Re: [PATCHv7 9/9] ARM: dma-mapping: add support for IOMMU mapper

[KyongHo Cho <pullip.cho@samsung.com>]

On Thu, Mar 1, 2012 at 12:04 AM, Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
> +/**
> + * arm_iommu_map_sg - map a set of SG buffers for streaming mode DMA
> + * @dev: valid struct device pointer
> + * @sg: list of buffers
> + * @nents: number of buffers to map
> + * @dir: DMA transfer direction
> + *
> + * Map a set of buffers described by scatterlist in streaming mode for DMA.
> + * The scatter gather list elements are merged together (if possible) and
> + * tagged with the appropriate dma address and length. They are obtained via
> + * sg_dma_{address,length}.
> + */
> +int arm_iommu_map_sg(struct device *dev, struct scatterlist *sg, int nents,
> +                    enum dma_data_direction dir, struct dma_attrs *attrs)
> +{
> +       struct scatterlist *s = sg, *dma = sg, *start = sg;
> +       int i, count = 0;
> +       unsigned int offset = s->offset;
> +       unsigned int size = s->offset + s->length;
> +       unsigned int max = dma_get_max_seg_size(dev);
> +
> +       for (i = 1; i < nents; i++) {
> +               s->dma_address = ARM_DMA_ERROR;
> +               s->dma_length = 0;
> +
> +               s = sg_next(s);
> +
> +               if (s->offset || (size & ~PAGE_MASK) || size + s->length > max) {
> +                       if (__map_sg_chunk(dev, start, size, &dma->dma_address,
> +                           dir) < 0)
> +                               goto bad_mapping;
> +
> +                       dma->dma_address += offset;
> +                       dma->dma_length = size - offset;
> +
> +                       size = offset = s->offset;
> +                       start = s;
> +                       dma = sg_next(dma);
> +                       count += 1;
> +               }
> +               size += s->length;
> +       }
> +       if (__map_sg_chunk(dev, start, size, &dma->dma_address, dir) < 0)
> +               goto bad_mapping;
> +
> +       dma->dma_address += offset;
> +       dma->dma_length = size - offset;
> +
> +       return count+1;
> +
> +bad_mapping:
> +       for_each_sg(sg, s, count, i)
> +               __iommu_remove_mapping(dev, sg_dma_address(s), sg_dma_len(s));
> +       return 0;
> +}
> +
This looks that the given sg list specifies the list of physical
memory chunks and
the list of IO virtual memory chunks at the same time after calling
arm_dma_map_sg().
It can happen that dma_address and dma_length of a sg entry does not
correspond to
physical memory information of the sg entry.

I think it is beneficial for handling IO virtual memory.

However, I worry about any other problems caused by a single sg entry contains
information from 2 different context.

Regards,

Cho KyongHo.

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