[RFC] ARM: dma-mapping: Add DMA attribute to skip iommu mapping

Message ID 1357639944-12050-1-git-send-email-abhinav.k@samsung.com
State New
Headers show

Commit Message

Abhinav Jan. 8, 2013, 10:12 a.m.
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(-)

Comments

Marek Szyprowski Jan. 15, 2013, 3:07 p.m. | #1
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
Arnd Bergmann Jan. 15, 2013, 7:05 p.m. | #2
On Tuesday 15 January 2013, Marek Szyprowski wrote:
> 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.

Makes sense. DRM drivers and KVM are the two cases where you typically
want to use the iommu API rather than the dma-mapping API, because you
need protection between multiple concurrent user contexts.

	Arnd

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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,