Re: [PATCH 09/10] swiotlb: add support for non-coherent DMA

Robin Murphy
2018-10-22T17:11:10Z

On 08/10/2018 09:02, Christoph Hellwig wrote:
> Handle architectures that are not cache coherent directly in the main
> swiotlb code by calling arch_sync_dma_for_{device,cpu} in all the right
> places from the various dma_map/unmap/sync methods when the device is
> non-coherent.
>
> Because swiotlb now uses dma_direct_alloc for the coherent allocation
> that side is already taken care of by the dma-direct code calling into
> arch_dma_{alloc,free} for devices that are non-coherent.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> kernel/dma/swiotlb.c | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 475a41eff3dc..52885b274368 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -21,6 +21,7 @@
>
> #include <linux/cache.h>
> #include <linux/dma-direct.h>
> +#include <linux/dma-noncoherent.h>
> #include <linux/mm.h>
> #include <linux/export.h>
> #include <linux/spinlock.h>
> @@ -677,6 +678,10 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
> dma_addr = swiotlb_bounce_page(dev, &phys, size, dir, attrs);
> }
>
> + if (!dev_is_dma_coherent(dev) &&
> + (attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)

Nit: other parts of the file are already using the "!(...)" style rather
than "(...) == 0".

> + arch_sync_dma_for_device(dev, phys, size, dir);
> +
> return dma_addr;
> }
>
> @@ -696,6 +701,10 @@ void swiotlb_unmap_page(struct device *hwdev, dma_addr_t dev_addr,
>
> BUG_ON(dir == DMA_NONE);
>
> + if (!dev_is_dma_coherent(hwdev) &&
> + (attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
> + arch_sync_dma_for_cpu(hwdev, paddr, size, dir);
> +
> if (is_swiotlb_buffer(paddr)) {
> swiotlb_tbl_unmap_single(hwdev, paddr, size, dir, attrs);
> return;
> @@ -732,15 +741,17 @@ swiotlb_sync_single(struct device *hwdev, dma_addr_t dev_addr,
>
> BUG_ON(dir == DMA_NONE);
>
> - if (is_swiotlb_buffer(paddr)) {
> + if (!dev_is_dma_coherent(hwdev) && target == SYNC_FOR_CPU)
> + arch_sync_dma_for_cpu(hwdev, paddr, size, dir);
> +
> + if (is_swiotlb_buffer(paddr))
> swiotlb_tbl_sync_single(hwdev, paddr, size, dir, target);
> - return;
> - }
>
> - if (dir != DMA_FROM_DEVICE)
> - return;
> + if (!dev_is_dma_coherent(hwdev) && target == SYNC_FOR_DEVICE)
> + arch_sync_dma_for_device(hwdev, paddr, size, dir);
>
> - dma_mark_clean(phys_to_virt(paddr), size);
> + if (!is_swiotlb_buffer(paddr) && dir == DMA_FROM_DEVICE)
> + dma_mark_clean(phys_to_virt(paddr), size);
> }

All these "if"s end up pretty hard to follow at first glance :(

I had a quick play at moving the cache maintenance here out into the
callers, which comes out arguably looking perhaps a little cleaner (only
+1 source line overall, and actually reduces text size by 32 bytes for
my build), but sadly I can't really see any way of doing the equivalent
for map/unmap short of duplicating the whole 3-line arch_sync_*() block,
which just makes for a different readability problem. As you mentioned
on patch #7, I guess this really is just one of those things which has
no nice solution, so cosmetics aside,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

FWIW, below is my "cleanup" attempt (diff on top of the
swiotlb-noncoherent.3 branch).

Robin.

----->8-----
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 52885b274368..43ee29969fdd 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -741,23 +741,24 @@ swiotlb_sync_single(struct device *hwdev,
dma_addr_t dev_addr,

BUG_ON(dir == DMA_NONE);

- if (!dev_is_dma_coherent(hwdev) && target == SYNC_FOR_CPU)
- arch_sync_dma_for_cpu(hwdev, paddr, size, dir);
-
- if (is_swiotlb_buffer(paddr))
+ if (is_swiotlb_buffer(paddr)) {
swiotlb_tbl_sync_single(hwdev, paddr, size, dir, target);
+ return;
+ }

- if (!dev_is_dma_coherent(hwdev) && target == SYNC_FOR_DEVICE)
- arch_sync_dma_for_device(hwdev, paddr, size, dir);
+ if (dir != DMA_FROM_DEVICE)
+ return;

- if (!is_swiotlb_buffer(paddr) && dir == DMA_FROM_DEVICE)
- dma_mark_clean(phys_to_virt(paddr), size);
+ dma_mark_clean(phys_to_virt(paddr), size);
}

void
swiotlb_sync_single_for_cpu(struct device *hwdev, dma_addr_t dev_addr,
size_t size, enum dma_data_direction dir)
{
+ if (!dev_is_dma_coherent(hwdev))
+ arch_sync_dma_for_cpu(hwdev, dma_to_phys(hwdev, dev_addr),
+ size, dir);
swiotlb_sync_single(hwdev, dev_addr, size, dir, SYNC_FOR_CPU);
}

@@ -766,6 +767,9 @@ swiotlb_sync_single_for_device(struct device *hwdev,
dma_addr_t dev_addr,
size_t size, enum dma_data_direction dir)
{
swiotlb_sync_single(hwdev, dev_addr, size, dir, SYNC_FOR_DEVICE);
+ if (!dev_is_dma_coherent(hwdev))
+ arch_sync_dma_for_device(hwdev, dma_to_phys(hwdev, dev_addr),
+ size, dir);
}

/*
@@ -828,31 +832,28 @@ swiotlb_unmap_sg_attrs(struct device *hwdev,
struct scatterlist *sgl,
* The same as swiotlb_sync_single_* but for a scatter-gather list,
same rules
* and usage.
*/
-static void
-swiotlb_sync_sg(struct device *hwdev, struct scatterlist *sgl,
- int nelems, enum dma_data_direction dir,
- enum dma_sync_target target)
+void
+swiotlb_sync_sg_for_cpu(struct device *hwdev, struct scatterlist *sgl,
+ int nelems, enum dma_data_direction dir)
{
struct scatterlist *sg;
int i;

for_each_sg(sgl, sg, nelems, i)
- swiotlb_sync_single(hwdev, sg->dma_address,
- sg_dma_len(sg), dir, target);
+ swiotlb_sync_single_for_cpu(hwdev, sg->dma_address,
+ sg_dma_len(sg), dir);
}

void
-swiotlb_sync_sg_for_cpu(struct device *hwdev, struct scatterlist *sg,
- int nelems, enum dma_data_direction dir)
-{
- swiotlb_sync_sg(hwdev, sg, nelems, dir, SYNC_FOR_CPU);
-}
-
-void
-swiotlb_sync_sg_for_device(struct device *hwdev, struct scatterlist *sg,
+swiotlb_sync_sg_for_device(struct device *hwdev, struct scatterlist *sgl,
int nelems, enum dma_data_direction dir)
{
- swiotlb_sync_sg(hwdev, sg, nelems, dir, SYNC_FOR_DEVICE);
+ struct scatterlist *sg;
+ int i;
+
+ for_each_sg(sgl, sg, nelems, i)
+ swiotlb_sync_single_for_device(hwdev, sg->dma_address,
+ sg_dma_len(sg), dir);
}

/*