staging: comedi: prepare support for per-file read and write subdevices

staging: comedi: prepare support for per-file read and write subdevices

Comedi devices may have several subdevices that support "read" and/or
"write" asynchronous commands that use the "read" or "write" file
operations for data transfer.  The low-level Comedi drivers may nominate
a default "read" subdevice and/or a default "write" subdevice, but it
may have other subdevices that support asynchronous commands.

The Comedi core provides a somewhat clunky mechanism to provide access
to the asynchronous command support of the non-default subdevices.  When
a low-level device is "attached" to a core Comedi device, it dynamically
allocates a minor device number for each of the subdevices that support
asynchrounous commands and associates them with files created in SysFS
named "comediX_subdY", where "X" is the minor device number of the main
comedi device, and "Y" is the subdevice number.  An application can open
these subdevice-specific files and they behave like the regular
"comediX" files except that the "read" and/or "write" subdevice may be
different to the default chosen by the low-level driver.

This patch adds a layer of indirection between the file object and the
comedi device object to allow the current "read" and/or "write"
subdevice to be altered after opening the Comedi device, on a per-file
object basis.  The advantage is that an application only needs to open
the main Comedi device file and can then choose which subdevice it wants
to "read" or "write".  The main Comedi device file can be opened more
than once, and each file object can choose the "read" and "write"
subdevices independently.

The new `struct comedi_file` is created on "open" and freed on
"release".  It includes pointers to the main Comedi device structure,
and to the current "read" and "write" subdevice structures (which may be
NULL).  It also has information to keep track of when a low-level device
has been attached or detached since the previous time the file object
was used.  In that case, the current "read" and "write" subdevices in
the `struct comedi_file` will be changed to the new defaults (or set to
NULL).  (The change to new defaults is done by `comedi_file_reset()`.
The checking for attach/detach is done by `comedi_file_check()` which
will call `comedi_file_reset()` if there have been any attach/detach
operations since the previous call.)

A subsequent patch will add the ioctls to change the current "read" and
"write" subdevices.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>