Re: Alternative approach to solve the deferred probe

[Frank Rowand]

On 10/20/2015 8:46 AM, Russell King - ARM Linux wrote:
> On Mon, Oct 19, 2015 at 06:21:40PM +0200, Geert Uytterhoeven wrote:
>> Hi Russell,
>>
>> On Mon, Oct 19, 2015 at 5:35 PM, Russell King - ARM Linux
>> <linux@xxxxxxxxxxxxxxxx> wrote:
>>>>> What you can do is print those devices which have failed to probe at
>>>>> late_initcall() time - possibly augmenting that with reports from
>>>>> subsystems showing what resources are not available, but that's only
>>>>> a guide, because of the "it might or might not be in a kernel module"
>>>>> problem.
>>>>
>>>> Well, adding those reports would give you a changelog similar to the
>>>> one in this series...
>>>
>>> I'm not sure about that, because what I was thinking of is adding
>>> a flag which would be set at late_initcall() time prior to running
>>> a final round of deferred device probing.
>>
>> Which round is the final round?
>> That's the one which didn't manage to bind any new devices to drivers,
>> which is something you only know _after_ the round has been run.
>>
>> So I think we need one extra round to handle this.
>>
>>> This flag would then be used in a deferred_warn() printk function
>>> which would normally be silent, but when this flag is set, it would
>>> print the reason for the deferral - and this would replace (or be
>>> added) to the subsystems and drivers which return -EPROBE_DEFER.
>>>
>>> That has the effect of hiding all the deferrals up until just before
>>> launching into userspace, which should then acomplish two things -
>>> firstly, getting rid of the rather useless deferred messages up to
>>> that point, and secondly printing the reason why the remaining
>>> deferrals are happening.
>>>
>>> That should be a small number of new lines plus a one-line change
>>> in subsystems and drivers.
>>
>> Apart from the extra round we probably can't get rid of, that sounds OK to me.
> 
> Something like this.  I haven't put a lot of effort into it to change all
> the places which return an -EPROBE_DEFER, and it also looks like we need
> some helpers to report when we have only an device_node (or should that
> be fwnode?)  See the commented out of_warn_deferred() in
> drivers/gpio/gpiolib-of.c.  Adding this stuff in the subsystems searching
> for resources should make debugging why things are getting deferred easier.
> 
> We could make driver_deferred_probe_report something that can be
> deactivated again after the last deferred probe run, and provide the
> user with a knob that they can turn it back on again.
> 
> I've tried this out on two of my platforms, including forcing
> driver_deferred_probe_report to be enabled, and I get exactly one
> deferred probe, so not a particularly good test.
> 
> The patch won't apply as-is to mainline for all files; it's based on my
> tree which has some 360 additional patches (which seems to be about
> normal for my tree now.)

I like the concept (I have been thinking along similar lines lately).
But I think this might make the console messages more confusing than
they are now.  The problem is that debug, warn, and error messages
come from a somewhat random set of locations at the moment.  Some
come from the driver probe routines and some come from the subsystems
that the probe routines call.  So the patch is suppressing some
messages, but not others.

One thing that seemed pretty obvious from the patches is that the
current probe routines are somewhat inconsistent in terms of messages,
and that there is room for a set of best practices for messaging.  That
is on my long term wish list, but I'm not sure I'll ever chase after
those windmills.

A couple of specific comments below.


> 
>  drivers/base/dd.c                       | 29 +++++++++++++++++++++++++++++
>  drivers/base/power/domain.c             |  7 +++++--
>  drivers/clk/clkdev.c                    |  9 ++++++++-
>  drivers/gpio/gpiolib-of.c               |  5 +++++
>  drivers/gpu/drm/bridge/dw_hdmi.c        |  2 +-
>  drivers/gpu/drm/exynos/exynos_drm_dsi.c |  2 +-
>  drivers/gpu/drm/imx/imx-ldb.c           |  5 +++--
>  drivers/gpu/drm/msm/dsi/dsi.c           |  2 +-
>  drivers/gpu/drm/msm/msm_drv.c           |  3 ++-
>  drivers/gpu/drm/rcar-du/rcar_du_crtc.c  |  3 ++-
>  drivers/of/irq.c                        |  5 ++++-
>  drivers/pci/host/pci-mvebu.c            |  1 +
>  drivers/pinctrl/core.c                  |  5 +++--
>  drivers/pinctrl/devicetree.c            |  4 ++--
>  drivers/regulator/core.c                |  5 +++--
>  include/linux/device.h                  |  1 +
>  16 files changed, 71 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index be0eb4639128..bb12224f2901 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -129,7 +129,29 @@ void driver_deferred_probe_del(struct device *dev)
>  	mutex_unlock(&deferred_probe_mutex);
>  }
>  
> +static bool driver_deferred_probe_report;
> +
> +/**
> + * dev_warn_deferred() - report why a probe has been deferred
> + */
> +void dev_warn_deferred(struct device *dev, const char *fmt, ...)
> +{
> +	if (driver_deferred_probe_report) {
> +		struct va_format vaf;
> +		va_list ap;
> +
> +		va_start(ap, fmt);
> +		vaf.fmt = fmt;
> +		vaf.va = &ap;
> +
> +		dev_warn(dev, "deferring probe: %pV", &vaf);
> +		va_end(ap);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(dev_warn_deferred);

The places where dev_warn_deferred() replaces dev_dbg(), we lose the
ability to turn on debugging and observe the driver reporting the
specific reason the deferral is occurring. So it would be useful to
add an "else dev_dbg()" in dev_warn_deferred() to retain that capability.

> +
>  static bool driver_deferred_probe_enable = false;
> +
>  /**
>   * driver_deferred_probe_trigger() - Kick off re-probing deferred devices
>   *
> @@ -188,6 +210,13 @@ static int deferred_probe_initcall(void)
>  	driver_deferred_probe_trigger();

Couldn't you put the "driver_deferred_probe_report = true" here?  And then
not add another round of probes.


>  	/* Sort as many dependencies as possible before exiting initcalls */
>  	flush_workqueue(deferred_wq);
> +
> +	/* Now one final round, reporting any devices that remain deferred */
> +	driver_deferred_probe_report = true;
> +	driver_deferred_probe_trigger();
> +	/* Sort as many dependencies as possible before exiting initcalls */
> +	flush_workqueue(deferred_wq);
> +
>  	return 0;
>  }

< snip >

-Frank

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