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    SubjectRe: Hang during boot, bisected to commit 96d60303fd
    On Thu, Jun 3, 2010 at 11:42 PM, Robert Hancock <> wrote:
    > On 06/03/2010 07:21 PM, Marc Dionne wrote:
    >> On Wed, Jun 2, 2010 at 10:00 PM, Matthew Garrett<>
    >>  wrote:
    >>> On Wed, Jun 02, 2010 at 09:49:27PM -0400, Marc Dionne wrote:
    >>>>    Since somewhere early in the 2.6.35 merge window I've been seeing a
    >>>> hang
    >>>>    during boot.  The hang last roughly 30 seconds and is followed by
    >>>> this:
    >>>>    ata4.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x6 frozen
    >>>>    ata4.00: failed command: READ FPDMA QUEUED
    >>>>    ata4.00: cmd 60/08:00:00:00:00/00:00:00:00:00/40 tag 0 ncq 4096 in
    >>>>             res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
    >>>>    ata4.00: status: { DRDY }
    >>>>    ata4: hard resetting link
    >>>>    ahci_is_device_present: status is: 80
    >>>>    ahci_is_device_present: status(2) is: 3
    >>>>    ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
    >>>>    ata4.00: configured for UDMA/133
    >>>>    ata4.00: device reported invalid CHS sector 0
    >>>>    ata4: EH complete
    >>>>    The system works normally once booted.  I had some time to do a
    >>>> bisect
    >>>>    today and it cleanly ended up pointing to this commit:
    >>>>        96d60303fd: ahci: Turn off DMA engines when there's no device
    >>>> attached
    >>>>    Reverting the commit makes the system boot cleanly as before.  System
    >>>> has
    >>>>    an Intel ICH10 based board, in AHCI mode, disk is an Intel X25M SSD.
    >>> Hm, interesting. Jeff, any immediate ideas? I don't see this behaviour
    >>> on my ICH10. Is there actually a device on ata4?
    >>> --
    >>> Matthew Garrett |
    >> If it's helpful, attached are the dmesg output following a normal boot
    >> and one with the hang.
    >> The only difference in the kernel used for those two boots is that the
    >> return statement in ahci_start_engine following the call to
    >> ahci_is_device_present is commented out for the normal boot case.
    >> When the return statement is active, the 30 second hang occurs.
    > Can you try and put a printk in where it's returning out of the
    > ahci_start_engine method? I wonder if that's getting triggered somewhere
    > unexpected..

    Attached is a dmesg output with some printks added to show the status
    when ahci_is_device_present causes ahci_start_engine to return early..

    It looks to me like there's two passes through ahci_start_engine for
    all ports, and during the first pass the port with the disk is showing
    a status of 127, causing the function to return early and skipping the
    start DMA part. This seems to cause the hang later on.

    I noticed that during this first pass all the print_id fields for the
    ports are still set to -1, so as a test I made ahci_start_engine only
    do the check for ahci_is_device_present if print_id != -1, and this
    makes the system boot normally.

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