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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: