DMA Survival Guide

When we’re done, it’s done!

DMA Survival Guide

Property of Tandem Group


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

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Agenda What is DMA DMA Buffer allocation

— Coherent— Streaming

Scatter Gather mapping DMA pools DMA Triggering

— PCI— dmaengine



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.



What is DMA

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



DMA Buffer allocation DMA controller works on physical

addresses Physical memory needs to be accessible

by DMA controller Memory must be continuous



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.



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);



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);



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);



Streaming DMA mapping (cont’d) enum dma_data_direction

— DMA_TO_DEVICE— DMA_FROM_DEVICE— DMA_BIDIRECTIONAL



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()



Scatter gather buffers

Special type of streaming DMA writev, readv, clustered buffers (YUV

plannar, non continuous memory, etc.)



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);



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);



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()



Start the DMA operation



Triggering the DMA operation PCI dmaengine



PCI Wrappers PCI wrappers

— pci_alloc_consistent()— pci_free_consistent()— pci_set_dma_mask()— pci_pool_create()— …



PCI DMA transaction exampledma_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);



DMA Engine Subsystem to handle memory-to-device

transfers Exists since 2.6.18 (2006) Code in “drivers/dma” Documentation in “dmaengine/*”

Poorly documented

THANK YOU!