staging: comedi: Add module parameters for default buffer size

For comedi subdevices that support asynchronous transfer commands, the
initial buffer size and maximum buffer size for the transfer are both
set to 64 KiB when the comedi device is "attached" to the hardware
device.  For many applications with reasonable fast sample rates and
slow user-space (e.g. Python) these sizes are a bit too small.

A task with CAP_SYS_ADMIN privileges can change the maximum buffer size
for a comedi subdevice with an ioctl call or by writing to a device
attribute file in sysfs, but that's not very convenient.  For comedi
devices attached during system startup, this could be done by a start-up
script, but for hot-plugged devices it would require scripts run by udev
rules, etc.

Rather than use hardwired values, this patch introduces a couple of
module parameters to set the defaults for the initial buffer size
(comedi_default_buf_size_kb) and maximum buffer size
(comedi_default_buf_maxsize_kb).  These values are applied in place of
the previous hard-wired values when the comedi device is "attached".
The module parameter values are in units of KiB for consistency with the
existing device attribute files.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -55,17 +55,40 @@ MODULE_AUTHOR("http://www.comedi.org");
 MODULE_DESCRIPTION("Comedi core module");
 MODULE_LICENSE("GPL");
 
+#define DEFAULT_BUF_MAXSIZE_KB 64
+#define DEFAULT_BUF_SIZE_KB 64
+
 #ifdef CONFIG_COMEDI_DEBUG
 int comedi_debug;
 EXPORT_SYMBOL(comedi_debug);
-module_param(comedi_debug, int, 0644);
+module_param(comedi_debug, int, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(comedi_debug,
+		 "enable comedi core and driver debugging if non-zero (default 0)"
+		);
 #endif
 
 bool comedi_autoconfig = 1;
-module_param(comedi_autoconfig, bool, 0444);
+module_param(comedi_autoconfig, bool, S_IRUGO);
+MODULE_PARM_DESC(comedi_autoconfig,
+		 "enable drivers to auto-configure comedi devices (default 1)");
 
 static int comedi_num_legacy_minors;
-module_param(comedi_num_legacy_minors, int, 0444);
+module_param(comedi_num_legacy_minors, int, S_IRUGO);
+MODULE_PARM_DESC(comedi_num_legacy_minors,
+		 "number of comedi minor devices to reserve for non-auto-configured devices (default 0)"
+		);
+
+unsigned int comedi_default_buf_size_kb = DEFAULT_BUF_SIZE_KB;
+module_param(comedi_default_buf_size_kb, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(comedi_default_buf_size_kb,
+		 "default asynchronous buffer size in KiB (default "
+		 __MODULE_STRING(DEFAULT_BUF_SIZE_KB) ")");
+
+unsigned int comedi_default_buf_maxsize_kb = DEFAULT_BUF_MAXSIZE_KB;
+module_param(comedi_default_buf_maxsize_kb, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(comedi_default_buf_maxsize_kb,
+		 "default maximum size of asynchronous buffer in KiB (default "
+		 __MODULE_STRING(DEFAULT_BUF_MAXSIZE_KB) ")");
 
 static DEFINE_SPINLOCK(comedi_file_info_table_lock);
 static struct comedi_device_file_info

drivers/staging/comedi/drivers.c

@@ -239,6 +239,8 @@ static int postconfig(struct comedi_device *dev)
 			s->len_chanlist = 1;
 
 		if (s->do_cmd) {
+			unsigned int buf_size;
+
 			BUG_ON((s->subdev_flags & (SDF_CMD_READ |
 						   SDF_CMD_WRITE)) == 0);
 			BUG_ON(!s->do_cmdtest);
@@ -254,19 +256,20 @@ static int postconfig(struct comedi_device *dev)
 			async->subdevice = s;
 			s->async = async;
 
-#define DEFAULT_BUF_MAXSIZE (64*1024)
-#define DEFAULT_BUF_SIZE (64*1024)
-
-			async->max_bufsize = DEFAULT_BUF_MAXSIZE;
+			async->max_bufsize =
+				comedi_default_buf_maxsize_kb * 1024;
+			buf_size = comedi_default_buf_size_kb * 1024;
+			if (buf_size > async->max_bufsize)
+				buf_size = async->max_bufsize;
 
 			async->prealloc_buf = NULL;
 			async->prealloc_bufsz = 0;
-			if (comedi_buf_alloc(dev, s, DEFAULT_BUF_SIZE) < 0) {
+			if (comedi_buf_alloc(dev, s, buf_size) < 0) {
 				printk(KERN_INFO "Buffer allocation failed\n");
 				return -ENOMEM;
 			}
 			if (s->buf_change) {
-				ret = s->buf_change(dev, s, DEFAULT_BUF_SIZE);
+				ret = s->buf_change(dev, s, buf_size);
 				if (ret < 0)
 					return ret;
 			}

drivers/staging/comedi/internal.h

@@ -1,5 +1,5 @@
 /*
- * various internal comedi functions
+ * various internal comedi stuff
  */
 int do_rangeinfo_ioctl(struct comedi_device *dev,
 		       struct comedi_rangeinfo __user *arg);
@@ -11,3 +11,6 @@ int comedi_find_board_minor(struct device *hardware_device);
 void comedi_reset_async_buf(struct comedi_async *async);
 int comedi_buf_alloc(struct comedi_device *dev, struct comedi_subdevice *s,
 		     unsigned long new_size);
+
+extern unsigned int comedi_default_buf_size_kb;
+extern unsigned int comedi_default_buf_maxsize_kb;