staging: comedi: introduce comedi_timeout()

staging: comedi: introduce comedi_timeout()

Introduce a comedi core helper function to handle the boilerplate
needed by the drivers to busy- wait for a condition to occur.
Typically this condition is the analog input/output end-of-conversion
used with the comedi (*insn_read) and (*insn_write) operations.

To use this function, the drivers just need to provide a callback
that checks for the desired condition. The callback should return
0 if the condition is met or -EBUSY if it is still waiting. Any
other errno will be returned to the caller. If the timeout occurs
before the condition is met -ETIMEDOUT will be returned.

The parameters to the callback function are the comedi_device,
comedi_subdevice, and comedi_insn pointers that were passed to the
(*insn_read) or (*insn_write) as well as an unsigned long, driver
specific, 'context' that can be used to pass any other information
that might be needed in the callback. This 'context' could be anything
such as the register offset to read the status or the bits needed
to check the status. The comedi_timeout() function itself does not
use any of these parameters.

This will help remove all the crazy "wait this many loops" used by
some of the drivers. It also creates a common errno for comedi to
detect when a timeout occurs.

ADC/DAC conversion times are typically pretty fast, usually around
100K samples/sec (10 usec). A conservative timeout of 1 second is used
in comedi_timeout().

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