Hi Robin,
On Fri, Jan 27, 2017 at 6:50 PM, Robin Murphy <robin.mur...@arm.com> wrote:
> On 27/01/17 15:34, Geert Uytterhoeven wrote:
>> Add helpers for allocating physically contiguous DMA buffers to the
>> generic IOMMU DMA code. This can be useful when two or more devices
>> with different memory requirements are involved in buffer sharing.
>>
>> The iommu_dma_{alloc,free}_contiguous() functions complement the existing
>> iommu_dma_{alloc,free}() functions, and allow architecture-specific code
>> to implement support for the DMA_ATTR_FORCE_CONTIGUOUS attribute on
>> systems with an IOMMU. As this uses the CMA allocator, setting this
>> attribute has a runtime-dependency on CONFIG_DMA_CMA.
>>
>> Note that unlike the existing iommu_dma_alloc() helper,
>> iommu_dma_alloc_contiguous() has no callback to flush pages.
>> Ensuring the returned region is made visible to a non-coherent device is
>> the responsibility of the caller.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+rene...@glider.be>
>> ---
>> v2:
>> - Provide standalone iommu_dma_{alloc,free}_contiguous() functions, as
>> requested by Robin Murphy,
>> - Simplify operations by getting rid of the page array/scatterlist
>> dance, as the buffer is contiguous,
>> - Move CPU cache magement into the caller, which is much simpler with
>> a single contiguous buffer.
>
> Thanks for the rework, that's a lot easier to make sense of! Now, please
> don't hate me, but...
I don't hate you at all. I'm just a drivers/iommu rookie ;-)
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> +/**
>> + * iommu_dma_alloc_contiguous - Allocate and map a buffer contiguous in IOVA
>> + * and physical space
>> + * @dev: Device to allocate memory for. Must be a real device attached to an
>> + * iommu_dma_domain
>> + * @size: Size of buffer in bytes
>> + * @prot: IOMMU mapping flags
>> + * @handle: Out argument for allocated DMA handle
>> + *
>> + * Return: Buffer page pointer, or NULL on failure.
>> + *
>> + * Note that unlike iommu_dma_alloc(), it's the caller's responsibility to
>> + * ensure the returned region is made visible to the given non-coherent
>> device.
>> + */
>> +struct page *iommu_dma_alloc_contiguous(struct device *dev, size_t size,
>> + int prot, dma_addr_t *handle)
>> +{
>> + struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
>> + struct iova_domain *iovad = cookie_iovad(domain);
>> + dma_addr_t dma_addr;
>> + unsigned int count;
>> + struct page *page;
>> + struct iova *iova;
>> + int ret;
>> +
>> + *handle = DMA_ERROR_CODE;
>> +
>> + size = PAGE_ALIGN(size);
>> + count = size >> PAGE_SHIFT;
>> + page = dma_alloc_from_contiguous(dev, count, get_order(size));
>> + if (!page)
>> + return NULL;
>> +
>> + iova = __alloc_iova(domain, size, dev->coherent_dma_mask);
>> + if (!iova)
>> + goto out_free_pages;
>> +
>> + size = iova_align(iovad, size);
>> + dma_addr = iova_dma_addr(iovad, iova);
>> + ret = iommu_map(domain, dma_addr, page_to_phys(page), size, prot);
>> + if (ret < 0)
>> + goto out_free_iova;
>> +
>> + *handle = dma_addr;
>> + return page;
>> +
>> +out_free_iova:
>> + __free_iova(iovad, iova);
>> +out_free_pages:
>> + dma_release_from_contiguous(dev, page, count);
>> + return NULL;
>> +}
>
> ...now that I can see it clearly, isn't this more or less just:
>
> page = dma_alloc_from_contiguous(dev, ...);
> if (page)
> dma_addr = iommu_dma_map_page(dev, page, ...);
> ?
>
> Would it not be even simpler to just make those two calls directly from
> the arm64 code?
You're right. Will update. Thanks a lot!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
