staging: comedi: check for more errors for zero-length read
If the "read" file operation handler, `comedi_read()` is passed 0 for the amount to read, some error conditions are currently skipped and the function just returns 0. Change it to check those error conditions and return an error value if appropriate. The trickiest case is the check for when the previously set up asynchronous command has terminated with an error. In that case, `-EPIPE` is returned (as it is for a read of non-zero length) and the subdevice gets marked as non-busy. A zero-length read that returns 0 has no other effects, in particular, it does not cause the subdevice to be marked as non-busy, and the return value does not indicate an "end-of-file" condition. 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>
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