[PATCH v3 07/11] fblog: allow selecting fbs via sysfs

[David Herrmann <dh.herrmann@googlemail.com>]

fblog is mainly useful during boot, reboot, panics and maintenance. In all
cases you often want to control which monitors are used for console
output. Moreover, in multi-seat environments it is desireable to reduce
system-overhead by not drawing the console to all framebuffers. Two
mechanisms to select framebuffers for fblog are added:

1) "active" module parameter: This parameter selects whether new
framebuffers are opened automatically. By default this is on, that is, all
framebuffers are automatically used by fblog during boot. By passing
fblog.active=0 you can deactivate this.
The init process can set this to 0 via
/sys/modules/fblog/parameters/active, too. However, this does not affect
already available and used framebuffers in any way.

2) "active" sysfs attribute for each fblog object. Reading this value
returns whether a framebuffer is currently active. Writing it opens/closes
the framebuffer. This allows runtime control which fbs are used. For
instance, init can set these to 0 after bootup.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
---
 drivers/video/console/fblog.c | 53 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 52 insertions(+), 1 deletion(-)

diff --git a/drivers/video/console/fblog.c b/drivers/video/console/fblog.c
index 1c526c5..5519f91 100644
--- a/drivers/video/console/fblog.c
+++ b/drivers/video/console/fblog.c
@@ -44,6 +44,7 @@ struct fblog_fb {
 
 static DEFINE_MUTEX(fblog_registration_lock);
 static struct fblog_fb *fblog_fbs[FB_MAX];
+static bool active = 1;
 
 #define to_fblog_dev(_d) container_of(_d, struct fblog_fb, dev)
 
@@ -115,6 +116,40 @@ static void fblog_close(struct fblog_fb *fb, bool kill_dev)
 	mutex_unlock(&fb->lock);
 }
 
+static ssize_t fblog_dev_active_show(struct device *dev,
+				     struct device_attribute *attr,
+				     char *buf)
+{
+	struct fblog_fb *fb = to_fblog_dev(dev);
+
+	return snprintf(buf, PAGE_SIZE, "%d\n",
+			!!test_bit(FBLOG_OPEN, &fb->flags));
+}
+
+static ssize_t fblog_dev_active_store(struct device *dev,
+				      struct device_attribute *attr,
+				      const char *buf,
+				      size_t count)
+{
+	struct fblog_fb *fb = to_fblog_dev(dev);
+	unsigned long num;
+	int ret = 0;
+
+	num = simple_strtoul(buf, NULL, 10);
+
+	mutex_lock(&fb->info->lock);
+	if (num)
+		ret = fblog_open(fb);
+	else
+		fblog_close(fb, false);
+	mutex_unlock(&fb->info->lock);
+
+	return ret ? ret : count;
+}
+
+static DEVICE_ATTR(active, S_IRUGO | S_IWUSR | S_IWGRP, fblog_dev_active_show,
+		   fblog_dev_active_store);
+
 /*
  * fblog framebuffer list
  * The fblog_fbs[] array contains all currently registered framebuffers. If a
@@ -148,6 +183,7 @@ static void fblog_do_unregister(struct fb_info *info)
 	fblog_fbs[info->node] = NULL;
 
 	fblog_close(fb, true);
+	device_remove_file(&fb->dev, &dev_attr_active);
 	device_del(&fb->dev);
 	put_device(&fb->dev);
 }
@@ -156,6 +192,7 @@ static void fblog_do_register(struct fb_info *info, bool force)
 {
 	struct fblog_fb *fb;
 	int ret;
+	bool do_open = true;
 
 	fb = fblog_fbs[info->node];
 	if (fb && fb->info != info) {
@@ -186,7 +223,18 @@ static void fblog_do_register(struct fb_info *info, bool force)
 		return;
 	}
 
-	fblog_open(fb);
+	ret = device_create_file(&fb->dev, &dev_attr_active);
+	if (ret) {
+		pr_err("fblog: cannot create sysfs entry");
+		/* do not open fb if we cannot create control file */
+		do_open = false;
+	}
+
+	if (!active)
+		do_open = false;
+
+	if (do_open)
+		fblog_open(fb);
 }
 
 static void fblog_register(struct fb_info *info, bool force)
@@ -321,6 +369,9 @@ static void __exit fblog_exit(void)
 	}
 }
 
+module_param(active, bool, S_IRUGO | S_IWUSR | S_IWGRP);
+MODULE_PARM_DESC(active, "Activate fblog by default");
+
 module_init(fblog_init);
 module_exit(fblog_exit);
 MODULE_LICENSE("GPL");
-- 
1.7.11.2