Describe how the comedi minor device numbers are split across comedi
devices and comedi subdevices.

Replace the current, long dead URL with an official URL for the Comedi
project.

Signed-off-by: Ian Abbott <abbo...@mev.co.uk>
---
 Documentation/admin-guide/devices.txt | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/devices.txt 
b/Documentation/admin-guide/devices.txt
index e56e00655153..1c5d2281efc9 100644
--- a/Documentation/admin-guide/devices.txt
+++ b/Documentation/admin-guide/devices.txt
@@ -1647,8 +1647,17 @@
                  0 = /dev/comedi0      First comedi device
                  1 = /dev/comedi1      Second comedi device
                    ...
+                47 = /dev/comedi47     48th comedi device
 
-               See http://stm.lbl.gov/comedi.
+               Minors 48 to 255 are reserved for comedi subdevices with
+               pathnames of the form "/dev/comediX_subdY", where "X" is the
+               minor number of the associated comedi device and "Y" is the
+               subdevice number.  These subdevice minors are assigned
+               dynamically, so there is no fixed mapping from subdevice
+               pathnames to minor numbers.
+
+               See http://www.comedi.org/ for information about the Comedi
+               project.
 
   98 block     User-mode virtual block device
                  0 = /dev/ubda         First user-mode block device
-- 
2.23.0

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