linux-kernel - Re: [PATCH 2/2] dma: pl330: add DMA

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Date:	Tue, 25 Nov 2014 14:41:40 +0100
From:	Krzysztof Kozłowski <k.kozlowski@...sung.com>
To:	Robert Baldyga <r.baldyga@...sung.com>, vinod.koul@...el.com
Cc:	dan.j.williams@...el.com, ars@...afoo.de,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	m.szyprowski@...sung.com, l.czerwinski@...sung.com
Subject: Re: [PATCH 2/2] dma: pl330: add DMA_PAUSE feature

On 25.11.2014 13:25, Robert Baldyga wrote:
> DMA_PAUSE command is used for halting DMA transfer on chosen channel.
> It can be useful when we want to safely read residue before terminating
> all requests on channel. Otherwise there can be situation when some data
> is transferred before channel termination but after reading residue,
> which obviously results with data loss. To avoid this situation we can
> pause channel, read residue and then terminate all requests.
> This scenario is common, for example, in serial port drivers.
> 
> Signed-off-by: Robert Baldyga <r.baldyga@...sung.com>
> ---
>  drivers/dma/pl330.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> index c32806d..9c64421 100644
> --- a/drivers/dma/pl330.c
> +++ b/drivers/dma/pl330.c
> @@ -2115,6 +2115,26 @@ static int pl330_control(struct dma_chan *chan, enum dma_ctrl_cmd cmd, unsigned
>  		list_splice_tail_init(&pch->completed_list, &pl330->desc_pool);
>  		spin_unlock_irqrestore(&pch->lock, flags);
>  		break;
> +	case DMA_PAUSE:

pm_runtime_get_sync() here? If transfer is ongoing this shouldn't be
needed but if this is called after all transfers complete then device
will be suspended.

> +		/*
> +		 * We don't support DMA_RESUME command because of hardware
> +		 * limitations, so after pausing the channel we cannot restore
> +		 * it to active state. We have to terminate channel and setup
> +		 * DMA transfer again. This pause feature was implemented to
> +		 * allow safely read residue before channel termination.
> +		 */
> +		spin_lock_irqsave(&pch->lock, flags);
> +
> +		spin_lock(&pl330->lock);
> +		_stop(pch->thread);
> +		spin_unlock(&pl330->lock);
> +
> +		pch->thread->req[0].desc = NULL;
> +		pch->thread->req[1].desc = NULL;
> +		pch->thread->req_running = -1;
> +
> +		spin_unlock_irqrestore(&pch->lock, flags);

symmetric pm_runtime_put + autosuspend.

Best regards,
Krzysztof


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