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

[Will Deacon <will.deacon@arm.com>]

On Thu, Sep 25, 2014 at 11:43:35AM +0100, Marek Szyprowski wrote:
> On 2014-09-24 19:06, Will Deacon wrote:
> > 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 <m.szyprowski@samsung.com>
> >> ---
> >>   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?
> 
> Right, good idea. This patch predates your arch_setup_dma_ops changes, so I
> had to use something. The value taken from dma_mask is much better than 
> hardcoded
> DMA_BIT_MASK(32). Do you want to include an updated patch in next 
> version of your
> arch_setup_dma_ops patchset?

Sure, I'll have a go at rolling that in.

Cheers,

Will