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Linux设备驱动开发-linux驱动中的非阻塞访问方式
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前面已經詳細分析過了阻塞訪問方式，下面就來繼續分析一下非阻塞的訪問方式。
什么是非阻塞的訪問方式呢？非阻塞操作的進程在不能進行設備操作時，并不掛起，他或者是放棄當前的進程執行，或者是不停地進行查詢，知道進程可以進行操作為止。實際上就是常說的輪詢的方式進行設備的訪問。
select()和poll()調用的本質是一樣的，在性能上也不存在明顯的差異，只是select監視的文件描述符數量有限，下面分別看下這兩個系統調用。
首先看下select()系統調用，其函數原型為：
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int?select(int?nfds,?fd_set?*readfds,?fd_set?*writefds,?? ??????????????????fd_set?*exceptfds,?struct?timeval?*timeout);??
readfds、writefds、exceptfds是被select()監視的讀、寫、異常處理的文件描述符集合，nfds是需要檢查的號碼最高的文件描述符加1，timeout是一個超時值得設定，當在timeout的時間里沒有文件描述符準備好則返回。struct timeval結構體如下：
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struct?timeval?{?? ????????__kernel_time_t?????????tv_sec;??????????? ????????__kernel_suseconds_t????tv_usec;?????????? };??
在select()函數后面還提供了一些API函數，如：
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void?FD_CLR(int?fd,?fd_set?*set);?????????????????????????????????? int??FD_ISSET(int?fd,?fd_set?*set);???????????????????????????????? void?FD_SET(int?fd,?fd_set?*set);????????????????????????????????? void?FD_ZERO(fd_set?*set);????????????????????????????????????????
select()的接口主要是建立在叫fd_set類型的基礎上，它是一組文件描述符的集合，來看下面的定義：
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typedef?struct?{?? ????????unsigned?long?fds_bits?[__FDSET_LONGS];?? }?__kernel_fd_set;??
而__FDSET_LONGS是
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#define?__FDSET_LONGS???(__FD_SETSIZE/__NFDBITS)??
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#define?__FD_SETSIZE????1024?? #define?__NFDBITS???????(8?*?sizeof(unsigned?long))??
這里計算得__FDSET_LONGS的值是32，不同平臺上這個值不一定相同。
上面大概介紹了一下selece()，下面來看poll()。
poll()系統調用的原型是：
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int?poll(struct?pollfd?*fds,?nfds_t?nfds,?int?timeout);??
其中pollfd結構體如下：
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struct?pollfd?{?? ????????int?fd;?????????????????????????????????????? ????????short?events;????????????????????????????? ????????short?revents;???????????????????????????? };??
起執行過程如下：
首先將用戶傳入的pollfd數組拷貝到內核空間，然后查詢每個文件描述符對應的設備的狀態，最后將獲得的數據傳送到用戶空間并執行釋放內存和剝離等待隊列等工作。
設計上無論是select()還是poll()系統調用，其最終都會引發設備驅動中的poll()被執行，那么下面就來看下設備驅動中的poll()函數：
首先來看下在設備驅動中的poll()函數的原型：
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unsigned?int?(*poll)?(struct?file?*,?struct?poll_table_struct?*);??
其在file_operations結構體中。參數file是file結構體指針，第二個參數是輪詢表指針。這個函數主要完成的工作有：
對可能引起設備文件狀態變化的等待隊列調用poll_wait()函數，將對應的等待隊列頭添加到poll_table。最后返回表示是否能對設備進行無阻塞讀、寫訪問的掩碼。
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typedef?void?(*poll_queue_proc)(struct?file?*,?wait_queue_head_t?*,?struct?poll_table_struct?*);?? ?????????? typedef?struct?poll_table_struct?{?? ????????poll_queue_proc?qproc;?? ????????unsigned?long?key;?? }?poll_table;??
poll_queue_proc是一個函數指針，其在整個過程中都不變化，而key根據不同的fd的檢測要求而變化。
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static?inline?void?poll_wait(struct?file?*?filp,?wait_queue_head_t?*?wait_address,?poll_table?*p)?? {?? ????????if?(p?&&?wait_address)?? ????????????????p->qproc(filp,?wait_address,?p);?? }??
