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    SubjectRe: [PATCH] libsas: support NCQ for SATA disks
    James Bottomley wrote:

    > This doesn't seem to quite work for me on a SATA-1 disc:
    >
    > sas: DOING DISCOVERY on port 1, pid:1897
    > sas: sas_ata_phy_reset: Found ATA device.
    > ata1.00: ATA-7, max UDMA/133, 781422768 sectors: LBA48 NCQ (depth 31/32)
    > ata1.00: configured for UDMA/133
    > scsi 2:0:1:0: Direct-Access ATA ST3400832AS 3.03 PQ: 0
    > ANSI: 5
    > SCSI device sdc: 781422768 512-byte hdwr sectors (400088 MB)
    > sdc: Write Protect is off
    > SCSI device sdc: drive cache: write back
    > SCSI device sdc: 781422768 512-byte hdwr sectors (400088 MB)
    > sdc: Write Protect is off
    > SCSI device sdc: drive cache: write back
    > sdc: unknown partition table
    > sd 2:0:1:0: Attached scsi disk sdc
    > sas: DONE DISCOVERY on port 1, pid:1897, result:0
    > sas: command 0xf785f3c0, task 0x00000000, timed out: EH_HANDLED
    > sas: command 0xf785f3c0, task 0x00000000, timed out: EH_HANDLED
    > [...]
    >
    > It looks like the first few commands get through (read capacity, ATA
    > IDENTIFY etc) and it hangs up on the read for the partition table.

    Hm... if I put in some debug printks in the qc_issue code, I get the
    same symptoms. I've observed that once again we get hung up on ATA
    commands where the tag number > 0. I also noticed this pattern:

    1. ATA command w/ tag 0 (command A) issued.
    2. Command A goes out to sas-ata.
    2. ATA command w/ tag 1 (command B) issued.
    3. Command A completes
    4. Command B goes out to sas-ata.
    [...]
    5. Command B times out.

    Very odd that this all works if there are no printks. I don't see
    anything obvious that would suggest why this apparent race seems to
    happen--unless there's some conflict between issuing an ATA command
    while completing another one.

    --D
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