Neal and
Vicky,
The COMWAKE_Received state machine variable
addresses a timing issue that only applies to the SP0 and SP1 states. Detection
of COMWAKE in the SP3 and SP4 states was added for a different
reason.
The COMWAKE_Received state machine variable was
added by proposal 04-357r1 (http://www.t10.org/ftp/t10/document.04/04-357r1.pdf), and addresses a condition resulting from the fact
that the duration of the COMWAKE signal is much shorter than that of the COMINIT
signal, and it is possible that a port selector can respond to a received
COMINIT, and the SAS phy can detect the COMWAKE, even before it has completed
transmission of the COMINIT, and following the transition SP0->SP1, SP1 would erroneously determine the
port selector is no longer present, except for the COMWAKE_Received state
machine variable which communicates prior detection of the COMWAKE detected
message.
Detection of COMWAKE was added to SP3 and SP4 to
handle corner timing cases that don't necessarily follow the nominal
flow. There are places in the SATA 2.5
specification of the Port Selector behavior that indicate a COMINIT received on
the phy connected to the SATA device might cause a COMINIT to be transmitted on
the selected phy attached to the host (granted, this is subject to
interpretation). At the time the COMWAKE detected message text was added to the
SP state machine (see 03-273r2: http://www.t10.org/ftp/t10/document.03/03-273r2.pdf),
it was believed that a SATA port selector could, indeed, transmit COMWAKE while
the SP state machine is in the SP3:OOB_AwaitCOMX state or in the SP4:OOB_COMSAS
state. Whether this is the prevailing interpretation of the Port Selector state
machine in SATA 2.5 or not, maintaining the capability to detect COMWAKE in
these two states does not impede interoperability, and could, perhaps,
facilitate interoperability.
Here is one
example where during normal operation, both the SATA device and the SAS phy
experience an internal reset at roughly the same time. In this case, the SATA
device wins the race, marginally, and (subject to interpretation of the Port
Selector description in SATA 2.5) the
COMINIT transmitted by the SATA device is "propogated" through the port selector to the
selected SAS phy before the SAS phy completes transmission of it's COMINIT, but
after the SAS phy has transitioned to the SP0:OOB_COMINIT
state.
Then, having both transmitted and received COMINIT
in the SP0:OOB_COMINIT state, the SP state machine transitions to the
SP4:OOB_COMSAS state, where it receives the COMWAKE transmitted by the port
selector in response to the COMINIT transmitted by the SAS
phy.
The attached sequence diagram shows the
interaction.
Best regards,
Bob