Hello,

On 1/8/2013 11:12 AM, Abhinav Kochhar wrote:

Adding a new dma attribute which can be used by the platform drivers to avoid creating iommu mappings. In some cases the buffers are allocated by display controller driver using dma alloc apis but are not used for scanout. Though the buffers are allocated by display controller but are only used for sharing among different devices. With this attribute the platform drivers can choose not to create iommu mapping at the time of buffer allocation and only create the mapping when they access this buffer.

Change-Id: I2178b3756170982d814e085ca62474d07b616a21 Signed-off-by: Abhinav Kochhar abhinav.k@samsung.com

---

arch/arm/mm/dma-mapping.c | 8 +++++--- include/linux/dma-attrs.h | 1 + 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c index c0f0f43..e73003c 100644 --- a/arch/arm/mm/dma-mapping.c +++ b/arch/arm/mm/dma-mapping.c @@ -1279,9 +1279,11 @@ static void *arm_iommu_alloc_attrs(struct device *dev, size_t size, if (!pages) return NULL;

• *handle = __iommu_create_mapping(dev, pages, size);
• if (*handle == DMA_ERROR_CODE)

• goto err_buffer;
  

• if (!dma_get_attr(DMA_ATTR_NO_IOMMU_MAPPING, attrs)) {

• *handle = __iommu_create_mapping(dev, pages, size);
  

• if (*handle == DMA_ERROR_CODE)
  

• 	goto err_buffer;
  

• }

if (dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs)) return pages; diff --git a/include/linux/dma-attrs.h b/include/linux/dma-attrs.h index c8e1831..1f04419 100644 --- a/include/linux/dma-attrs.h +++ b/include/linux/dma-attrs.h @@ -15,6 +15,7 @@ enum dma_attr { DMA_ATTR_WEAK_ORDERING, DMA_ATTR_WRITE_COMBINE, DMA_ATTR_NON_CONSISTENT,

• DMA_ATTR_NO_IOMMU_MAPPING, DMA_ATTR_NO_KERNEL_MAPPING, DMA_ATTR_SKIP_CPU_SYNC, DMA_ATTR_FORCE_CONTIGUOUS,

I'm sorry, but from my perspective this patch and the yet another dma attribute shows that there is something fishy happening in the exynos-drm driver. Creating a mapping in DMA address space is the MAIN purpose of the DMA mapping subsystem, so adding an attribute which skips this operation already should give you a sign of warning that something is not used right.

It looks that dma-mapping in the current state is simply not adequate for this driver. I noticed that DRM drivers are already known for implementing a lots of common code for their own with slightly changed behavior, like custom page manager/allocator. It looks that exynos-drm driver grew to the point where it also needs such features. It already contains custom code for CPU cache handling, IOMMU and contiguous memory special cases management. I would advise to drop DMA-mapping API completely, avoid adding yet another dozen of DMA attributes useful only for one driver and implement your own memory manager with direct usage of IOMMU API, alloc_pages() and dma_alloc_pages_from_contiguous(). This way DMA mapping subsystem can be kept simple, robust and easy to understand without confusing or conflicting parts.

Best regards