Enable widgets to summarize alarm severities from multiple PVs

[shilorigins]

What's the problem this feature will solve?

Some displays need to summarize alarms from multiple PVs e.g. a button that opens a valve status panel will show the highest alarm present in any of the valves. The current workaround is to create a drawing for each alarm state and use visibility rules with calc plugins to show the correct alarm. This involves additional work to set up, is less intuitive than using channels directly, and obstructs common design patterns like layouts.

Describe the solution you'd like

Ideally, I would be able to set a widget's alarm severity using a calc plugin such as calc://summary_alarm?A=PV1.SEVR&B=PV2.SEVR&expr=max(A, B). Maybe widgets could have an additional property called e.g. color channel that accepts the example calc. Maybe a new plugin type could take a list of PVs and summarize their alarms internally, in a way that's compatible with channel's current alarm support. The exact interface is less important than ease of use.

Additional context

[mattgibbs]

Hi @shilorigins,

I think in most cases at SLAC, the best thing to do is use the alarm handler and create a new alarm summary PV, rather than handling this at the display layer. This is more work up front (editing the alarm tree files, restarting the alarm IOC), but there are several advantages:

• The alarm summary PV can in turn be used in alarm trees one level up from your display, and eventually propagate to the top-level alarms (like the boxes on LCLSHome).
• Handling alarm bypasses is more robust with an alarm summary PV, and we have infrastructure in place to help track what is bypassed and why. (This could use improvement, but is better than nothing)
• Other displays can trivially use the alarm summary PV if they have links to the same display, and don't have to re-invent their own summary.
• The display doesn't need to connect to additional PVs just to calculate the summary - that is handled in an IOC instead. This makes the display faster, and lowers the load on client CPUs as well as the network.

[shilorigins]

Got it, thanks for the explanation!