wait_address是驅動程序需要提供的等待隊列頭。
下面把poll()系統調用涉及的主要代碼在這里貼出來，大家可以深入解析一下：
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SYSCALL_DEFINE3(poll,?struct?pollfd?__user?*,?ufds,?unsigned?int,?nfds,?? ????????????????long,?timeout_msecs)?? {?? ????????struct?timespec?end_time,?*to?=?NULL;?? ????????int?ret;?? ?? ????????if?(timeout_msecs?>=?0)?{?? ????????????????to?=?&end_time;?? ????????????????poll_select_set_timeout(to,?timeout_msecs?/?MSEC_PER_SEC,?? ????????????????????????NSEC_PER_MSEC?*?(timeout_msecs?%?MSEC_PER_SEC));?? ????????}?? ?? ????????ret?=?do_sys_poll(ufds,?nfds,?to);?? ?? ????????if?(ret?==?-EINTR)?{?? ????????????????struct?restart_block?*restart_block;?? ?? ????????????????restart_block?=?¤t_thread_info()->restart_block;?? ????????????????restart_block->fn?=?do_restart_poll;?? ????????????????restart_block->poll.ufds?=?ufds;?? ????????????????restart_block->poll.nfds?=?nfds;?? ?? ????????????????if?(timeout_msecs?>=?0)?{?? ????????????????????????restart_block->poll.tv_sec?=?end_time.tv_sec;?? ????????????????????????restart_block->poll.tv_nsec?=?end_time.tv_nsec;?? ????????????????????????restart_block->poll.has_timeout?=?1;?? ????????????????}?else?? ????????????????????????restart_block->poll.has_timeout?=?0;?? ?? ????????????????ret?=?-ERESTART_RESTARTBLOCK;?? ????????}?? ????????return?ret;?? }??
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int?do_sys_poll(struct?pollfd?__user?*ufds,?unsigned?int?nfds,?? ????????????????struct?timespec?*end_time)?? {?? ????????struct?poll_wqueues?table;?? ????????int?err?=?-EFAULT,?fdcount,?len,?size;?? ????????? ? ?? ????????long?stack_pps[POLL_STACK_ALLOC/sizeof(long)];?? ????????struct?poll_list?*const?head?=?(struct?poll_list?*)stack_pps;?? ????????struct?poll_list?*walk?=?head;?? ????????unsigned?long?todo?=?nfds;?? ?? ????????if?(nfds?>?current->signal->rlim[RLIMIT_NOFILE].rlim_cur)?? ????????????????return?-EINVAL;?? ?? ????????len?=?min_t(unsigned?int,?nfds,?N_STACK_PPS);?? ????????for?(;;)?{?? ????????????????walk->next?=?NULL;?? ????????????????walk->len?=?len;?? ????????????????if?(!len)?? ????????????????????????break;?? ?? ????????????????if?(copy_from_user(walk->entries,?ufds?+?nfds-todo,?? ????????????????????????????????????????sizeof(struct?pollfd)?*?walk->len))?? ????????????????????????goto?out_fds;?? ?? ????????????????todo?-=?walk->len;?? ????????????????if?(!todo)?? ????????????????????????break;?? ?? ????????????????len?=?min(todo,?POLLFD_PER_PAGE);?? ????????????????size?=?sizeof(struct?poll_list)?+?sizeof(struct?pollfd)?*?len;?? ????????????????walk?=?walk->next?=?kmalloc(size,?GFP_KERNEL);?? ????????????????if?(!walk)?{?? ????????????????????????err?=?-ENOMEM;?? ????????????????????????goto?out_fds;?? ????????????????}?? ????????}?? ?? ????????poll_initwait(&table);?? ????????fdcount?=?do_poll(nfds,?head,?&table,?end_time);?? ????????poll_freewait(&table);?? ?? ????????for?(walk?=?head;?walk;?walk?=?walk->next)?{?? ????????????????struct?pollfd?*fds?=?walk->entries;?? ????????????????int?j;?? ?? ????????????????for?(j?=?0;?j?<?walk->len;?j++,?ufds++)?? ????????????????????????if?(__put_user(fds[j].revents,?&ufds->revents))?? ????????????????????????????????goto?out_fds;?? ????????}?? ?? ????????err?=?fdcount;?? out_fds:?? ????????walk?=?head->next;?? ????????while?(walk)?{?? ????????????????struct?poll_list?*pos?=?walk;?? ????????????????walk?=?walk->next;?? ????????????????kfree(pos);?? ????????}?? ?? ????????return?err;?? }??
