vcs: add poll/fasync support
The /dev/vcs* devices are used, amongst other things, by accessibility applications such as BRLTTY to display the screen content onto refreshable braille displays. Currently this is performed by constantly reading from /dev/vcsa0 whether or not the screen content has changed. Given the default braille refresh rate of 25 times per second, this easily qualifies as the biggest source of wake-up events preventing laptops from entering deeper power saving states. To avoid this periodic polling, let's add support for select()/poll() and SIGIO with the /dev/vcs* devices. The implemented semantic is to report data availability whenever the corresponding vt has seen some update after the last read() operation. The application still has to lseek() back as usual in order to read() the new data. Not to create unwanted overhead, the needed data structure is allocated and the vt notification callback is registered only when the poll or fasync method is invoked for the first time per file instance. Signed-off-by: Nicolas Pitre <nicolas.pitre@canonical.com> Cc: Andrew Morton <akpm@linux-foundation.org> Acked-by: Alan Cox <alan@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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@ -35,6 +35,12 @@
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#include <linux/console.h>
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#include <linux/device.h>
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#include <linux/smp_lock.h>
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#include <linux/sched.h>
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#include <linux/fs.h>
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#include <linux/poll.h>
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#include <linux/signal.h>
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#include <linux/slab.h>
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#include <linux/notifier.h>
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#include <asm/uaccess.h>
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#include <asm/byteorder.h>
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@ -45,6 +51,86 @@
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#undef addr
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#define HEADER_SIZE 4
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struct vcs_poll_data {
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struct notifier_block notifier;
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unsigned int cons_num;
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bool seen_last_update;
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wait_queue_head_t waitq;
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struct fasync_struct *fasync;
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};
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static int
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vcs_notifier(struct notifier_block *nb, unsigned long code, void *_param)
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{
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struct vt_notifier_param *param = _param;
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struct vc_data *vc = param->vc;
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struct vcs_poll_data *poll =
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container_of(nb, struct vcs_poll_data, notifier);
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int currcons = poll->cons_num;
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if (code != VT_UPDATE)
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return NOTIFY_DONE;
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if (currcons == 0)
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currcons = fg_console;
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else
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currcons--;
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if (currcons != vc->vc_num)
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return NOTIFY_DONE;
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poll->seen_last_update = false;
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wake_up_interruptible(&poll->waitq);
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kill_fasync(&poll->fasync, SIGIO, POLL_IN);
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return NOTIFY_OK;
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}
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static void
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vcs_poll_data_free(struct vcs_poll_data *poll)
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{
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unregister_vt_notifier(&poll->notifier);
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kfree(poll);
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}
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static struct vcs_poll_data *
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vcs_poll_data_get(struct file *file)
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{
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struct vcs_poll_data *poll = file->private_data;
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if (poll)
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return poll;
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poll = kzalloc(sizeof(*poll), GFP_KERNEL);
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if (!poll)
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return NULL;
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poll->cons_num = iminor(file->f_path.dentry->d_inode) & 127;
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init_waitqueue_head(&poll->waitq);
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poll->notifier.notifier_call = vcs_notifier;
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if (register_vt_notifier(&poll->notifier) != 0) {
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kfree(poll);
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return NULL;
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}
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/*
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* This code may be called either through ->poll() or ->fasync().
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* If we have two threads using the same file descriptor, they could
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* both enter this function, both notice that the structure hasn't
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* been allocated yet and go ahead allocating it in parallel, but
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* only one of them must survive and be shared otherwise we'd leak
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* memory with a dangling notifier callback.
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*/
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spin_lock(&file->f_lock);
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if (!file->private_data) {
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file->private_data = poll;
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} else {
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/* someone else raced ahead of us */
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vcs_poll_data_free(poll);
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poll = file->private_data;
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}
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spin_unlock(&file->f_lock);
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return poll;
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}
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static int
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vcs_size(struct inode *inode)
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{
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@ -102,6 +188,7 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
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struct inode *inode = file->f_path.dentry->d_inode;
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unsigned int currcons = iminor(inode);
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struct vc_data *vc;
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struct vcs_poll_data *poll;
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long pos;
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long viewed, attr, read;
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int col, maxcol;
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@ -134,6 +221,9 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
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ret = -EINVAL;
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if (pos < 0)
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goto unlock_out;
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poll = file->private_data;
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if (count && poll)
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poll->seen_last_update = true;
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read = 0;
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ret = 0;
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while (count) {
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@ -457,6 +547,37 @@ unlock_out:
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return ret;
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}
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static unsigned int
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vcs_poll(struct file *file, poll_table *wait)
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{
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struct vcs_poll_data *poll = vcs_poll_data_get(file);
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int ret = 0;
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if (poll) {
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poll_wait(file, &poll->waitq, wait);
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if (!poll->seen_last_update)
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ret = POLLIN | POLLRDNORM;
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}
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return ret;
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}
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static int
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vcs_fasync(int fd, struct file *file, int on)
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{
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struct vcs_poll_data *poll = file->private_data;
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if (!poll) {
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/* don't allocate anything if all we want is disable fasync */
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if (!on)
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return 0;
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poll = vcs_poll_data_get(file);
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if (!poll)
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return -ENOMEM;
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}
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return fasync_helper(fd, file, on, &poll->fasync);
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}
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static int
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vcs_open(struct inode *inode, struct file *filp)
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{
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@ -470,11 +591,23 @@ vcs_open(struct inode *inode, struct file *filp)
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return ret;
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}
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static int vcs_release(struct inode *inode, struct file *file)
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{
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struct vcs_poll_data *poll = file->private_data;
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if (poll)
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vcs_poll_data_free(poll);
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return 0;
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}
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static const struct file_operations vcs_fops = {
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.llseek = vcs_lseek,
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.read = vcs_read,
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.write = vcs_write,
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.poll = vcs_poll,
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.fasync = vcs_fasync,
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.open = vcs_open,
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.release = vcs_release,
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};
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static struct class *vc_class;
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