On 4/13/22 09:51, Scott Cheloha wrote:
This series adds a driver for PAPR hypercall-based watchdog timers, tentatively named "pseries-wdt". I wanted to get some clarification on a few things before submitting the series as a patch, hence the RFC. The first patch adding the hypercall to hvcall.h is straightforward, but I have questions about the second patch (the driver). In particular: - In pseries_wdt_probe() we register the watchdog device with devm_watchdog_register_device(). However, in pseries_wdt_remove(), calling watchdog_unregister_devce() causes a kernel panic later, so I assume this is the wrong thing to do.
The whole point of using devm_ functions is to handle cleanup (or removal) automatically. I would suggest to make yourself familiar with the concept.
Do we need to do anything to clean up the watchdog device during pseries_wdt_remove()? Or does devm_watchdog_register_device() ensure the cleanup is handled transparently? - In pseries_wdt_probe(), is it incorrect to devm_kfree() my allocation in the event that devm_watchdog_register_device() fails?
No. Same thing.
- The enormous hypercall input/output comment is mostly for my edification. It seems like the sort of thing that will rot over time. I intend to remove most of it. However, as far as I know the PAPR revision containing these details is not published yet. Should I leave the comment in to ease review for now and remove it later? Or should I omit it from the initial commit entirely? - Should we print something to the console when probing/removing the watchdog0 device or is that just noise?
It is just noise, but some developers insist on it.
Most drivers (as distinct from devices) seem to print something during initialization, so that's what I've done in pseries_wdt_module_init() when the capability query succeeds.
No. If you have to print something, print it during probe. module init noise is even worse. And those error messages in the init function are completely unacceptable. Anyway, doesn't pseries support devicetree ? Why is this driver not instantiated through a devicetree node ? Guenter
- The timeout action is currently hardcoded to a hard reset. This could be made configurable through a module parameter. I intend to do this in a later patch unless someone needs it included in the initial patch. - We set EIO if the hypercall fails in pseries_wdt_start() or pseries_wdt_stop(). There is nothing userspace can do if this happens. All hypercall failures in these contexts are unexpected. Given all of that, is there is a more appropriate errno than EIO? - The H_WATCHDOG spec indicates that H_BUSY is possible. Is it probable, though? Should we spin and retry the hypercall in the event that we see it? Or is that pointless?
