Re: [PATCH v6 20/22] driver core: Allow deferring probes until late init

From: Tomeu Vizoso <tomeu.vizoso@collabora.com>
To: Rob Herring <robh@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Stephen Warren <swarren@wwwdotorg.org>,
	Javier Martinez Canillas <javier@osg.samsung.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mark Brown <broonie@kernel.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	linux-acpi@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v6 20/22] driver core: Allow deferring probes until late init
Date: Tue, 29 Sep 2015 10:05:38 +0200	[thread overview]
Message-ID: <CAAObsKBoBgP_y161qWC_U+yVtbWxudFgBNM7map0jz2WcJws_w@mail.gmail.com> (raw)
In-Reply-To: <5606E0DA.2040900@kernel.org>

On 26 September 2015 at 20:15, Rob Herring <robh@kernel.org> wrote:
> On 09/21/2015 09:03 AM, Tomeu Vizoso wrote:
>> Add a field to struct device that instructs the device-driver core to
>> defer the probe of this device until the late_initcall level.
>
> This is true until the next patch with moves deferred probe processing
> to device_initcall_sync. So disabling this option alone won't totally
> revert to current behaviour. I guess patch 21 could be reverted if
> necessary.

Actually, the goal with that commit was to prevent potential problems
due to the increased pressure on late_initcall, as suggested by
Grygorii Strashko, but I haven't found yet any evidence of it being
needed, and in my testing the series boot all boards in kernelci with
or without this commit. So I would just not commit it for now and only
consider applying it later if someone reports a problem.

>> By letting all built-in drivers to register before starting to probe, we
>> can avoid any deferred probes by probing dependencies on demand.
>>
>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
>>
>> ---
>>
>> Changes in v4:
>> - Add Kconfig DELAY_DEVICE_PROBES to allow disabling delayed probing in
>>   machines with initcalls that depend on devices probing at a given time.
>>
>>  drivers/base/Kconfig   | 18 ++++++++++++++++++
>>  drivers/base/dd.c      |  7 +++++++
>>  include/linux/device.h |  2 ++
>>  3 files changed, 27 insertions(+)
>>
>> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
>> index 98504ec99c7d..44b5d33b1f49 100644
>> --- a/drivers/base/Kconfig
>> +++ b/drivers/base/Kconfig
>> @@ -324,4 +324,22 @@ config CMA_ALIGNMENT
>>
>>  endif
>>
>> +config DELAY_DEVICE_PROBES
>> +     bool "Allow delaying the probe of some devices"
>
> I'd like to hide visibility of this behind EXPERT.

Done.

>> +     default y
>> +     help
>> +       Devices can be matched to a driver and probed from the moment they
>> +       are registered, but early during boot their probes are likely to be
>> +       deferred because some dependency isn't available yet because most
>> +       drivers haven't been registered yet.
>> +
>> +       Enabling this option allows the device registration code to delay the
>> +       probing of a specific device until device_initcall_sync, when all
>> +       built-in drivers have been registered already.
>> +
>> +       In some platforms there may be implicit assumptions about when some
>> +       devices are probed, so enabling this option could cause problems there.
>> +
>> +       If unsure, say Y here.
>> +
>>  endmenu
>> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>> index 7dc04ee81c8b..269ea76c1a4f 100644
>> --- a/drivers/base/dd.c
>> +++ b/drivers/base/dd.c
>> @@ -417,6 +417,13 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
>>       if (!device_is_registered(dev))
>>               return -ENODEV;
>>
>> +#if IS_ENABLED(CONFIG_DELAY_DEVICE_PROBES)
>
> This can be moved to the if.

Done.

Thanks,

Tomeu

>> +     if (!driver_deferred_probe_enable && dev->probe_late) {
>> +             driver_deferred_probe_add(dev);
>> +             return 0;
>> +     }
>> +#endif
>> +
>>       pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
>>                drv->bus->name, __func__, dev_name(dev), drv->name);
>>
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 8e7b806f0744..e64f4c7e243d 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -744,6 +744,7 @@ struct device_dma_parameters {
>>   *
>>   * @offline_disabled: If set, the device is permanently online.
>>   * @offline: Set after successful invocation of bus type's .offline().
>> + * @probe_late:      If set, device will be probed in the late initcall level.
>>   *
>>   * At the lowest level, every device in a Linux system is represented by an
>>   * instance of struct device. The device structure contains the information
>> @@ -828,6 +829,7 @@ struct device {
>>
>>       bool                    offline_disabled:1;
>>       bool                    offline:1;
>> +     bool                    probe_late:1;
>>  };
>>
>>  static inline struct device *kobj_to_dev(struct kobject *kobj)
>>
>
> --
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