Re: [PATCH 04/10] swiotlb: remove the overflow buffer

Robin Murphy
2018-10-11T18:19:40Z

On 08/10/18 09:02, Christoph Hellwig wrote:
> Like all other dma mapping drivers just return an error code instead
> of an actual memory buffer. The reason for the overflow buffer was
> that at the time swiotlb was invented there was no way to check for
> dma mapping errors, but this has long been fixed.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> arch/arm64/mm/dma-mapping.c | 2 +-
> arch/powerpc/kernel/dma-swiotlb.c | 4 +--
> include/linux/dma-direct.h | 2 ++
> include/linux/swiotlb.h | 3 --
> kernel/dma/direct.c | 2 --
> kernel/dma/swiotlb.c | 59 ++-----------------------------
> 6 files changed, 8 insertions(+), 64 deletions(-)
>
> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
> index 072c51fb07d7..8d91b927e09e 100644
> --- a/arch/arm64/mm/dma-mapping.c
> +++ b/arch/arm64/mm/dma-mapping.c
> @@ -324,7 +324,7 @@ static int __swiotlb_dma_supported(struct device *hwdev, u64 mask)
> static int __swiotlb_dma_mapping_error(struct device *hwdev, dma_addr_t addr)
> {
> if (swiotlb)

From a bit of archaeology, I think io_tlb_overflow_buffer being
uninitialised was the reason this needed fixing in adbe7e26f425. Given
that, you could as well just nuke this whole helper and install
dma_direct_mapping_error in the ops like the other users, although I
appreciate it's only really a bisection thing so may not matter much.

With the caveat that it's late and I may technically have missed
something, but have at least found the branch to boot-test OK:

