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    Subject[BUG] Deferred probing in driver model is racy, resulting in lost probes
    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
    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.

    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
    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.)

    I do not know all the details of the locking in this area, so I'm not
    going to try to come up with a patch... but as this stands, it's
    unworkable. I have not yet had the system come up once with these
    drivers as modules and had it work without having to manually remove
    and re-add the Kirkwood SPDIF driver.


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