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authorJason Wessel <jason.wessel@windriver.com>2019-08-29 07:00:31 -0700
committerRichard Purdie <richard.purdie@linuxfoundation.org>2019-08-31 08:44:19 +0100
commitf0f359ec9210759f6b4dbfb35d3fba8af208c43a (patch)
tree86d1aa33871e28129bb948e430510db06108154e /meta/recipes-devtools/build-compare
parent7a3b36401f86afa13426b302ce31bbd20dd871be (diff)
serial-getty@.service: Allow device to fast fail if it does not exist
Some BSPs use a USB serial port which may or may not actually be plugged all the time. It is quite useful to have a USB serial port have a getty running but it does not make sense to wait for it for 90 seconds before completing the system startup if it might never get plugged in. The typical example is that a USB serial device might only need to be plugged in when debugging, upgrading, or initially configuring a device. This change is somewhat subtle. Systemd uses the "BindsTo" directive to ensure existence of the device in order to start the service as well as to terminate the service if the device goes away. The "After" directive makes that same relationship stronger. When used together this has the undesired side effect that systemd will wait until its internal time out value of 90 seconds for the device to come on line before executing a fail operation or letting other tasks and groups continue. This is certainly the kind of behavior we want for a disk, but not for serial ports in general. The "BindsTo" directive is replaced by the combination of the "PartOf" and the "ConditionPathExists" directives. The "After" directive is unchanged because that will wait for the udev rules to process. The "PartOf" directive will issue a stop to the getty service if the device goes away, similar to the "BindsTo" directive. The "ConditionPathExists" is what allows the service to fail fast vs waiting for the timeout. When a USB device is not plugged in at boot you would find a message in the system journal like: systemd[1]: Condition check resulted in Serial Getty on \ ttyUSB0 being skipped. If you want to observe the problem with qemu, it is easy to replicate. Simply add the following line to your local.conf for a x86-64 qemu build. SERIAL_CONSOLES="115200;ttyS0 115200;ttyUSB0" Login right after the system boots and observe: root@qemux86-64:~# systemctl list-jobs |cat JOB UNIT TYPE STATE 1 multi-user.target start waiting 69 serial-getty@ttyUSB0.service start waiting 64 getty.target start waiting 71 dev-ttyUSB0.device start running 62 systemd-update-utmp-runlevel.service start waiting 5 jobs listed. You can see above that the dev-ttyUSB0.device will block for 1min 30 seconds. While that might not be a problem for this reference build. It is certainly a problem for images that have software watchdogs that verify the system booted up all the way to systemd completion in less than 90 seconds. This other nice effect of this change is that the fast fail device extend to additional serial ports that may not exist on ARM BSPs or that might be configured in or out by the dtb files on different boards. Signed-off-by: Jason Wessel <jason.wessel@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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