linux-kernel - Re: [RFC PATCH 1/2] driver core: export driver_deferred_probe

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Date:   Wed, 18 Aug 2021 07:59:35 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        alsa-devel@...a-project.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Takashi Iwai <tiwai@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Vinod Koul <vkoul@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH 1/2] driver core: export driver_deferred_probe_trigger()

On Wed, Aug 18, 2021 at 7:52 AM Pierre-Louis Bossart
<pierre-louis.bossart@...ux.intel.com> wrote:
>
>
>
> >>> The issue is that the driver core is using drivers completing probe as a
> >>> proxy for resources becoming available.  That works most of the time
> >>> because most probes are fully synchronous but it breaks down if a
> >>> resource provider registers resources outside of probe, we might still
> >>> be fine if system boot is still happening and something else probes but
> >>> only through luck.
> >
> >> The driver core is not using that as a proxy, that is up to the driver
> >> itself or not.  All probe means is "yes, this driver binds to this
> >> device, thank you!" for that specific bus/class type.  That's all, if
> >> the driver needs to go off and do real work before it can properly
> >> control the device, wonderful, have it go and do that async.
> >
> > Right, which is what is happening here - but the deferred probe
> > machinery in the core is reading more into the probe succeeding than it
> > should.
>
> I think Greg was referring to the use of the PROBE_PREFER_ASYNCHRONOUS
> probe type. We tried just that and got a nice WARN_ON because we are
> using request_module() to deal with HDaudio codecs. The details are in
> [1] but the kernel code is unambiguous...
>
>         /*
>          * We don't allow synchronous module loading from async.  Module
>          * init may invoke async_synchronize_full() which will end up
>          * waiting for this task which already is waiting for the module
>          * loading to complete, leading to a deadlock.
>          */
>         WARN_ON_ONCE(wait && current_is_async());
>
>
> The reason why we use a workqueue is because we are otherwise painted in
> a corner by conflicting requirements.
>
> a) we have to use request_module()
> b) we cannot use the async probe because of the request_module()
> c) we have to avoid blocking on boot
>
> I understand the resistance to exporting this function, no one in our
> team was really happy about it, but no one could find an alternate
> solution. If there is something better, I am all ears.

Additionally you mentioned that the consumer is unknown to the
producer, so you are not able, for example, to use the newly exported
device_driver_attach() to directly trigger the unblocked dependency.