+EXPORT_SYMBOL(is_dmabuf_sync_supported);
_GPL ?
I would also prefix it with 'dmabuf_is_sync_supported' just to make all of the libraries call start with 'dmabuf'
Seems better. Will change it to dmabuf_is_sync_supported, and use EXPORT_SYMBOL_GPL.
One thing thought - while I suggest that you use GPL variant I think you should check who the consumers are. As in, if nvidia wants to use it it might make their lawyers unhappy - and in turn means that their engineers won't be able to use these symbols.
So - if there is a strong argument to not have it GPL - then please say so.