Subject: [PATCH v2 01/18] arm: dma-mapping: arm_iommu_attach_device: automatically set max_seg_size

From: [email-masked] (Will Deacon)
Date: Wed, 24 Sep 2014 18:06:27 +0100

Hi Marek,

On Tue, Sep 16, 2014 at 12:54:28PM +0100, Marek Szyprowski wrote:
> If device has no max_seg_size set, we assume that there is no limit and
> force it to DMA_BIT_MASK(32) to always use contiguous mappings in DMA
> address space.
> 
> Signed-off-by: Marek Szyprowski <[email-masked]>
> ---
>  arch/arm/mm/dma-mapping.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index bcd5f836f27e..84705e24571b 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -2050,6 +2050,22 @@ int arm_iommu_attach_device(struct device *dev,
>  {
>  	int err;
>  
> +	/*
> +	 * if device has no max_seg_size set, we assume that there is no limit
> +	 * and force it to DMA_BIT_MASK(32) to always use contiguous mappings
> +	 * in DMA address space
> +	 */
> +	if (!dev->dma_parms) {
> +		dev->dma_parms = kzalloc(sizeof(*dev->dma_parms), GFP_KERNEL);
> +		if (!dev->dma_parms)
> +			return -ENOMEM;
> +	}
> +	if (!dev->dma_parms->max_segment_size) {
> +		err = dma_set_max_seg_size(dev, DMA_BIT_MASK(32));

Would it make more sense to base this default value off the dma_mask?
In my IOMMU series, of_dma_configure passes back a size parameter to
arch_setup_dma_ops which is calculated from the dma-ranges property or the
coherent dma mask if the ranges property is absent, so maybe we should set
this there too?

Will