Specifications

This is the Title of the Book, eMatter Edition

Direct Memory Access

453

void pci_dac_dma_sync_single_for_device(struct pci_dev *pdev,

dma64_addr_t dma_addr,

size_t len,

int direction);

A simple PCI DMA example

As an example of how the DMA mappings might be used, we present a simple exam-

ple of DMA coding for a PCI device. The actual form of DMA operations on the PCI

bus is very dependent on the device being driven. Thus, this example does not apply

to any real device; instead, it is part of a hypothetical driver called dad (DMA Acqui-

sition Device). A driver for this device might define a transfer function like this:

int dad_transfer(struct dad_dev *dev, int write, void *buffer,

size_t count)

{

dma_addr_t bus_addr;

/* Map the buffer for DMA */

dev->dma_dir = (write ? DMA_TO_DEVICE : DMA_FROM_DEVICE);

dev->dma_size = count;

bus_addr = dma_map_single(&dev->pci_dev->dev, buffer, count,

dev->dma_dir);

dev->dma_addr = bus_addr;

/* Set up the device */

writeb(dev->registers.command, DAD_CMD_DISABLEDMA);

writeb(dev->registers.command, write ? DAD_CMD_WR : DAD_CMD_RD);

writel(dev->registers.addr, cpu_to_le32(bus_addr));

writel(dev->registers.len, cpu_to_le32(count));

/* Start the operation */

writeb(dev->registers.command, DAD_CMD_ENABLEDMA);

return 0;

}

This function maps the buffer to be transferred and starts the device operation. The

other half of the job must be done in the interrupt service routine, which looks some-

thing like this:

void dad_interrupt(int irq, void *dev_id, struct pt_regs *regs)

{

struct dad_dev *dev = (struct dad_dev *) dev_id;

/* Make sure it's really our device interrupting */

/* Unmap the DMA buffer */

dma_unmap_single(dev->pci_dev->dev, dev->dma_addr,

dev->dma_size, dev->dma_dir);

/* Only now is it safe to access the buffer, copy to user, etc. */

...

}

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