[Linaro-mm-sig] [PATCH 1/2] ARM: initial proof-of-concept IOMMU mapper for DMA-mapping

[Krishna Reddy]

> >>It looks that You have simplified arm_iommu_map_sg() function too much.
> >>The main advantage of the iommu is to map scattered memory pages into
> >>contiguous dma address space. DMA-mapping is allowed to merge consecutive
> >>entries in the scatter list if hardware supports that.
> >>http://article.gmane.org/gmane.linux.kernel/1128416
> >I would update arm_iommu_map_sg() back to coalesce the sg list.
> >>MMC drivers seem to be aware of coalescing the SG entries together as they are using
> dma_sg_len().
> 
> I have updated the arm_iommu_map_sg() back to coalesce and fixed the issues with it. During
> testing, I found out that mmc host driver doesn't support buffers bigger than 64K. To get the
> device working, I had to break the sg entries coalesce when dma_length is about to go beyond
> 64KB. Looks like Mmc host driver(sdhci.c) need to be fixed to handle buffers bigger than 64KB.
> Should the clients be forced to handle bigger buffers or is there any better way to handle
> these kind of issues?

>There is struct device_dma_parameters *dma_parms member of struct device. You can specify
>maximum segment size for the dma_map_sg function. This will of course complicate this function
>even more...

dma_get_max_seg_size() seem to take care of this issue already. This returns default max_seg_size as 64K unless device has defined its own size.


Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center