Subject: Re: [PATCH v2 12/15] ath10k: use new module_firmware_crashed()

From: Luis Chamberlain <[email-masked]>
Date: Tue, 19 May 2020 14:02:12 +0000

On Mon, May 18, 2020 at 06:23:33PM -0700, Brian Norris wrote:
> On Sat, May 16, 2020 at 6:51 AM Johannes Berg <[email-masked]> wrote:
> > In addition, look what we have in iwl_trans_pcie_removal_wk(). If we
> > detect that the device is really wedged enough that the only way we can
> > still try to recover is by completely unbinding the driver from it, then
> > we give userspace a uevent for that. I don't remember exactly how and
> > where that gets used (ChromeOS) though, but it'd be nice to have that
> > sort of thing as part of the infrastructure, in a sort of two-level
> > notification?
> 
> <slight side track>
> We use this on certain devices where we know the underlying hardware
> has design issues that may lead to device failure 

Ah, after reading below I see you meant for iwlwifi.

If userspace can indeed grow to support this, that would be fantastic.

I should note that I don't discourage hiding firmware or hardware
issues. Quite the contrary, I suspect that taking pride in being
trasnparent about it, and dealing with it fast can help lead the pack.
I wrote about this long ago in 2015 [0], and stand by it.

[0] https://www.do-not-panic.com/2015/04/god-complex-why-open-models-will-win.html

> -- then when we see
> this sort of unrecoverable "firmware-death", we remove the
> device[*]+driver, force-reset the PCI device (SBR), and try to
> reload/reattach the driver. This all happens by way of a udev rule.

So you've sprikled your own udev event here as part of your kernel delta?

> We
> also log this sort of stuff (and metrics around it) for bug reports
> and health statistics, since we really hope to not see this happen
> often.

Assuming perfection is ideal but silly. So, what infrastructure do you
use for this sort of issue?

> [*] "We" (user space) don't actually do this...it happens via the
> 'remove_when_gone' module parameter abomination found in iwlwifi.

Holy moly.. but hey, at least it may seem a bit more seemless than forcing
a reboot / manual driver removal / addition to the user.

BTW is this likely a place on iwlwifi where the firmware likely crashed?

> I'd
> personally rather see the EVENT=INACESSIBLE stuff on its own, and let
> user space deal with when and how to remove and reset the device. But
> I digress too much here ;)
> </slight side track>

This is all useful information. We are just touching the surface of the
topic by addressing networking first. Imagine when we address other
subsystems.

> I really came to this thread to say that I also love the idea of a
> generic mechanism (a la $subject) to report firmware crashes, but I
> also have no interest in seeing a taint flag for it. For Chrome OS, I
> would readily (as in, we're already looking at more-hacky /
> non-generic ways to do this for drivers we care about) process these
> kinds of stats as they happen, logging metrics for bug reports and/or
> for automated crash statistics, when we see a firmware crash.

Great!

> A uevent
> would suit us very well I think, although it would be nice if drivers
> could also supply some small amount of informative text along with it

A follow up to this series was to add a uevent to add_taint(), however
since a *count* is not considered I think it is correct to seek
alternatives at this point. The leaner the solution the better though.

Do you have a pointer to what guys use so I can read?

> (e.g., a sort of "reason code", in case we can possibly aggregate
> certain failure types). We already do this sort of thing for WARN()
> and friends (not via uevent, but via log parsing; at least it has nice
> "cut here" markers!).

Indeed, similar things can indeed be argued about WARN*()... this
however can be non-device specific. With panic-on-warn becoming a
"thing", the more important it becomes to really tally exactly *why*
these WARN*()s may trigger.

> Perhaps 

Note below.

> devlink (as proposed down-thread) would also fit the bill. I
> don't think sysfs alone would fit our needs, as we'd like to process
> these things as they happen, not only when a user submits a bug
> report.

I think we've reached a point where using "*Perhaps*" does not suffice,
and if there is already a *user* of similar desired infrastructure I
think we should jump on the opportunity to replace what you have with
something which could be used by other devices / subsystems which
require firmware. And indeed, also even consider in the abstract sense,
the possibility to leverage something like this for WARN*()s later too.

> > Level 1: firmware crashed, but we're recovering, at least mostly, and
> > it's more informational
> 
> Chrome OS would love to track these things too, since we'd like to see
> these minimized, even if they're usually recoverable ;)
> 
> > Level 2: device is wedged, going to try to recover by some more forceful
> > means (perhaps some devices can be power-cycled? etc.) but (more) state
> > would be lost in these cases?
> 
> And we'd definitely want to know about these. We already get this for
> the iwlwifi case described above, in a non-generic way.
> 
> In general, it's probably not that easy to tell the difference between
> 1 and 2, since even as you and Luis have noted, with the same driver
> (and the same driver location), you find the same crashes may or may
> not be recoverable. iwlwifi has extracted certain level 2 cases into
> iwl_trans_pcie_removal_wk(), but even iwlwifi doesn't know all the
> ways in which level 1 crashes actually lead to severe
> (non-recoverable) failure.

And that is fine, accepting these for what they are will help. However,
leaving the user in the *dark*, is what we should *not do*.

  Luis