Re: [PATCH] libsas: support NCQ for SATA disks

["Darrick J. Wong" <djwong@us.ibm.com>]

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