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From:   Rob Herring <robh@kernel.org>
Date:   Thu, 24 May 2018 14:42:43 -0500
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Subject: Re: [PATCH v2 1/8] driver core: make deferring probe after init optional
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Alexander Graf <agraf@suse.de>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Kevin Hilman <khilman@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Joerg Roedel <joro@8bytes.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        devicetree@vger.kernel.org,
        Architecture Mailman List <boot-architecture@lists.linaro.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>
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On Thu, May 24, 2018 at 1:56 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Thu, May 24, 2018 at 12:50:17PM -0500, Rob Herring wrote:
>> Deferred probe will currently wait forever on dependent devices to probe,
>> but sometimes a driver will never exist. It's also not always critical for
>> a driver to exist. Platforms can rely on default configuration from the
>> bootloader or reset defaults for things such as pinctrl and power domains.
>> This is often the case with initial platform support until various drivers
>> get enabled. There's at least 2 scenarios where deferred probe can render
>> a platform broken. Both involve using a DT which has more devices and
>> dependencies than the kernel supports. The 1st case is a driver may be
>> disabled in the kernel config. The 2nd case is the kernel version may
>> simply not have the dependent driver. This can happen if using a newer DT
>> (provided by firmware perhaps) with a stable kernel version.
>>
>> Subsystems or drivers may opt-in to this behavior by calling
>> driver_deferred_probe_check_init_done() instead of just returning
>> -EPROBE_DEFER. They may use additional information from DT or kernel's
>> config to decide whether to continue to defer probe or not.
>>
>> Cc: Alexander Graf <agraf@suse.de>
>> Signed-off-by: Rob Herring <robh@kernel.org>
>> ---
>>  drivers/base/dd.c      | 17 +++++++++++++++++
>>  include/linux/device.h |  2 ++
>>  2 files changed, 19 insertions(+)
>>
>> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>> index c9f54089429b..d6034718da6f 100644
>> --- a/drivers/base/dd.c
>> +++ b/drivers/base/dd.c
>> @@ -226,6 +226,16 @@ void device_unblock_probing(void)
>>       driver_deferred_probe_trigger();
>>  }
>>
>> +int driver_deferred_probe_check_init_done(struct device *dev, bool optional)
>> +{
>> +     if (optional && initcalls_done) {
>> +             dev_WARN(dev, "ignoring dependency for device, assuming no driver");
>
> You really only need dev_warn(), here, right?

No, the screaming is on purpose.

Bjorn had concerns about debugging/supporting subtle problems that
could stem from this. Such as electrical settings not quite right that
cause intermittent errors.

Rob