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static?int?do_poll(unsigned?int?nfds,??struct?poll_list?*list,?? ???????????????????struct?poll_wqueues?*wait,?struct?timespec?*end_time)?? {?? ????????poll_table*?pt?=?&wait->pt;?? ????????ktime_t?expire,?*to?=?NULL;?? ????????int?timed_out?=?0,?count?=?0;?? ????????unsigned?long?slack?=?0;?? ?? ?????????? ????????if?(end_time?&&?!end_time->tv_sec?&&?!end_time->tv_nsec)?{?? ????????????????pt?=?NULL;?? ????????????????timed_out?=?1;?? ????????}?? ?? ????????if?(end_time?&&?!timed_out)?? ????????????????slack?=?estimate_accuracy(end_time);?? ?? ????????for?(;;)?{?? ????????????????struct?poll_list?*walk;?? ?? ????????????????for?(walk?=?list;?walk?!=?NULL;?walk?=?walk->next)?{?? ????????????????????????struct?pollfd?*?pfd,?*?pfd_end;?? ?? ????????????????????????pfd?=?walk->entries;?? ????????????????????????pfd_end?=?pfd?+?walk->len;?? ????????????????????????for?(;?pfd?!=?pfd_end;?pfd++)?{?? ????????????????????????????????? ? ? ? ? ? ?? ????????????????????????????????if?(do_pollfd(pfd,?pt))?{?? ????????????????????????????????????????count++;?? ????????????????????????????????????????pt?=?NULL;?? ????????????????????????????????}?? ????????????????????????}?? ????????????????}?? ????????????????? ? ? ?? ????????????????pt?=?NULL;?? ????????????????if?(!count)?{?? ????????????????????????count?=?wait->error;?? ????????????????????????if?(signal_pending(current))?? ????????????????????????????????count?=?-EINTR;?? ????????????????}?? ????????????????if?(count?||?timed_out)?? ????????????????????????break;?? ?? ????????????????? ? ? ? ?? ????????????????if?(end_time?&&?!to)?{?? ????????????????????????expire?=?timespec_to_ktime(*end_time);?? ????????????????????????to?=?&expire;?? ????????????????}?? ?? ??????????????????if?(!poll_schedule_timeout(wait,?TASK_INTERRUPTIBLE,?to,?slack))?? ????????????????????????timed_out?=?1;?? ????????}?? ????????return?count;?? }??
下面還是根據前面所舉的globalmem例子寫一個poll函數
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static?unsigned?int?globalmem_poll(struct?file?*filp,poll_table?*wait)?? {?? ????unsigned?int?mask?=?0;?? ????struct?globalmem_dev?*dev?=?filp->private_data;?? ?? ????down(&dev->sem);?? ?? ????poll_wait(filp,&dev->r_wait,wait);?????????????????????? ????poll_wait(filp,&dev->w_wait,wait);????????????????????? ?????? ????if(dev->current_len?!=?0)?? ????????mask?|=?POLLIN?|?POLLRDNORM;???????????? ????if(dev->current_len?!=?GLOBALFIFO_SIZE)?????????? ????????mask?|=?POLLOUT?|?POLLWRNORM;????????? ?? ????up(&dev->sem);?? ????return?mask;?? }??
最后將其付給poll函數指針：
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static?const?struct?file_operations?globalmem_fops?=?{?? ????.poll?=?globalmem_poll,?? }??
而關于上面的標識符<poll.h>中有定義。
下面再編寫一個應用程序用于監控globalmem的可讀可寫狀態，代碼如下（未加頭文件）：
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#define?FIFO_CLEAR?0x1;?? #define?BUFFER_LEN?20;?? ?? mian()?? {?? ????int?fd,num;?? ????char?rd_ch[BUFFER_LEN];?? ????fd_set?rfds,wfds;?? ?? ????fd?=?open("/dev/globalmem",O_RDONLY?|?O_NONBLOCK);?? ????if(fd?!=?-1){?? ????????if(ioctl(fd,FIFO_CLEAR,0)<0)?? ????????????printf("ioctl?command?failed\n");?? ?? ????while(1){?? ????????FD_ZERO(&rfds);?? ????????FD_ZERO(&wfds);?? ????????FD_SET(fd,&rfds);?? ????????FD_SET(fd,&wfds);?? ?? ????????select(fd+1,&rfds,&wfds,NULL,NULL);?? ????????if(FD_ISSET(fd,&rfds))?? ????????????printf("poll?monitor:can?be?read\n");?? ????????if(FD_ISSET(fd,&wfds))?? ????????????printf("poll?monitor:can?be?written\n");?????? ????????}?? ????}else{?? ????????printf("Device?open?failure\n");?????? ????}?? } ?
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