Potentially wrong assumption of SATA spec in SAS standard

Karthikeyan, Kishore K kishore.k.karthikeyan at intel.com
Mon Sep 22 07:18:37 PDT 2008


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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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