对DMA 通道的操作分析之一（3） structure, address, invalid, active, count

yuyang911220

/* Set DMA address
*
* Copy address to the structure, and set the invalid bit
*/
void __set_dma_addr (unsigned int chan, void *addr)
{
dma_t *dma = dma_channel(chan); if (dma->active)
  printk(KERN_ERR "dma%d: altering DMA address while "
         "DMA active\n", chan);
dma->sg = NULL;
dma->addr = addr;
dma->invalid = 1;
}
---------------------------------------------------------------------   
   /* Set the DMA byte count for this channel
    *
    * This should not be called if a DMA channel is enabled,
    * especially since some DMA architectures don't update the
    * DMA count immediately, but defer it to the enable_dma().
    */
   extern void set_dma_count(unsigned int chan, unsigned long count);
   
   该函数对传输的字节赋值，count表示16位通道的字节数
----------------------------------------------------------------------  
   /* Set the transfer direction for this channel
    *
    * This should not be called if a DMA channel is enabled,
    * especially since some DMA architectures don't update the
    * DMA transfer direction immediately, but defer it to the
    * enable_dma().
    */
   extern void set_dma_mode(unsigned int chan, unsigned int mode);
   指出通道从设备读(DMA_MODE_WRITE)或写(DMA_MODE_READ),注意这里的
   write或read都是对cpu而言的，向cpu写--当然就是从设备读取数据了。
   当mode设置为DMA_MODE_CASCADE时，表示释放对总线的控制。
void set_dma_mode (unsigned int chan, unsigned int mode)
{
dma_t *dma = dma_channel(chan);
if (dma->active)
  printk(KERN_ERR "dma%d: altering DMA mode while "
         "DMA active\n", chan);
dma->dma_mode = mode;
dma->invalid = 1;
}
------------------------------------------------------------   
   /* Set the transfer speed for this channel
    */
   extern void set_dma_speed(unsigned int chan, int cycle_ns);
--------------------------------------------------------------------   
   /* Get DMA residue count. After a DMA transfer, this
    * should return zero. Reading this while a DMA transfer is
    * still in progress will return unpredictable results.
    * If called before the channel has been used, it may return 1.
    * Otherwise, it returns the number of _bytes_ left to transfer.
    */
   extern int  get_dma_residue(unsigned int chan);
   
   该函数查询一个DMA传输还有多少字节没有传输完，
   函数返回剩余的字节数，当传输成功时，该函数返回0.
-------------------------------------------------------------  
   /* enable/disable a specific DMA channel */
   static inline void enable_dma(unsigned int dmanr)
   {
    if (dmanr <= 3)
     dma_outb(dmanr, DMA1_MASK_REG);
    else
     dma_outb(dmanr & 3, DMA2_MASK_REG);
   }
   在DMA通道中包含了合法的数据时，该函数激活DMA控制器
----------------------------------------------------   
   static inline void disable_dma(unsigned int dmanr)
   {
    if (dmanr <= 3)
     dma_outb(dmanr | 4, DMA1_MASK_REG);
    else
     dma_outb((dmanr & 3) | 4, DMA2_MASK_REG);
   }
   该函数禁止DMA通道，对它的设置应该在配置DMA之前