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SubjectRE: [PATCH RESEND] iwlwifi, Do not implement thermal zone unless ucode is loaded
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> 
> Prarit Bhargava <prarit@redhat.com> writes:
>
> > On 07/13/2016 03:24 AM, Luca Coelho wrote:
> >
> >> I totally agree with Emmanuel and Kalle. We should not change this.
> >> It is a design decision to return an error when the interface is
> >> down, this is very common with other subsystems as well.
> >
> > Please show me another subsystem or driver that does this. I've
> > looked around the kernel but cannot find one that updates the firmware
> > and implements new features on the fly like this. I have come across
> > several drivers that allow for an update, but they do not implement
> > new features based on the firmware.
> >
> > Additionally, what happens when someone back revs firmware versions
> > (which happens far more than you and I would expect)? Does that mean
> > I now go from a functional system to a non-functional system wrt to
> userspace?
>
> I'm not following, what do you mean exactly? Why are you talking updating
> the firmware?
>
> So when we talk about "loading firmware" we mean that the driver pushes
> the firmware image to to the chipset. And then the interface is down the
> chipset is powered down and the RAM on it will be erased. That's the general
> idea anyway, I haven't checked how iwlwifi exactly works in this case but
> Luca or Emmanuel can correct me.

This is correct.


>
> >> The userspace should be able to handle errors and report something
> >> like "unavailable" when this kind of error is returned.
> >
> > I myself have made the same arguments wrt to cpufreq code & bad
> > userspace choices. I just went through this a few months back with
> > what went from a simple patch and turned out to be a hideous patch in
> > cpufreq. You cannot break userspace like this.
>
> Don't get me wrong, I'm a strong supporter of stable user space interfaces
> and I always try to adher to that. But there's a limit for everything. If I'm
> understanding correctly, what you mean is that the kernel should never
> return an error because an application doesn't handle errors gracefully.
> Sorry, but that doesn't make sense to me.
>
> > See commit 51443fbf3d2c ("cpufreq: intel_pstate: Fix intel_pstate
> > powersave min_perf_pct value"). What should have been a trivial
> > change resulted in a massive change because of broken userspace.
>
> In that cpufreq case I understand, it was about a combination of
> configuration values which broke the user space. But here we are just
> dealing with a simple error value, nothing fancy.
>
> >> I'm not sure EIO is the best we can have, but for me that's exactly
> >> what it is. The thermal zone *is* there, but cannot be accessed
> >> because the firmware is not available. I'm okay to change it to
> >> EBUSY, if that would help userspace, but I think that's a bit
> >> misleading. The device is not busy, on the contrary, it's not even running
> at all.
> >>
> >
> > I understand that, but by returning -EIO we end up with an error.
> >
> >> Furthermore, I don't think this is "breaking userspace" in the sense
> >> of being a regression.
> >
> > I run (let's say 4.5 kernel). sensors works. I update to 4.7.
> > sensors doesn't work. How is that not a regression? That's _exactly_
> > what it should be reported as.
>
> Sure, it's a regression in a way. But that's how the user space app you are
> using is implemented, the same problem would happen with any driver
> returning errors.
>
> >> The userspace API has always been implemented with the possibility of
> >> returning errors. It's not a good design if a single device
> >> returning an error causes all the other devices to also fail.
> >>
> >
> > If that were the case we would never have to worry about "breaking
> userspace"?
> > For any kernel change I could just say that the userspace design was
> > bad and be done with it. Why fix anything then?
>
> Because we are talking about a simple error value.
>
> > I don't see any harm in waiting to register the sysfs files for hwmon
> > until the firmware has been validated.
>
> I'm against of that because it's bad software design. It's standard practise in
> Linux that drivers register their capabilities during driver probe time so that
> user space can query them whenever needed. I assume a properly behaving
> user space app would want to know about all the available sensors once the
> driver is initialised and your suggestion would break that.
>
> > IIUC, the up/down'ing of the device doesn't happen that often (during
> > initial boot, and suspend/resume, switching wifi connections,
> > shutdown?).
>
> Basically it can happen anytime, this is fully controlled by user space.
> There's no point of trying to make any assumptions as they won't hold
> anyway.
>
> > This would make the iwlwifi community happy (IMO) and sensors would
> > still work. At the same time I could write a patch for lm-sensors to
> > fix this issue if it comes up in future versions.
> > [Aside: I'm going to have the reproducing system available today and
> > will test this out. It looks like just moving some code around.]
>
> Another option, but still a bad one I don't like, is that you change the kernel
> interface to ignore all errors from drivers (like iwlwifi). This way drivers don't
> need to make ugly workarounds.
>
> > The bottom line is that lm-sensors is currently broken with this
> > change in iwlwifi. AFAICT, no other thermal device returns an error
> > this way, and IMO that means the iwlwifi driver is doing something new
> > and unexpected wrt to userspace.
>
> I haven't checked but I suspect ath10k has a similar problem when interface
> is down.
>
> --
> Kalle Valo

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