUSE flag can be set (even for MEM-TO-MEM tranfers)
> only if channel supports slave transactions.
> 
> Now we can use DMA_CTRL_REUSE flag for all channel types.
> Also it allows to test reusing mechanism with simply mem-to-mem dma
> test.

We do not want to allow that actually. Slave is always treated as a special
case, so resue was allowed.

With memcpy the assumptions are different and clients can do reuse.

> 
> Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
> ---
>  include/linux/dmaengine.h | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index 5336808..92cf8b0 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -1376,11 +1376,7 @@ static inline int dma_get_slave_caps(struct dma_chan *chan,
>  
>  static inline int dmaengine_desc_set_reuse(struct dma_async_tx_descriptor *tx)
>  {
> -	struct dma_slave_caps caps;
> -
> -	dma_get_slave_caps(tx->chan, &caps);
> -
> -	if (caps.descriptor_reuse) {
> +	if (tx->chan->device->descriptor_reuse) {
>  		tx->flags |= DMA_CTRL_REUSE;
>  		return 0;
>  	} else {
> -- 
> 2.9.3
> 
> --
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-- 
~Vinod