Ramon Fried covers the following topics: * What DMA is. * DMA Buffer Allocations and Management. * Cache Coherency. * PCI and DMA. * dmaengine Framework. Ramon is an Embedded Linux team leader in TandemG, leading various cutting edge projects in the Linux kernel. He has years of experience in embedded systems, operating systems and Linux kernel.

1. Whenwe’redone, it’sdone!
DMA Survival Guide

2. Property of Tandem Group
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Ramon Fried
 Developer and hacker since 1983
 B.sc in computer science
 Expertise in Embedded systems and Linux
system and kernel development.
 Currently Embedded Linux Team Leader in
TandemG

3. Property of Tandem Group
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TandemG
 TandemG is Israel’s leading Software, Hardware and Systems R&D
center, acting as a one-stop-shop for our range of partners, from
prominent start-ups through to market leaders
 In the embedded domain, TandemG tailors solutions spanning
across RTOS, Embedded Linux, low-level Android and DSP.
 For the second year in a row, TandemG has been selected to
Delloite’s Israel Technology Fast50 list in the 23rd place
 Visit us at www.tandemG.com, for additional details.

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Agenda
 What is DMA
 DMA Buffer allocation
— Coherent
— Streaming
 Scatter Gather mapping
 DMA pools
 DMA Triggering
— PCI
— dmaengine

5. Property of Tandem Group
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What is DMA
 Direct memory access
 Feature of computer systems that allows
certain hardware subsystems to access main
system memory (RAM), independent of the
central processing unit (CPU).
 CPU can be notified on the end of operation by
IRQ.

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What is DMA
Photo from: http://encyclopedia2.thefreedictionary.com/DMA

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DMA Buffer allocation
 DMA controller works on physical addresses
 Physical memory needs to be accessible by
DMA controller
 Memory must be continuous

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DMA Buffer allocation
 Coherent DMA mapping
— Usually long lasting.
— Can be accessed by both ends.
— No-caching *
— At least page sized.
 Streaming DMA mapping
— Usually singly used and freed.
— Architecture/Platform optimized.
— direction must be defined explicitly.

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DMA access mask
#include <linux/dma-mapping.h>
int dma_set_mask_and_coherent(struct device
*dev, u64 mask);
int dma_set_mask (struct device *dev, u64
mask);
int dma_set_coherent_mask(struct device *dev,
u64 mask);

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Coherent DMA mapping
#include <linux/dma-mapping.h>
void *dma_alloc_coherent(struct device *dev,
size_t size, dma_addr_t
*dma_handle, gfp_t flag);
void dma_free_coherent(struct device *dev,
size_t size, void *cpu_addr,
dma_addr_t dma_handle);

11. Property of Tandem Group
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Streaming DMA mapping
#include <linux/dma-mapping.h>
dma_addr_t dma_map_single(struct device
*dev, void *ptr,size_t size,
enum dma_data_direction dir);
void dma_unmap_single(struct device
*dev, dma_addr_t addr,
size_t size,
enum dma_data_direction dir);

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Streaming DMA mapping (cont’d)
 enum dma_data_direction
— DMA_TO_DEVICE
— DMA_FROM_DEVICE
— DMA_BIDIRECTIONAL

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Streaming DMA usage
 Buffer Ownership
— Buffer is owned by the device.
— Altering the buffer can be done only after acquiring
ownership
 dma_sync_single_for_cpu()
— After altering the buffer, the ownership needs to be
returned to the device.
 dma_sync_single_for_device()

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Scatter gather buffers
 Special type of streaming DMA
 writev, readv, clustered buffers (YUV plannar,
non continuous memory, etc.)

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Scatter/Gather API
#include linux/dma-mapping.h
int dma_map_sg( struct device *dev,
struct scatterlist *sg,
int nents,
enum dma_data_direction dir);
void dma_unmap_sg(struct device *dev,
struct scatterlist *list,
int nents,
enum dma_data_direction dir);

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Don’t forget to sync
#include linux/dma-mapping.h
void dma_sync_sg_for_cpu(struct device *dev,
struct scatterlist *sg,
int nelems,
enum dma_data_direction dir);
void dma_sync_sg_for_device(struct device *dev,
struct scatterlist *sg,
int nelems,
enum dma_data_direction dir);

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DMA Pools
 DMA Pools
— Coherent
— Used to allocate buffers smaller than a page.
— dma_pool_create()
— dma_pool_destroy()
— dma_pool_alloc()
— dma_pool_free()

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Start the DMA operation

19. Property of Tandem Group
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Triggering the DMA operation
 PCI
 dmaengine

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PCI Wrappers
 PCI wrappers
— pci_alloc_consistent()
— pci_free_consistent()
— pci_set_dma_mask()
— pci_pool_create()
— …

21. Property of Tandem Group
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PCI DMA transaction example
dma_addr_t bus_addr;
bus_addr = dma_map_single(&dev->pci_dev->dev, buffer, count, DMA_TO_DEVICE);
writeb(dev->registers.command, DAD_CMD_DISABLEDMA);
writeb(dev->registers.command, DAD_CMD_WR);
writel(dev->registers.addr, cpu_to_le32(bus_addr));
writel(dev->registers.len, cpu_to_le32(count));
writeb(dev->registers.command, DAD_CMD_ENABLEDMA);

22. Property of Tandem Group
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DMA Engine
 Subsystem to handle memory-to-device
transfers
 Exists since 2.6.18 (2006)
 Code in “drivers/dma”
 Documentation in “dmaengine/*”
 Poorly documented

23. THANK YOU!