comedi: kcomedilib: Add loop checking variants of open and close

Add `comedi_open_from(path, from)` and `comedi_close_from(dev, from)` as
variants of the existing `comedi_from(path)` and `comedi_close(dev)`.
The additional `from` parameter is a minor device number that tells the
function that the COMEDI device is being opened or closed from another
COMEDI device if the value is in the range [0,
`COMEDI_NUM_BOARD_MINORS`-1].  In that case the function will refuse to
open the device if it would lead to a chain of devices opening each
other.  (It will also impose a limit on the number of simultaneous opens
from one device to another because we need to count those.)

The new functions are intended to be used by the "comedi_bond" driver,
which is the only driver that uses the existing `comedi_open()` and
`comedi_close()` functions.  The new functions will be used to avoid
some possible deadlock situations.

Replace the existing, exported `comedi_open()` and `comedi_close()`
functions with inline wrapper functions that call the newly exported
`comedi_open_from()` and `comedi_close_from()` functions.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20251027153748.4569-2-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 32 insertions, 2 deletions

diff --git a/include/linux/comedi/comedilib.h b/include/linux/comedi/comedilib.h
index 0223c9cd9215..1f2b22b383cc 100644
--- a/include/linux/comedi/comedilib.h
+++ b/include/linux/comedi/comedilib.h
@@ -10,8 +10,38 @@
 #ifndef _LINUX_COMEDILIB_H
 #define _LINUX_COMEDILIB_H
 
-struct comedi_device *comedi_open(const char *path);
-int comedi_close(struct comedi_device *dev);
+struct comedi_device *comedi_open_from(const char *path, int from);
+
+/**
+ * comedi_open() - Open a COMEDI device from the kernel
+ * @filename: Fake pathname of the form "/dev/comediN".
+ *
+ * Converts @filename to a COMEDI device number and "opens" it if it exists
+ * and is attached to a low-level COMEDI driver.
+ *
+ * Return: A pointer to the COMEDI device on success.
+ * Return %NULL on failure.
+ */
+static inline struct comedi_device *comedi_open(const char *path)
+{
+	return comedi_open_from(path, -1);
+}
+
+int comedi_close_from(struct comedi_device *dev, int from);
+
+/**
+ * comedi_close() - Close a COMEDI device from the kernel
+ * @dev: COMEDI device.
+ *
+ * Closes a COMEDI device previously opened by comedi_open().
+ *
+ * Returns: 0
+ */
+static inline int comedi_close(struct comedi_device *dev)
+{
+	return comedi_close_from(dev, -1);
+}
+
 int comedi_dio_get_config(struct comedi_device *dev, unsigned int subdev,
 			  unsigned int chan, unsigned int *io);
 int comedi_dio_config(struct comedi_device *dev, unsigned int subdev,