linux-kernel - Re: [RFCv3 1/2] device: add dma_params->max_segment

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Date:	Wed, 28 Jan 2015 15:09:30 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	Sumit Semwal <sumit.semwal@...aro.org>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org
Cc:	linaro-kernel@...ts.linaro.org, robdclark@...il.com,
	daniel@...ll.ch, stanislawski.tomasz@...glemail.com,
	robin.murphy@....com
Subject: Re: [RFCv3 1/2] device: add dma_params->max_segment_count

Hello,

On 2015-01-27 09:25, Sumit Semwal wrote:
> From: Rob Clark <robdclark@...il.com>
>
> For devices which have constraints about maximum number of segments in
> an sglist.  For example, a device which could only deal with contiguous
> buffers would set max_segment_count to 1.
>
> The initial motivation is for devices sharing buffers via dma-buf,
> to allow the buffer exporter to know the constraints of other
> devices which have attached to the buffer.  The dma_mask and fields
> in 'struct device_dma_parameters' tell the exporter everything else
> that is needed, except whether the importer has constraints about
> maximum number of segments.
>
> Signed-off-by: Rob Clark <robdclark@...il.com>
>   [sumits: Minor updates wrt comments]
> Signed-off-by: Sumit Semwal <sumit.semwal@...aro.org>

This feature is definitely needed to start thinking of real buffer
sharing between devices.

Acked-by: Marek Szyprowski <m.szyprowski@...sung.com>

> ---
>
> v3: include Robin Murphy's fix[1] for handling '0' as a value for
>       max_segment_count
> v2: minor updates wrt comments on the first version
>
> [1]: http://article.gmane.org/gmane.linux.kernel.iommu/8175/
>
>   include/linux/device.h      |  1 +
>   include/linux/dma-mapping.h | 19 +++++++++++++++++++
>   2 files changed, 20 insertions(+)
>
> diff --git a/include/linux/device.h b/include/linux/device.h
> index fb506738f7b7..a32f9b67315c 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -647,6 +647,7 @@ struct device_dma_parameters {
>   	 * sg limitations.
>   	 */
>   	unsigned int max_segment_size;
> +	unsigned int max_segment_count;    /* INT_MAX for unlimited */
>   	unsigned long segment_boundary_mask;
>   };
>   
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index c3007cb4bfa6..d3351a36d5ec 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -154,6 +154,25 @@ static inline unsigned int dma_set_max_seg_size(struct device *dev,
>   		return -EIO;
>   }
>   
> +#define DMA_SEGMENTS_MAX_SEG_COUNT ((unsigned int) INT_MAX)
> +
> +static inline unsigned int dma_get_max_seg_count(struct device *dev)
> +{
> +	if (dev->dma_parms && dev->dma_parms->max_segment_count)
> +		return dev->dma_parms->max_segment_count;
> +	return DMA_SEGMENTS_MAX_SEG_COUNT;
> +}
> +
> +static inline int dma_set_max_seg_count(struct device *dev,
> +						unsigned int count)
> +{
> +	if (dev->dma_parms) {
> +		dev->dma_parms->max_segment_count = count;
> +		return 0;
> +	}
> +	return -EIO;
> +}
> +
>   static inline unsigned long dma_get_seg_boundary(struct device *dev)
>   {
>   	return dev->dma_parms ?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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