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

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Message-ID: <556E9B27.20006@xs4all.nl>
Date:	Wed, 03 Jun 2015 08:13:59 +0200
From:	Hans Verkuil <hverkuil@...all.nl>
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, stanislawski.tomasz@...glemail.com,
	robdclark@...il.com, daniel@...ll.ch, robin.murphy@....com
Subject: Re: [Linaro-mm-sig] [RFCv3 1/2] device: add dma_params->max_segment_count

Hi Sumit, Rob,

Is there any reason why this patch hasn't been merged yet? It makes perfect
sense to me and I would really like to use this in the media drivers.

Many DMA engines do have a limit to the number of segments (obviously
a max count of 1 being the most common limitation, but other limits are
definitely possible), so this patch seems a no-brainer to me.

So:

Acked-by: Hans Verkuil <hans.verkuil@...co.com>

Regards,

	Hans

On 01/27/2015 09:25 AM, 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>
> ---
> 
> 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 ?
> 

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