linux-kernel - Re: [PATCH 02/13] driver-core: defer all probes until late

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Date:	Fri, 19 Jun 2015 15:36:46 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Alexander Holler <holler@...oftware.de>,
	Alexandre Courbot <gnurou@...il.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Kumar Gala <galak@...eaurora.org>, Len Brown <lenb@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lv Zheng <lv.zheng@...el.com>, Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Robert Moore <robert.moore@...el.com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	Terje Bergström <tbergstrom@...dia.com>,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 02/13] driver-core: defer all probes until late_initcall

On 18 June 2015 at 23:50, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Wednesday, June 17, 2015 03:42:12 PM Tomeu Vizoso wrote:
>> To decrease the chances of devices deferring their probes because of
>> dependencies not having probed yet because of their drivers not having
>> registered yet, delay all probing until the late initcall level.
>>
>> This will allow us to avoid deferred probes completely later by probing
>> dependencies on demand, or by probing them in dependency order.
>>
>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
>> ---
>>  drivers/base/dd.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>> index a638bbb..18438aa 100644
>> --- a/drivers/base/dd.c
>> +++ b/drivers/base/dd.c
>> @@ -407,6 +407,12 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
>>       if (!device_is_registered(dev))
>>               return -ENODEV;
>>
>> +     /* Defer all probes until we start processing the queue */
>> +     if (!driver_deferred_probe_enable) {
>> +             driver_deferred_probe_add(dev);
>
> Do I think correctly that this will effectively force everybody to use deferred
> probing?

Guess it depends on the meaning of "using deferred probing". It will
defer the probe of the first device to late_initcall (which will
happen much earlier in time than before), but afterwards all built-in
drivers will be available and depending on the order in which we try
to probe devices, none may actually ask to defer its probe.

But what this patch achieves has nothing to do with drivers returning
-EPROBE_DEFER, it just delays device probe until all built-in drivers
have been registered.

I could have avoided reusing any of the deferred probe code by
creating a new queue of devices that need probing, and by registering
a new late_initcall to start processing them, but because that is
always enabled unconditionally, it seemed silly to not reuse that code
that already does exactly that.

Thanks,

Tomeu

>> +             return 0;
>> +     }
>> +
>>       pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
>>                drv->bus->name, __func__, dev_name(dev), drv->name);
>>
>> @@ -585,7 +591,7 @@ EXPORT_SYMBOL_GPL(device_attach);
>>
>>  void device_initial_probe(struct device *dev)
>>  {
>> -     __device_attach(dev, true);
>> +     __device_attach(dev, driver_deferred_probe_enable);
>>  }
>>
>>  static int __driver_attach(struct device *dev, void *data)
>>
>
> --
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
> --
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