Re: [gpsd-dev] [PATCH] Fixes various flakiness in PPS handling.

[Fred Wright]

On Mon, 26 Sep 2016, Gary E. Miller wrote:
> On Sat, 24 Sep 2016 16:23:45 -0700 (PDT)
> Fred Wright <address@hidden> wrote:
>
> > On Wed, 21 Sep 2016, Fred Wright wrote:
> >
> > [...]
> >
> > It looks like this one fell through a crack.
>
> Which one?  Can you just resend it?

I just did.  I also BCCed you on it, so you should have received a copy
that bypassed the listserver.  And it's also an attachment here, though
there's something fishy about why that should be necessary.

> > Note in particular that the MAGIC_HAT/MAGIC_LINK hack is not only
> > kludgy, but also a complete disaster when used with a kernel that's
> > built with CONFIG_PPS_CLIENT_KTIMER=y, since the latter
> > causes /dev/pps0 to be the fake software-generated PPS source, which
> > doesn't even run at quite 1Hz.
>
> It solves a real problem for RasPi and BeagleBone users.  Any change
> now would break a lot of HOWTO's and existing systems.  I'd consider
> a patch to implement an opt-out option.

You still haven't answered the question of what that "real problem" was,
and neither the comments nor the commit message provide any documentation.

It certainly never helped Beaglebone users using the only off-the-shelf
GPS cape that's available, since it's incompatible with the kernel
provided by the vendor.

An opt-out approach makes sense as an interim fix that doesn't break
existing setups, but everyone should be strongly encouraged to opt out and
stop depending on it.  Any HOWTO needs to include a section on identifying
the correct PPS device, since it can't be assumed to be /dev/pps0.  Once
that determination has been made, the subsequent instructions can proceed
in one of two ways, depending on whether the MAGIC_HAT/MAGIC_LINK kludge
is present:

With MAGIC_HAT:

1) If your GPS serial port is named something other than /dev/ttyAMA0 or
/dev/gpsd0, then include the name of your PPS device on the command line
in addition to the serial device name.

2) If your GPS serial port is named /dev/ttyAMA0 or /dev/gpsd0 and your
GPS PPS device is /dev/pps0, then specify just the serial device on the
command line, and gpsd will add /dev/pps0 automatically.

3) If your GPS serial port is named /dev/ttyAMA0 or /dev/gpsd0 and your
GPS PPS device is something other than /dev/pps0, then rebuild gpsd with
MAGIC_HAT disabled, or create a symlink to the GPS serial port that's
named something other than /dev/ttyAMA0 or /dev/gpsd0 and use that name
instead, and specify both the serial device and the PPS device on the
command line.

Without MAGIC_HAT:

Specify both the serial device and the PPS device for your receiver on the
command line.

Fred Wright

0001-Fixes-various-flakiness-in-PPS-handling.patch
Description: Text document