Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open

From: Maarten Brock
Date: Fri Dec 11 2020 - 10:53:45 EST

On 2020-12-10 19:59, Mychaela Falconia wrote:
Maarten Brock wrote:

I agree. And an application not configuring the required handshakes, but
still relying on them is an equal bug.

This comment can be interpreted in at least two different ways. Are
you referring to:

1) Mainstream existing applications that expect DTR and/or RTS to be
asserted on open without doing any explicit TIOCMBIS,

or

2) My fc-host-tools programs (fc-loadtool, fc-xram, rvinterf etc) that
operate on the second UART of my DUART28C adapter, expect to NOT have
auto-assertion of DTR/RTS on open, but rely on my custom USB ID and
the ftdi_sio driver patch that goes with it to suppress this auto-
assertion, without doing any explicit TIOCMBIC.

If you are referring to 1, it is difficult to fault the authors of
those applications, as they had every right to depend on the behaviour
that had been codified in numerous official standards like POSIX. Or
if you are referring to 2, what other choice do I have? With existing
unpatched Linux kernels of every currently existing version, it is
*impossible* to prevent DTR & RTS auto-assertion immediately on open
of a tty device, thus applying a patch to the kernel (or at least to
the ftdi_sio driver in my case) is the *only* way to make my hardware
work with Linux. Doing a TIOCMBIC after open won't help, as it will
be too late if the kernel already asserted DTR & RTS and thus caused
an unwanted deep reset.

I was referring to 1) And I still think that an application that *relies*
on handshakes working should configure the handshakes, even if POSIX
promises they should be set up.

Any device with a classic old-fashioned RS-232 has probably already
solved this in another way or is accepted as not working on Linux.

If someone built such a device for their own personal enjoyment rather
than for commercial sale, and needed to get it working with Linux, I
suspect that person most likely applied a local patch to the kernel
on their own system, likely implementing something similar to what is
being discussed in this thread.

A person might have done that, a company will probably just not support
Linux or do a redesign with a different solution.

Personally, I would prefer the VID:PID to enforce the quirk and an
O_DIRECT (or other) flag used on open() as general backup plan. To
me a sysfs solution seems illogical.

A sysfs solution could work as a sort of poor man's substitute for a
VID:PID-driven quirk: instead of a driver quirk in the kernel, there
is a udev rule that detects a particular USB-serial device (perhaps
based on textual manuf/product strings as opposed to VID:PID) and sets
the needed sysfs flag. But then if we are talking about a special
USB device as opposed to generic serial as in RS-232 etc, then I argue
for a driver quirk: if the device has a custom VID:PID, a patch to the
driver is needed in any case just to recognize that custom ID, so why
not support the custom hw device properly by setting the quirk bit
right there and then? Seen in this light, the sysfs approach indeed
makes little sense.

OTOH, if we are talking about RS-232 or similarly interfaced devices
which the user plugs into any random serial port (PC native, or a
random off-the-shelf USB-serial cable), then there is really nothing
that a udev rule can key onto, it is just a user plugging in some
serial device and then running custom userspace apps on it. In this
case asking the user to 'echo' something from the shell into /sys/blah
prior to running her userspace app seems illogical indeed, and asking
userspace app programmers to implement an equivalent sysfs write
internally is equally awkward. For this non-custom-USB-ID scenario I
thus agree that the O_DIRECT approach would be better - in this case
the userspace app programmer simply needs to add this one flag to
their open call, a trivial one line change.

Or use your option 3) mentioned below: open with O_DIRECT, use ioctl to
set the sticky flag and close before starting the application.

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.

My vote would be to implement 1 first, for reasons of utmost
simplicity, both interface and implementation, and then possibly
implement 3 at some later time if and when someone actually needs that
extra bit of complexity.

A strong argument can be made that if someone repurposed DTR and/or
RTS signals on a serial port for some very different non-standard
purpose, then they will almost certainly be writing their own custom
userspace sw to talk to that serial port, as opposed to using standard
existing userspace sw, hence asking them to include a non-standard
flag on every open of their serial port shouldn't be too burdensome.

M~

Option 3) looks best to me.

Maarten