Date:	Sun, 16 Sep 2012 14:41:28 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Arnd Bergmann <arnd@...db.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: [BUG] Deferred probing in driver model is racy, resulting in lost probes

On Sat, Aug 18, 2012 at 10:58 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> Okay, so EPROBE_DEFER seems to work when I build everything into the
> kernel, but when I build a pile of ASoC drivers as modules, it fails
> every time I've tried booting the platform so far.
>
> This is a v3.5 based kernel, with preempt enabled.
>
> Okay, what I have is a bunch of devices already pre-registered in the
> system:
>
>         kirkwood-spdif-audio.1 (requiring module snd_soc_kirkwood_spdif)
>         kirkwood-i2s.1 (requiring module snd_soc_kirkwood_i2s)
>         kirkwood-pcm-audio.1 (requiring module snd_soc_kirkwood)
>         spdif-dit (requiring module snd_soc_spdif)
>
> and the modules were loaded in this order:
>
> Module                  Size  Used by
> snd_soc_spdif           1020  0         (spdif-dit driver)
> dove                   24577  1         (dove-drm driver)
> snd_soc_kirkwood_i2s     5209  0        (kirkwood-i2s driver)
> snd_soc_kirkwood        3629  0         (kirkwood-pcm-audio driver)
> snd_soc_kirkwood_spdif     1449  0      (kirkwood-spdif-audio driver)
>
> snd_soc_kirkwood_spdif will return -EPROBE_DEFER if it can't find the
> devices registered into ASoC by the other three modules (referred to as
> the CPU DAI, and Codec DAI).
>
> What I see in the debugging is this (I've added some to increase the
> debuggability):
>
> bus: 'platform': add driver kirkwood-spdif-audio
> bus: 'platform': driver_probe_device: matched device kirkwood-spdif-audio.1 with driver kirkwood-spdif-audio
> bus: 'platform': really_probe: probing driver kirkwood-spdif-audio with device kirkwood-spdif-audio.1
> kirkwood-spdif-audio kirkwood-spdif-audio.1: CPU DAI kirkwood-i2s.1 not registered
> bus: 'platform': add driver kirkwood-pcm-audio
> bus: 'platform': add driver kirkwood-i2s
> kirkwood-spdif-audio kirkwood-spdif-audio.1: failed to register card
> bus: 'platform': add driver dove-drm
> platform kirkwood-spdif-audio.1: Driver kirkwood-spdif-audio requests probe deferral
> platform kirkwood-spdif-audio.1: Added to deferred list
> bus: 'platform': driver_probe_device: matched device kirkwood-pcm-audio.1 with driver kirkwood-pcm-audio
> bus: 'platform': really_probe: probing driver kirkwood-pcm-audio with device kirkwood-pcm-audio.1
> driver: 'kirkwood-pcm-audio.1': driver_bound: bound to device 'kirkwood-pcm-audio'
> bus: 'platform': really_probe: bound device kirkwood-pcm-audio.1 to driver kirkwood-pcm-audio
> platform kirkwood-spdif-audio.1: Retrying from deferred list
> bus: device_attach 'kirkwood-spdif-audio.1' unbound

Looks the above message is added by yourself, where did you add
the message?

> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'dove-temp'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'galcore'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'alarmtimer'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'mv_xor_shared'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'mv_xor'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'serial8250'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'sata_mv'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'orion_spi'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'mv643xx_eth'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'mv643xx_eth_port'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'ap510-vmeta'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'orion-ehci'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'rtc-mv'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'mv64xxx_i2c'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'gpio-rc-recv'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'sdhci-pxav2'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'sdhci-dove'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'leds-gpio'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'snd-soc-dummy'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'soc-audio'
> bus: __device_attach 'kirkwood-spdif-audio.1' probing with 'kirkwood-pcm-audio'
>
> What you notice here is that the kirkwood-spdif-audio driver has gone
> missing - it hasn't been unloaded, and it still appears in sysfs, and
> it can still be cleanly unloaded and reloaded.

It is a bit weird, and I am wondering why is the driver of  kirkwood-spdif-audio
deleted from bus->p->klist_drivers without being unregistered from
platform bus?

>
> bus: 'platform': driver_probe_device: matched device kirkwood-i2s.1 with driver kirkwood-i2s
> bus: 'platform': really_probe: probing driver kirkwood-i2s with device kirkwood-i2s.1
> kirkwood-i2s kirkwood-i2s.1: found external clock
> driver: 'kirkwood-i2s.1': driver_bound: bound to device 'kirkwood-i2s'
> bus: 'platform': really_probe: bound device kirkwood-i2s.1 to driver kirkwood-i2s
> bus: 'platform': driver_probe_device: matched device dove-drm with driver dove-drm
> bus: 'platform': really_probe: probing driver dove-drm with device dove-drm
> bus: 'platform': add driver spdif-dit
> [drm] Supports vblank timestamp caching Rev 1 (10.10.2010).
> [drm] No driver support for vblank timestamp query.
> fb0: dove-drmfb frame buffer device
> drm: registered panic notifier
> [drm] Initialized dove-drm 1.0.0 20120730 on minor 0
> driver: 'dove-drm': driver_bound: bound to device 'dove-drm'
> bus: 'platform': really_probe: bound device dove-drm to driver dove-drm
> bus: 'platform': driver_probe_device: matched device spdif-dit with driver spdif-dit
> bus: 'platform': really_probe: probing driver spdif-dit with device spdif-dit
> driver: 'spdif-dit': driver_bound: bound to device 'spdif-dit'
> bus: 'platform': really_probe: bound device spdif-dit to driver spdif-dit
>
> I suspect what is going on is that we have a race condition between the
> device being added to the deferred list vs other drivers successfully
> probing and running the deferred list, and the driver with the deferred
> device being added to the driver list.
>
> This allows the deferred list to be run *before* the driver is placed
> onto the bus driver list, which means when we try to re-probe deferred

Looks the above should not be a problem. After the driver is placed
on the bus, it still will call driver_attach to probe all unbound devices
on the bus.

> devices, we find no matching driver to probe the device with.  This
> then results in the device being dropped off the deferred probe list
> (as the only way it stays on that list is if a driver is probed _and_
> it returns -EPROBE_DEFER.)


Thanks,
-- 
Ming Lei
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