Hello, On 2016-02-19 11:30, Arnd Bergmann wrote:
On Friday 19 February 2016 09:22:44 Marek Szyprowski wrote:This patch replaces ARM-specific IOMMU-based DMA-mapping implementation with generic IOMMU DMA-mapping code shared with ARM64 architecture. The side-effect of this change is a switch from bitmap-based IO address space management to tree-based code. There should be no functional changes for drivers, which rely on initialization from generic arch_setup_dna_ops() interface. Code, which used old arm_iommu_* functions must be updated to new interface.Signed-off-by: Marek Szyprowski <[email protected]>I like the overall idea. However, this interface from the iommu subsystem into architecture specific code:+/* + * The DMA API is built upon the notion of "buffer ownership". A buffer + * is either exclusively owned by the CPU (and therefore may be accessed + * by it) or exclusively owned by the DMA device. These helper functions + * represent the transitions between these two ownership states. + * + * Note, however, that on later ARMs, this notion does not work due to + * speculative prefetches. We model our approach on the assumption that + * the CPU does do speculative prefetches, which means we clean caches + * before transfers and delay cache invalidation until transfer completion. + * + */ +extern void __dma_page_cpu_to_dev(struct page *, unsigned long, size_t, + enum dma_data_direction); +extern void __dma_page_dev_to_cpu(struct page *, unsigned long, size_t, + enum dma_data_direction); + +static inline void arch_flush_page(struct device *dev, const void *virt, + phys_addr_t phys) +{ + dmac_flush_range(virt, virt + PAGE_SIZE); + outer_flush_range(phys, phys + PAGE_SIZE); +} + +static inline void arch_dma_map_area(phys_addr_t phys, size_t size, + enum dma_data_direction dir) +{ + unsigned int offset = phys & ~PAGE_MASK; + __dma_page_cpu_to_dev(phys_to_page(phys & PAGE_MASK), offset, size, dir); +} + +static inline void arch_dma_unmap_area(phys_addr_t phys, size_t size, + enum dma_data_direction dir) +{ + unsigned int offset = phys & ~PAGE_MASK; + __dma_page_dev_to_cpu(phys_to_page(phys & PAGE_MASK), offset, size, dir); +} + +static inline pgprot_t arch_get_dma_pgprot(struct dma_attrs *attrs, + pgprot_t prot, bool coherent) +{ + if (coherent) + return prot; + + prot = dma_get_attr(DMA_ATTR_WRITE_COMBINE, attrs) ? + pgprot_writecombine(prot) : + pgprot_dmacoherent(prot); + return prot; +} + +extern void *arch_alloc_from_atomic_pool(size_t size, struct page **ret_page, + gfp_t flags); +extern bool arch_in_atomic_pool(void *start, size_t size); +extern int arch_free_from_atomic_pool(void *start, size_t size); + +doesn't feel completely right yet. In particular the arch_flush_page() interface is probably still too specific to ARM/ARM64 and won't work that way on other architectures. I think it would be better to do this either more generic, or less generic: a) leave the iommu_dma_map_ops definition in the architecture specific code, but make it call helper functions in the drivers/iommu to do all of the really generic parts. b) clarify that this is only applicable to arch/arm and arch/arm64, and unify things further between these two, as they have very similar requirements in the CPU architecture.
Some really generic parts are already in iommu/dma-iommu.c and one can build it's own, non-ARM CPU architecture based IOMMU/DMA-mapping code. Initially Ialso wanted to use that generic code on both ARM and ARM64, but it turned out
that both archs, ARM and ARM64 will duplicate 99% of code, which use this
'generic' functions. This was the reason why I dedided to move all that
common code from arch/{arm,arm64}/mm/dma-mapping.c to
drivers/iommu/dma-iommu-ops.c
I'm not sure if I can design all the changes that need to be made to
drivers/iommu/dma-iommu-ops.c to make it more generic. Maybe when one will
try to use that code with other, non-ARM architecture based arch glue code,
a better abstraction can be developed. For now I would like to keep all this
code in a common place so both arm and arm64 will benefit from improvements
done there.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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