Specifications

This is the Title of the Book, eMatter Edition

458

Chapter 15: Memory Mapping and DMA

In addition to these functions, there are a number of housekeeping facilities that

must be used when dealing with DMA devices:

void disable_dma(unsigned int channel);

A DMA channel can be disabled within the controller. The channel should be

disabled before the controller is configured to prevent improper operation. (Oth-

erwise, corruption can occur because the controller is programmed via 8-bit data

transfers and, therefore, none of the previous functions is executed atomically).

void enable_dma(unsigned int channel);

This function tells the controller that the DMA channel contains valid data.

int get_dma_residue(unsigned int channel);

The driver sometimes needs to know whether a DMA transfer has been com-

pleted. This function returns the number of bytes that are still to be transferred.

The return value is

0 after a successful transfer and is unpredictable (but not 0)

while the controller is working. The unpredictability springs from the need to

obtain the 16-bit residue through two 8-bit input operations.

void clear_dma_ff(unsigned int channel)

This function clears the DMA flip-flop. The flip-flop is used to control access to

16-bit registers. The registers are accessed by two consecutive 8-bit operations,

and the flip-flop is used to select the least significant byte (when it is clear) or the

most significant byte (when it is set). The flip-flop automatically toggles when

eight bits have been transferred; the programmer must clear the flip-flop (to set

it to a known state) before accessing the DMA registers.

Using these functions, a driver can implement a function like the following to pre-

pare for a DMA transfer:

int dad_dma_prepare(int channel, int mode, unsigned int buf,

unsigned int count)

{

unsigned long flags;

flags = claim_dma_lock( );

disable_dma(channel);

clear_dma_ff(channel);

set_dma_mode(channel, mode);

set_dma_addr(channel, virt_to_bus(buf));

set_dma_count(channel, count);

enable_dma(channel);

release_dma_lock(flags);

return 0;

}

Then, a function like the next one is used to check for successful completion of

DMA:

int dad_dma_isdone(int channel)

{

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