/* $Id: dma.c,v 1.5 1992/11/18 02:49:05 root Exp root $
* linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c.
* Written by Hennus Bergman, 1992.
*/
#include <linux/kernel.h>
#include <linux/errno.h>
#include <asm/dma.h>
/* A note on resource allocation:
*
* All drivers needing DMA channels, should allocate and release them
* through the public routines `request_dma()‘ and `free_dma()‘.
*
* In order to avoid problems, all processes should allocate resources in
* the same sequence and release them in the reverse order.
*
* So, when allocating DMAs and IRQs, first allocate the IRQ, then the DMA.
* When releasing them, first release the DMA, then release the IRQ.
* If you don‘t, you may cause allocation requests to fail unnecessarily.
* This doesn‘t really matter now, but it will once we get real semaphores
* in the kernel.
*/
/* Channel n is busy iff dma_chan_busy[n] != 0.
* DMA0 is reserved for DRAM refresh, I think.
* DMA4 is reserved for cascading (?).
*/
/* dma_chan_busy[n] != 0 表示该通道不可用
* DMA0 用作DRAM的刷新.
* DMA4 用作级连.
*/
static volatile unsigned int dma_chan_busy[MAX_DMA_CHANNELS] = {
1, 0, 0, 0, 1, 0, 0, 0
};
/* Atomically swap memory location [32 bits] with `newval‘.
* This avoid the cli()/sti() junk and related problems.
* [And it‘s faster too :-)]
* Maybe this should be in include/asm/mutex.h and be used for
* implementing kernel-semaphores as well.
*/
// 将*p指向的数据替换为newval.由关键字xchgl看，这个值是32位的值。
// xchgl是原子操作，可以省去cli和sti.
static __inline__ unsigned int mutex_atomic_swap(volatile unsigned int * p, unsigned int newval)
{
unsigned int semval = newval;
/* If one of the operands for the XCHG instructions is a memory ref,
* it makes the swap an uninterruptible RMW cycle.
*
* One operand must be in memory, the other in a register, otherwise
* the swap may not be atomic.
*/
asm __volatile__ ("xchgl %2, %0\n"
: /* outputs: semval */ "=r" (semval)
: /* inputs: newval, p */ "0" (semval), "m" (*p)
); /* p is a var, containing an address */
return semval;
} /* mutex_atomic_swap */
//请求DMA，核心由上个函数mutex_atomic_swap完成
int request_dma(unsigned int dmanr)
{
if (dmanr >= MAX_DMA_CHANNELS)
return -EINVAL;
if (mutex_atomic_swap(&dma_chan_busy[dmanr], 1) != 0)
return -EBUSY;
else
/* old flag was 0, now contains 1 to indicate busy */
return 0;
} /* request_dma */
//释放DMA，核心由函数mutex_atomic_swap完成
void free_dma(unsigned int dmanr)
{
if (dmanr >= MAX_DMA_CHANNELS) {
printk("Trying to free DMA%d\n", dmanr);
return;
}
if (mutex_atomic_swap(&dma_chan_busy[dmanr], 0) == 0)
printk("Trying to free free DMA%d\n", dmanr);
} /* free_dma */
原文地址：http://www.cnblogs.com/xiaofengwei/p/3774007.html
