[SAS] Frame Credit Managmeent

Seto, Pak-lung pak-lung.seto at intel.com
Fri Nov 8 09:55:14 PST 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* "Seto, Pak-lung" <pak-lung.seto at intel.com>
*
It is unclear that how frame credit should be managed in SAS.

First, a couple terms may need to be defined in the standard or at least for
this discussion.

Outstanding Credit - The number of R_RDY sent by the port minus the number
of frame received after connection has been established
Available Credit - The number of R_RDY received minus hte number of frame
sent after connection has been established

I don't believe there is a clause in the standard to describe all the Frame
Credit Management issues:

- Both Outstanding Credit and Available Credit must be initialized to zero
after connection has been established - when? After OPEN Address Frame has
been sent? or after connection has been accepted?

- If a transmitting frame encountered an internal error within the port and
the port will need to force the frame CRC error since partial frame has been
sent (since we did not accept the EOFa proposal).  Does this errored frame
used one Available Credit?

- If the receiving port received a frame with Outstanding Credit = 0, what
should the port do, especially it has buffers to store the frame (Missing
R_RDY?)?  Is the received frame OK if an R_RDY is sent before the EOF is
received/detected?

- When does the Outstanding Credit count should be decremented? After EOF
has been detected?

- When should the Available Credit count be decremented? After receive a
frame SOF-EOF, valid CRC, the right destination SAS address, etc?

- If a received frame does not have the matching destination SAS address,
does it used up one Outstanding Credit?

- If a received frame does not belong to one of the active outstanding IO,
does it used up one Outstanding Credit?

- If a frame has more than 1024 bytes of payload, does it used up one
Outstanding Credit?

- If a frame is terminated by the receiving port, does it used up one
Outstanding Credit?

- If a frame is received with the wrong protocol (SMP frame in SSP
connection), does it used up one Outstanding Credit?

- Any errored frame with good CRC or no decoding error, does it used up one
Outstanding Credit?

- Does any of the above conditions - especially credit unbalance - received
a frame without outstanding credit, should it cause the connection to be
terminated?

Any other situations?

Pak 
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