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

> - return swiotlb_dma_mapping_error(hwdev, addr);
> + return dma_direct_mapping_error(hwdev, addr);
> return 0;
> }
>
> diff --git a/arch/powerpc/kernel/dma-swiotlb.c b/arch/powerpc/kernel/dma-swiotlb.c
> index 88f3963ca30f..5fc335f4d9cd 100644
> --- a/arch/powerpc/kernel/dma-swiotlb.c
> +++ b/arch/powerpc/kernel/dma-swiotlb.c
> @@ -11,7 +11,7 @@
> *
> */
>
> -#include <linux/dma-mapping.h>
> +#include <linux/dma-direct.h>
> #include <linux/memblock.h>
> #include <linux/pfn.h>
> #include <linux/of_platform.h>
> @@ -59,7 +59,7 @@ const struct dma_map_ops powerpc_swiotlb_dma_ops = {
> .sync_single_for_device = swiotlb_sync_single_for_device,
> .sync_sg_for_cpu = swiotlb_sync_sg_for_cpu,
> .sync_sg_for_device = swiotlb_sync_sg_for_device,
> - .mapping_error = swiotlb_dma_mapping_error,
> + .mapping_error = dma_direct_mapping_error,
> .get_required_mask = swiotlb_powerpc_get_required,
> };
>
> diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
> index fbca184ff5a0..bd73e7a91410 100644
> --- a/include/linux/dma-direct.h
> +++ b/include/linux/dma-direct.h
> @@ -5,6 +5,8 @@
> #include <linux/dma-mapping.h>
> #include <linux/mem_encrypt.h>
>
> +#define DIRECT_MAPPING_ERROR 0
> +
> #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
> #include <asm/dma-direct.h>
> #else
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 7ef541ce8f34..f847c1b265c4 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -106,9 +106,6 @@ extern void
> swiotlb_sync_sg_for_device(struct device *hwdev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir);
>
> -extern int
> -swiotlb_dma_mapping_error(struct device *hwdev, dma_addr_t dma_addr);
> -
> extern int
> swiotlb_dma_supported(struct device *hwdev, u64 mask);
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 674a8da22844..12798abba55e 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -14,8 +14,6 @@
> #include <linux/pfn.h>
> #include <linux/set_memory.h>
>
> -#define DIRECT_MAPPING_ERROR 0
> -
> /*
> * Most architectures use ZONE_DMA for the first 16 Megabytes, but
> * some use it for entirely different regions:
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 69bf305ee5f8..11dbcd80b4a6 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -72,13 +72,6 @@ static phys_addr_t io_tlb_start, io_tlb_end;
> */
> static unsigned long io_tlb_nslabs;
>
> -/*
> - * When the IOMMU overflows we return a fallback buffer. This sets the size.
> - */
> -static unsigned long io_tlb_overflow = 32*1024;
> -
> -static phys_addr_t io_tlb_overflow_buffer;
> -
> /*
> * This is a free list describing the number of free entries available from
> * each index
> @@ -126,7 +119,6 @@ setup_io_tlb_npages(char *str)
> return 0;
> }
> early_param("swiotlb", setup_io_tlb_npages);
> -/* make io_tlb_overflow tunable too? */
>
> unsigned long swiotlb_nr_tbl(void)
> {
> @@ -194,16 +186,10 @@ void __init swiotlb_update_mem_attributes(void)
> bytes = PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT);
> set_memory_decrypted((unsigned long)vaddr, bytes >> PAGE_SHIFT);
> memset(vaddr, 0, bytes);
> -
> - vaddr = phys_to_virt(io_tlb_overflow_buffer);
> - bytes = PAGE_ALIGN(io_tlb_overflow);
> - set_memory_decrypted((unsigned long)vaddr, bytes >> PAGE_SHIFT);
> - memset(vaddr, 0, bytes);
> }
>
> int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
> {
> - void *v_overflow_buffer;
> unsigned long i, bytes;
>
> bytes = nslabs << IO_TLB_SHIFT;
> @@ -212,17 +198,6 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
> io_tlb_start = __pa(tlb);
> io_tlb_end = io_tlb_start + bytes;
>
> - /*
> - * Get the overflow emergency buffer
> - */
> - v_overflow_buffer = memblock_virt_alloc_low_nopanic(
> - PAGE_ALIGN(io_tlb_overflow),
> - PAGE_SIZE);
> - if (!v_overflow_buffer)
> - return -ENOMEM;
> -
> - io_tlb_overflow_buffer = __pa(v_overflow_buffer);
> -
> /*
> * Allocate and initialize the free list array. This array is used
> * to find contiguous free memory regions of size up to IO_TLB_SEGSIZE
> @@ -330,7 +305,6 @@ int
> swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs)
> {
> unsigned long i, bytes;
> - unsigned char *v_overflow_buffer;
>
> bytes = nslabs << IO_TLB_SHIFT;
>
> @@ -341,19 +315,6 @@ swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs)
> set_memory_decrypted((unsigned long)tlb, bytes >> PAGE_SHIFT);
> memset(tlb, 0, bytes);
>
> - /*
> - * Get the overflow emergency buffer
> - */
> - v_overflow_buffer = (void *)__get_free_pages(GFP_DMA,
> - get_order(io_tlb_overflow));
> - if (!v_overflow_buffer)
> - goto cleanup2;
> -
> - set_memory_decrypted((unsigned long)v_overflow_buffer,
> - io_tlb_overflow >> PAGE_SHIFT);
> - memset(v_overflow_buffer, 0, io_tlb_overflow);
> - io_tlb_overflow_buffer = virt_to_phys(v_overflow_buffer);
> -
> /*
> * Allocate and initialize the free list array. This array is used
> * to find contiguous free memory regions of size up to IO_TLB_SEGSIZE
> @@ -390,10 +351,6 @@ swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs)
> sizeof(int)));
> io_tlb_list = NULL;
> cleanup3:
> - free_pages((unsigned long)v_overflow_buffer,
> - get_order(io_tlb_overflow));
> - io_tlb_overflow_buffer = 0;
> -cleanup2:
> io_tlb_end = 0;
> io_tlb_start = 0;
> io_tlb_nslabs = 0;
> @@ -407,8 +364,6 @@ void __init swiotlb_exit(void)
> return;
>
> if (late_alloc) {
> - free_pages((unsigned long)phys_to_virt(io_tlb_overflow_buffer),
> - get_order(io_tlb_overflow));
> free_pages((unsigned long)io_tlb_orig_addr,
> get_order(io_tlb_nslabs * sizeof(phys_addr_t)));
> free_pages((unsigned long)io_tlb_list, get_order(io_tlb_nslabs *
> @@ -416,8 +371,6 @@ void __init swiotlb_exit(void)
> free_pages((unsigned long)phys_to_virt(io_tlb_start),
> get_order(io_tlb_nslabs << IO_TLB_SHIFT));
> } else {
> - memblock_free_late(io_tlb_overflow_buffer,
> - PAGE_ALIGN(io_tlb_overflow));
> memblock_free_late(__pa(io_tlb_orig_addr),
> PAGE_ALIGN(io_tlb_nslabs * sizeof(phys_addr_t)));
> memblock_free_late(__pa(io_tlb_list),
> @@ -790,7 +743,7 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
> /* Oh well, have to allocate and map a bounce buffer. */
> map = map_single(dev, phys, size, dir, attrs);
> if (map == SWIOTLB_MAP_ERROR)
> - return __phys_to_dma(dev, io_tlb_overflow_buffer);
> + return DIRECT_MAPPING_ERROR;
>
> dev_addr = __phys_to_dma(dev, map);
>
> @@ -801,7 +754,7 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
> attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> swiotlb_tbl_unmap_single(dev, map, size, dir, attrs);
>
> - return __phys_to_dma(dev, io_tlb_overflow_buffer);
> + return DIRECT_MAPPING_ERROR;
> }
>
> /*
> @@ -985,12 +938,6 @@ swiotlb_sync_sg_for_device(struct device *hwdev, struct scatterlist *sg,
> swiotlb_sync_sg(hwdev, sg, nelems, dir, SYNC_FOR_DEVICE);
> }
>
> -int
> -swiotlb_dma_mapping_error(struct device *hwdev, dma_addr_t dma_addr)
> -{
> - return (dma_addr == __phys_to_dma(hwdev, io_tlb_overflow_buffer));
> -}
> -
> /*
> * Return whether the given device DMA address mask can be supported
> * properly. For example, if your device can only drive the low 24-bits
> @@ -1033,7 +980,7 @@ void swiotlb_free(struct device *dev, size_t size, void *vaddr,
> }
>
> const struct dma_map_ops swiotlb_dma_ops = {
> - .mapping_error = swiotlb_dma_mapping_error,
> + .mapping_error = dma_direct_mapping_error,
> .alloc = swiotlb_alloc,
> .free = swiotlb_free,
> .sync_single_for_cpu = swiotlb_sync_single_for_cpu,
>