On 10. 12. 20, 19:59, Mychaela Falconia wrote:
O_DIRECT is an interesting hack, has anyone seen if it violates the
posix rules for us to use it on a character device like this?
According to open(2) Linux man page, O_DIRECT does not come from POSIX
at all, instead it is specific to Linux, FreeBSD and SGI IRIX. Thus
it seems like there aren't any POSIX rules to be violated here.
If we go with O_DIRECT, what semantics are we going to implement?
There are 3 possibilities that come to mind most readily:
1) O_DIRECT applies only to the open call in which this flag is set,
and suppresses DTR/RTS assertion on that open. If someone needs to do
multiple opens with DTR/RTS suppression being required every time,
then they need to include O_DIRECT every time.
2) O_DIRECT applies not only immediately, but also sets a latched flag
whereby all subsequent opens continue to suppress auto-assertion
without requiring O_DIRECT every time. This approach by itself runs
counter to the generic Unix way of doing things, but it may be OK if
there is also some ioctl to explicitly set or clear the latched flag.
3) O_DIRECT applies only to the open call in which it is set, no
built-in latching, but there is also some ioctl to control a flag
enabling or disabling DTR/RTS auto-assertion on subsequent opens.
3) -- to allow standard tools to work on the device after the quirk is
set up once.
thanks,
--
js
