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

Hans Verkuil
2015-06-03T06:14:22Z

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@cisco.com>

Regards,

Hans

On 01/27/2015 09:25 AM, Sumit Semwal wrote:
> From: Rob Clark <robdclark@gmail.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@gmail.com>
> [sumits: Minor updates wrt comments]
> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.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 ?
>