SCSI, MMC, SAS and SATA power management

[Douglas Gilbert <dougg@torque.net>]

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While looking into implementing the START STOP UNIT
SCSI command for SATA disks, I spent some time looking
at power management state machines in various standards.
It is pretty confusing stuff so I have made a web page to
summarise the various states. See:
http://www.torque.net/sg/power.html


One linux (or device) problem I noticed while looking
at this was how lk 2.6.13-rc6 reacts when an ATAPI
cd/dvd drive is placed into SLEEP state. This isn't
a normal thing to do but can be done thus:
   sg_start -pc=5 /dev/hdd
where sg_start is found in the sg3_utils package. The
ATA8-ACS draft says that the device should first respond
to a SLEEP ATA command and thereafter not respond to
any commands until a reset occurs. One would think that
the MMC equivalent (i.e. START STOP UNIT command
[power_condition=5]) should react the same way. What I
observed on my laptop was about 10 minutes of timeouts
(with a message about lost interrupts every minute)
before things stabilised. Same thing occurred on a desktop
with a recent LG DVD+-R DL drive. I don't think kscd
polling the drive was helping.

Doug Gilbert
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