Re: [PATCH v3 2/3] drm/bridge: parade-ps8640: Use regmap APIs

[Stephen Boyd]

TL;DR: Please try to reduce these error messages in drivers and
consolidate them into subsystems so that drivers stay simple.

Quoting Doug Anderson (2021-09-15 09:41:39)
> Hi,
>
> On Tue, Sep 14, 2021 at 7:50 PM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote:
> >
> >
> > I'd rather see any sort of error message in getter APIs be pushed into
> > the callee so that we reduce the text size of the kernel by having one
> > message instead of hundreds/thousands about "failure to get something".
> > As far as I can tell this API is designed to skip printing anything when
> > EPROBE_DEFER is returned, and only print something when it isn't that
> > particular error code. The other benefit of this API is it sets the
> > deferred reason in debugfs which is nice to know why some device failed
> > to probe. Of course now with fw_devlink that almost never triggers so
> > the feature is becoming useless.
>
> I guess we need to split this apart into two issues. One (1) is
> whether we should be printing errors like this in probe() and the
> other (2) is the use of dev_err_probe() for cases where err could
> never be -EPROBE_DEFER.
>
> So the argument about reducing the text size for thousands of slightly
> different errors is all about (1), right? In other words, you'd be
> equally opposed to a change that added a normal error print with
> dev_err(), right? IMO, this is a fair debate to have and it comes down
> to a choice that has pros and cons. Yes the error messages are not
> needed in the normal case and yes they bloat the kernel size, but when
> something inevitably goes wrong then you have a way to track it down
> instead of trying to guess or having to recompile the code to add
> prints everywhere. Often this can give you a quick clue about a
> missing Kconfig or a wrongly coded device tree file without tons of
> time adding prints and recompiling code. That seems like it's worth
> something...

Agreed. dev_err_probe() does that by putting that into the deferred
reason debugfs file. I'm saying that drivers shouldn't really be using
this API unless they're doing something exotic. The subsystems that are
implementing the 'get' operation that may defer should use this function
and then drivers should just return the error value to driver core so
that we can consolidate error messages and shrink the kernel size.

Maybe we can look for the defer reason in call_driver_probe() and print
a warning message if the string is set. Right now -EPROBE_DEFER is
handled but it's a dev_dbg() print that probably nobody enables and it
doesn't print the reason string.

Even better, we could make the defer reason the 'probe failed reason'
instead, and then jam the dev_err_probe() string into there regardless
of EPROBE_DEFER being returned or not. This would elevate this API to
any sort of device probe error. One more crazy idea is that we could
save the stack when the dev_err_probe() call is made and print out the
stacktrace when the error string is printed in driver core. I'm not sure
this is any better than making it a WARN_ON() though.

>
> One could also make the argument that if you don't care about all
> these similar errors bloating the text segment that it would be pretty
> easy to create a new Kconfig: "CONFIG_I_THINK_PROBE_ERRORS_ARE_BLOAT".
> If that config is set then it could throw away the strings for every
> dev_err_probe() that you compile in.

I'll leave this little CONFIG_PRINTK=n sledgehammer here.

>
>
> I'm not so convinced about the argument (2) that dev_err_probe()
> should only be used if the error code could be -EPROBE_DEFER. Compare
> these two:
>
> Old:
>   ret = do_something_that_cant_defer();
>   if (ret < 0) {
>     dev_err(dev, "The foo failed to bar (%pe)\n", ERR_PTR(ret));
>     return ret;
>   }
>
> New:
>   ret = do_something_that_cant_defer();
>   if (ret < 0)
>     return dev_err_probe(dev, ret, "The foo failed to bar\n");
>
> It seems clear to me that the "New" case is better. The error code is
> printed in a consistent fashion compared to all other error prints and
> the fact that it returns the error code makes it cleaner. It's fine
> that the error could never be -EPROBE_DEFER. Certainly we could add a
> new function called dev_err_with_code() that worked exactly like
> dev_err_probe() except that it didn't have special logic for
> -EPROBE_DEFER but why?
>
> Also note that the current function is dev_err_probe(), not
> dev_err_might_defer(). By the name, it should be useful / OK to use
> for any errors that come up in the probe path.

I looked at the documentation for dev_err_probe()

 * This helper implements common pattern present in probe functions for error
 * checking: print debug or error message depending if the error value is
 * -EPROBE_DEFER and propagate error upwards.
 * In case of -EPROBE_DEFER it sets also defer probe reason, which can be
 * checked later by reading devices_deferred debugfs attribute.

This seems to imply that it's all about EPROBE_DEFER. I'm just
reconstructing what I read from kernel-doc. If the intent is to use it
outside of probe defer, then please update the documentation to
alleviate confusion.

>
>
> > > Is there some bad thing about dev_err_probe() that makes it
> > > problematic to use? If not then the above advantages should be a net
> > > win, right?
> > >
> >
> > I view it as an anti-pattern. We should strive for driver probe to be
> > fairly simple so that it's basically getting resources and registering
> > with frameworks. The error messages in probe may help when you're trying
> > to get the driver to work and the resource APIs don't make any sense but
> > after that it's basically debug messages hiding as error messages.
> > They're never supposed to happen in practice, because the code is
> > tested, right?
>
> IMO they happen even after initial driver bringup. You can trip error
> cases from device tree problems and config problems pretty easily. It
> could also be that you're bringing up an old / tested / tried and true
> driver but on new hardware where some other thing (clock, regulators,
> etc) is returning an error. Being able to track these down easily can
> justify the error messages long term.
>
> ...or maybe what you're saying is that if it's clear that the only
> case that an error could be returned is due to a driver error then we
> should skip the error message? I guess, so, but only if it's somehow
> built-in to the concept of the function that the only error case is a
> driver error. Otherwise the function may change to check for more
> errors in the future and you're back to where you started with.

I didn't really follow this paragraph, sorry.

>
> In the case of devm_regmap_init_i2c(), the driver could be fine but
> you might be trying to instantiate it on a system whose i2c bus lacks
> the needed functionality. That's not a bug in the bridge driver but an
> error in system integration. Yeah, after bringup of the new system you
> probably don't need the error, but it will be useful during people's
> bringups year after year.
>

The point I'm trying to make is that these error messages in probe
almost never get printed after the driver is brought up on the hardware
that starts shipping out to non-kernel developers. Of course they happen
when kernel devs are enabling new hardware year after year on the same
tried and tested driver. They're worthwhile messages to have to make our
lives easier at figuring out some misconfiguration, etc. The problem is
they lead to bloat once the bringup/configuration phase is over.

At one point we directed driver authors at dev_dbg() for these prints so
that the strings would be removed from the kernel image if debugging
wasn't enabled. It looks like dev_err_probe() goes in the opposite
direction by printing an error message and passing the string to an
exported function, so dev_dbg() won't reduce the image size. Ugh!