我现在实际上正在做完全相同的事情，并且正在走ioctl()路线。一般的想法是让用户空间分配缓冲区，该缓冲区将用于DMA传输，并且ioctl()将用于将缓冲区的大小和地址传递给设备驱动程序。然后，驱动程序将使用分散收集列表以及流DMA API来直接与设备和用户空间缓冲区之间来回传输数据。
我正在使用的实现策略是，ioctl()驱动程序中的进入一个循环，该循环以256k的块（这是硬件对其可处理的分散/聚集条目的硬性限制）的形式进入DMA的用户空间缓冲区。这是隔离在一个函数内部的，该函数在每次传输完成之前都会阻塞（请参阅下文）。当所有字节传输完毕或增量传输函数返回错误时，ioctl()退出并返回用户空间
的伪代码 ioctl()
/*serialize all DMA transfers to/from the device*/
if (mutex_lock_interruptible( &device_ptr->mtx ) )
return -EINTR;
chunk_data = (unsigned long) user_space_addr;
while( *transferred < total_bytes && !ret ) {
chunk_bytes = total_bytes - *transferred;
if (chunk_bytes > HW_DMA_MAX)
chunk_bytes = HW_DMA_MAX; /* 256kb limit imposed by my device */
ret = transfer_chunk(device_ptr, chunk_data, chunk_bytes, transferred);
chunk_data += chunk_bytes;
chunk_offset += chunk_bytes;
}
mutex_unlock(&device_ptr->mtx);
/*Assuming the userspace pointer is passed as an unsigned long, */
/*calculate the first,last, and number of pages being transferred via*/
first_page = (udata & PAGE_MASK) >> PAGE_SHIFT;
last_page = ((udata+nbytes-1) & PAGE_MASK) >> PAGE_SHIFT;
first_page_offset = udata & PAGE_MASK;
npages = last_page - first_page + 1;
/* Ensure that all userspace pages are locked in memory for the */
/* duration of the DMA transfer */
down_read(¤t->mm->mmap_sem);
ret = get_user_pages(current,
current->mm,
udata,
npages,
is_writing_to_userspace,
0,
&pages_array,
NULL);
up_read(¤t->mm->mmap_sem);
/* Map a scatter-gather list to point at the userspace pages */
/*first*/
sg_set_page(&sglist[0], pages_array[0], PAGE_SIZE - fp_offset, fp_offset);
/*middle*/
for(i=1; i < npages-1; i++)
sg_set_page(&sglist[i], pages_array[i], PAGE_SIZE, 0);
/*last*/
if (npages > 1) {
sg_set_page(&sglist[npages-1], pages_array[npages-1],
nbytes - (PAGE_SIZE - fp_offset) - ((npages-2)*PAGE_SIZE), 0);
}
/* Do the hardware specific thing to give it the scatter-gather list
and tell it to start the DMA transfer */
/* Wait for the DMA transfer to complete */
ret = wait_event_interruptible_timeout( &device_ptr->dma_wait,
&device_ptr->flag_dma_done, HZ*2 );
if (ret == 0)
/* DMA operation timed out */
else if (ret == -ERESTARTSYS )
/* DMA operation interrupted by signal */
else {
/* DMA success */
*transferred += nbytes;
return 0;
}
中断处理程序非常简短：
/* Do hardware specific thing to make the device happy */
/* Wake the thread waiting for this DMA operation to complete */
device_ptr->flag_dma_done = 1;
wake_up_interruptible(device_ptr->dma_wait);
请注意，这只是一种通用方法，最近几周我一直在研究此驱动程序，但尚未进行实际测试。因此，请不要将此伪代码视为福音，并且一定要加倍检查所有逻辑和参数；-)。
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