Power Management Questions

Gene_Milligan at notes.seagate.com Gene_Milligan at notes.seagate.com
Thu Nov 12 10:39:44 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Gene_Milligan at notes.seagate.com
*
I will provide comments regarding your discussion which may address some of
your concerns. Since I am not an MMC expert I will not specifically address
your exact questions.

You mentioned that SCSI-3 uses the SCSI-2 power save functions. I don't
know what you had in mind by this statement. The power save functions were
not added until SCSI-3.

You are trying to sort out differences between SBC and MMC or MMC-2. But
MMC/MMC-2 had a basic goal of optimizing for those device types differently
than SBC. Which document you adhere to depends on which device type you
report to the initiator. Without looking it up as I recall MMC/MMC-2 may by
reference call out some specific SBC commands but only those portions of
SBC that are specifically called out.

The foregoing discussion forced me to open MMC-2 to see if it had its own
START/STOP unit command or used SBC by specific reference. What I found was
odd. MMC-2 has several requirements which augment or select specific
options along with making it mandatory. The peculiar thing was that only
the ATAPI type devices call out the SBC START/STOP unit command while the
other versions of MMC-2 devices call out the SPC START/STOP unit command.
SPC does not have a START/STOP unit command. I assume these instances are
typos that need to be corrected by the editor.

SBC device types do not respond to selection if they implement the optional
(for SBC device types) START/STOP unit command and the optional power save
features (for SBC device types) and use the SPI physical interface.
Consequently such devices do require RST to leave the sleep mode.

The DEVICE RESET command is a more complex issue and due to circumstances
below I did not bother to read what MMC-2 has to say about it but will pose
additional questions and comments regarding it. DEVICE RESET is an optional
ATA/ATAPI command. When it was first proposed T13 considered making it
mandatory for both ATA and ATAPI devices due to its beneficial feature of
resetting or waking up only the addressed device. But most of the ATA
manufacturer's determined that it was not desirable from a power saving
standpoint and perhaps from a related command automation (silicon)
standpoint to monitor bus activity while asleep. Consequently it was made
optional and as far as I know not implemented in ATA devices. Although the
ATAPI manufacturer's input to the ATA/ATAPI standards has been through a
very few of the companies, we understood it to be mandatory for ATAPI
devices and that they do indeed monitor for the command while sleeping (ATA
devices get more rest when sleeping). But recently it has been alleged that
there are numerous ATAPI devices that do not wake up with this command
although the standard requires it.

T13 has not changed the original DEVICE RESET requirement to monitor the
bus while sleeping but has requested that ATAPI manufacturers provide
definitive information to T13 as to what the DEVICE RESET requirements
should be. Since this item has been confusing in T13, I would not venture
to speculate on what it's actual function is in SPI versions of MMC/MMC-2.

Finally I presume by your several references to SPI-3 that you are
concerned only with a SCSI Parallel physical device and not the variants
arising from ATA/ATAPI, 1394, or Fibre Channel.

I will eagerly watch the reflector to see if specific answers to your
questions provide information that will answer the T13 questions regarding
DEVICE RESET.

Gene


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