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From: Dan Williams <dan.j.williams@intel.com>
To: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: Mark Brown <broonie@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	alsa-devel@alsa-project.org,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Takashi Iwai <tiwai@suse.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Liam Girdwood <liam.r.girdwood@linux.intel.com>,
	Vinod Koul <vkoul@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC PATCH 1/2] driver core: export driver_deferred_probe_trigger()
Date: Wed, 18 Aug 2021 07:59:35 -0700	[thread overview]
Message-ID: <CAPcyv4hWNk-avNOPdWBNND9Bd9nebzs2A4Weveqy289gM+A1Bw@mail.gmail.com> (raw)
In-Reply-To: <14235b8d-d375-6e2d-cae9-33adf9c48120@linux.intel.com>

On Wed, Aug 18, 2021 at 7:52 AM Pierre-Louis Bossart
<pierre-louis.bossart@linux.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.

  reply	other threads:[~2021-08-18 14:59 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-17 19:00 [RFC PATCH 0/2] driver core: kick deferred probe from delayed context Pierre-Louis Bossart
2021-08-17 19:00 ` [RFC PATCH 1/2] driver core: export driver_deferred_probe_trigger() Pierre-Louis Bossart
2021-08-18  5:44   ` Greg Kroah-Hartman
2021-08-18 11:57     ` Mark Brown
2021-08-18 13:22       ` Greg Kroah-Hartman
2021-08-18 13:48         ` Mark Brown
2021-08-18 14:51           ` Pierre-Louis Bossart
2021-08-18 14:59             ` Dan Williams [this message]
2021-08-18 15:28             ` Greg Kroah-Hartman
2021-08-18 15:53               ` Pierre-Louis Bossart
2021-08-18 16:49                 ` Greg Kroah-Hartman
2021-08-18 17:52                   ` Mark Brown
2021-08-18 18:09                   ` Pierre-Louis Bossart
2021-08-18 18:28                     ` Mark Brown
2021-08-17 19:00 ` [RFC PATCH 2/2] ASoC: SOF: trigger re-probing of deferred devices from workqueue Pierre-Louis Bossart
2021-08-18 12:07   ` Mark Brown
2021-08-18 15:25     ` Pierre-Louis Bossart

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