Re: [PATCH 8/8] scsi: libsas: support SATA phy link rate unmatch the pathway

[John Garry <john.garry@huawei.com>]

On 29/05/2018 03:23, Jason Yan wrote:
> If a SATA disk attached to a expander phy and it's linkrate is greater
> than the expander host phy's linkrate, the disk will failed to discover.
> The topology is like below:
>
>    +----------+           +----------+
>    |          |           |          |
>    |          |-- 3.0 G --|          |-- 6.0 G -- SAS  disk
>    |          |           |          |
>    |          |-- 3.0 G --|          |-- 6.0 G -- SAS  disk
>    |initiator |           |          |
>    | device   |-- 3.0 G --| Expander |-- 6.0 G -- SAS  disk
>    |          |           |          |
>    |          |-- 3.0 G --|          |-- 6.0 G -- SATA disk  -->failed to connect
>    |          |           |          |
>    |          |           |          |-- 6.0 G -- SATA disk  -->failed to connect
>    |          |           |          |
>    +----------+           +----------+
>
> And when we check the sas protocal spec, this scenario is described as
> this:
>
> 7.13 Rate matching
> ......
> If an expander phy attached to a SATA phy is using a physical link rate
> greater than the maximum connection rate supported by the pathway from
> an STP initiator port, a management application client should use the
> SMP PHY CONTROL function (see 10.4.3.10) to set the PROGRAMMED MAXIMUM
> PHYSICAL LINK RATE field of the expander phy to the maximum connection
> rate supported by the pathway from that STP initiator port.
>
> In order to support this scenario, checking the SATA disk's linkrate
> to see if it is greater than any phy's linkrate it may pass through.
> Remember the minimum linkrate of the pathway and set the SATA phy
> linkrate to it using the SMP PHY CONTROL function.

As we (re)discover the tree, can we keep track of the min pathway to the 
root PHY dynamically (per expander), and then take action for any SATA 
devices attached which have a negotiated linkrate greater (than the 
expanders min pathway)? This would be an alternate to your approach of 
finishing discovery and then checking the min pathway as a whole new step.