Potentially wrong assumption of SATA spec in SAS standard

Elliott, Robert (Server Storage) Elliott at hp.com
Thu Sep 25 13:02:50 PDT 2008


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I don't think any expanders try to push the limits here, but agree the
discrepency should be corrected.
1. I will change SAS-2 rules about transmission on the SATA physical link to:
"When a SATA host phy in an STP/SATA bridge is transmitting a SATA frame to a
SATA physical link, it shall transmit no more than 19 dwords (e.g., including
data dwords, deletable primitives, SATA_HOLD, and SATA_EOF) after receiving
SATA_HOLD before responding with SATA_HOLDA."
2. Although the budget for STP should not include deletable primitives or
transmit-direction SATA_HOLDs (there's no reason to put those in the buffer),
it should include both data dwords and SATA_EOF.  I'll add "or SATA_EOFs" in
several places, like this:
"After transmitting SATA_HOLD, it shall accept at least the following number
of data dwords or SATA_EOFs for the SATA frame into its STP flow control
buffer:
a) 24 data dwords or SATA_EOFs at the 1.5 Gbps connection rate;
b) 28 data dwords or SATA_EOFs at the 3 Gbps connection rate; or
c) 36 data dwords or SATA_EOFs at the 6 Gbps connection rate,
and shall expect to receive SATA_HOLDA within that number of data dwords or
SATA_EOFs."
________________________________
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Karthikeyan,
Kishore K
Sent: Monday, September 22, 2008 9:19 AM
To: t10 at t10.org
Subject: Potentially wrong assumption of SATA spec in SAS standard
All
In Section 7.18.2 STP Flow Control, it talks about 20 DATA dwords when
referring to frame transmission and reception on the SATA physical link of
the STP/SATA bridge.
But SATA spec does not say "data dwords" but simply "dwords" which means it
includes primitives too (for eg ALIGNs and EOF) essentially reducing the
number of data dwords that can be transmitted after reception of HOLD and
before transmission of HOLDA and some SATA disk drive vendors have
interpreted the SATA spec as such. Hence these drives cannot accept more than
20 dwords (including primitives like ALIGNs and EOF) after it has transmitted
HOLD. Such SATA drives will have incompatibility with expanders whose
STP/SATA bride is designed according to SAS standard (which says 20 DATA
dwords instead of just 20 dwords). For eg. if the expander is forwarding a
frame to the SATA drive and it receives a HOLD from this drive when only 18
dwords were remaining to be transmitted, it can potentially just transmit all
the remaining 18 dwords + 2 CLKSKEW ALIGNs + CRC + EOF = 22 dwords and not
transmit HOLDA.
SAS standard can assume data dwords on STP phys because it is talking to a
SAS device but when referring to traffic on SATA physical links; it has to
match whatever is mentioned in the SATA specs to be compatible with existing
SATA drives in the market.
I would think that this needs to be fixed to prevent incompatiblity between
STP/SATA bridge designs in expanders and SATA drives in market.
Given below is the text from 7.18.2 in SAS Standard rev sas2r14d
When a SATA host phy in an STP/SATA bridge is receiving a SATA frame from a
SATA physical link, it shall
transmit a SATA_HOLD when it is only capable of receiving 21 more data
dwords. It shall stop transmitting
SATA_HOLD (e.g., return to transmitting SATA_R_IP) when it is capable of
receiving at least 21 more data
dwords.
NOTE 83 - SATA requires that frame transmission cease and SATA_HOLDA be
transmitted within 20 data
dwords of receiving SATA_HOLD. Since the SATA physical link has non-zero
propagation time, one dword of
margin is included.
When a SATA host phy in an STP/SATA bridge is transmitting a SATA frame to a
SATA physical link, it shall
transmit no more than 19 data dwords after receiving SATA_HOLD.
NOTE 84 - SATA assumes that once a SATA_HOLD is transmitted, frame
transmission ceases and
SATA_HOLDA arrives within 20 dwords. Since the SATA physical link has
non-zero propagation time, one
dword of margin is included.
Given below is the text from the SATA 2.6 spec
9.4.7 Flow Control Signaling Latency
In the case where the receiver wants to flow control the incoming data, it
transmits HOLDP
characters on the back channel. Some number of received Dwords later, valid
data ceases, and
HOLDAP characters are received. The larger the latency between transmitting
HOLDP until
receiving HOLDAP, the larger the receive FIFO needs to be. Within a single
HOLDP/ HOLDAP
sequence, the maximum allowed latency from the time the MSB of the initial
HOLDP is on the
wire, until the MSB of the initial HOLDAP is on the wire shall be no more
than 20 Dword times.
The LSB is transmitted first. A receiver shall be able to accommodate
reception of 20 Dwords of
additional data after the time it transmits the HOLDP flow control character
to the transmitter, and
the transmitter shall respond with a HOLDAP in response to receiving a HOLDP
within 20 Dword
times. The 20 Dword latency specification is not applicable to any subsequent
transmissions of
the HOLDP flow control character within the same sequence. Upon each new
instantiation of a
HOLDP/ HOLDAP sequence, the receiver and transmitter shall meet the 20 Dword
latency
specification



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