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

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Message-ID: <20210818115736.GA4177@sirena.org.uk>
Date:   Wed, 18 Aug 2021 12:57:36 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        alsa-devel@...a-project.org, tiwai@...e.de, vkoul@...nel.org,
        liam.r.girdwood@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@....de>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [RFC PATCH 1/2] driver core: export
 driver_deferred_probe_trigger()

On Wed, Aug 18, 2021 at 07:44:39AM +0200, Greg Kroah-Hartman wrote:
> On Tue, Aug 17, 2021 at 02:00:56PM -0500, Pierre-Louis Bossart wrote:

> > In these cases, there is no way to notify the deferred probe
> > infrastructure of the enablement of resources after the driver
> > binding.

> Then just wait for it to happen naturally?

Through what mechanism will it happen naturally?  Deferred probe
currently only does things if things are being registered or if probes
complete.

> > The driver_deferred_probe_trigger() function is currently used
> > 'anytime a driver is successfully bound to a device', this patch
> > suggest exporing by exporting it so that drivers can kick-off
> > re-probing of deferred devices at the end of a deferred processing.

> I really do not want to export this as it will get really messy very
> quickly with different drivers/busses attempting to call this.

I'm not sure I see the mess here - it's just queueing some work, one of
the things that the workqueue stuff does well is handle things getting
scheduled while they're already queued.  Honestly having understood
their problem I think we need to be adding these calls into all the
resource provider APIs.

> Either handle it in your driver (why do you have to defer probe at all,
> just succeed and move on to register the needed stuff after you are
> initialized) or rely on the driver core here.

That's exactly what they're doing currently and the driver core isn't
delivering.

Driver A is slow to start up and providing a resource to driver B, this
gets handled in driver A by succeeding immediately and then registering
the resource once the startup has completed.  Unfortunately while that
was happening not only has driver B registered and deferred but the rest
of the probes/defers in the system have completed so the deferred probe
mechanism is idle.  Nothing currently tells the deferred probe mechanism
that a new resource is now available so it never retries the probe of
driver B.  The only way I can see to fix this without modifying the
driver core is to make driver A block during probe but that would at
best slow down boot.

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.

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