The log message "Unable to do two-sided printing" comes from the "ipp"
CUPS backend, part of CUPS. It seems that the backend does not find the
"sides" attribute in the printer's IPP attributes.
See the code here:
----------
if (ipp_status == IPP_STATUS_OK_IGNORED_OR_SUBSTITUTED ||
ipp_status == IPP_STATUS
_OK_CONFLICTING)
{
/*
* One or more options are not supported...
*/
if (ippFindAttribute(response, "sides", IPP_TAG_ZERO))
{
/*
* The sides value is not supported, revert to one-sided as
needed...
*/
const char *sides = cupsGetOption("sides", num_options, options);
if (!sides || !strncmp(sides, "two-sided-", 10))
{
fputs("DEBUG: Unable to do two-sided printing, setting sides
to 'one-sided'.\n", stderr);
num_options = cupsAddOption("sides", "one-sided",
num_options, &options);
}
}
----------
