[PATCH v5 2/3] arm64: Add IOMMU dma_ops

[Daniel Kurtz]

Hi Robin,

On Sat, Aug 1, 2015 at 1:18 AM, Robin Murphy <robin.murphy at arm.com> wrote:
> Taking some inspiration from the arch/arm code, implement the
> arch-specific side of the DMA mapping ops using the new IOMMU-DMA layer.
>
> Unfortunately the device setup code has to start out as a big ugly mess
> in order to work usefully right now, as 'proper' operation depends on
> changes to device probe and DMA configuration ordering, IOMMU groups for
> platform devices, and default domain support in arm/arm64 IOMMU drivers.
> The workarounds here need only exist until that work is finished.
>
> Signed-off-by: Robin Murphy <robin.murphy at arm.com>
> ---

[snip]

> +static void __iommu_sync_sg_for_cpu(struct device *dev,
> +                                   struct scatterlist *sgl, int nelems,
> +                                   enum dma_data_direction dir)
> +{
> +       struct scatterlist *sg;
> +       int i;
> +
> +       if (is_device_dma_coherent(dev))
> +               return;
> +
> +       for_each_sg(sgl, sg, nelems, i)
> +               __dma_unmap_area(sg_virt(sg), sg->length, dir);
> +}

In an earlier review [0], Marek asked you to change the loop in
__iommu_sync_sg_for_cpu loop() to loop over the virtual areas when
invalidating/cleaning memory ranges.

[0] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/328232.html

However, this changed the meaning of the 'nelems' argument from what
was for arm_iommu_sync_sg_for_cpu() in arch/arm/mm/dma-mapping.c:
 "number of buffers to sync (returned from dma_map_sg)"
to:
 "number of virtual areas to sync (same as was passed to dma_map_sg)"

This has caused some confusion by callers of dma_sync_sg_for_device()
that must work for both arm & arm64 as illustrated by [1].
[1] https://lkml.org/lkml/2015/9/21/250

Based on the implementation of debug_dma_sync_sg_for_cpu() in
lib/dma-debug.c, I think the arm interpretation of nelems (returned
from dma_map_sg) is correct.

Therefore, I think we need an outer iteration over dma chunks, and an
inner iteration that calls __dma_map_area() over the set virtual areas
that correspond to that dma chunk, both here and for
__iommu_sync_sg_for_device().  This will be complicated by the fact
that iommu pages could actually be smaller than PAGE_SIZE, and offset
within a single physical page.  Also, as an optimization, we would
want to combine contiguous virtual areas into a single call to
__dma_unmap_area().

-Dan