AHCI Patsburg SATA controller and slow transfer speed

[Jeremy Chadwick]

On Thu, Jun 27, 2013 at 06:38:27PM -0700, Jeremy Chadwick wrote:
> Next, this statement by ahci(4) then confuses the user:
> 
> > ahci0: AHCI v1.30 with 6 6Gbps ports, Port Multiplier not supported
> 
> You see, when AHCI was invented, the existing idea was that all ports
> would have the same speed (and that was the case at the time).  Only
> somewhat recently have some vendors begun to mix-match speeds on the
> same controller -- like this one.
> 
> The AHCI specification probably (I have not read it even recently) only
> provides a number indicating "the total number of ports" followed by a
> single number indicating "the speed".
> 
> There may be support somewhere within AHCI to provide an updated way to
> get more granular information, but I do not know if that's the case.
> 
> If there is, FreeBSD's ahci(4) driver does not support such at this
> time (see sys/dev/ahci/ahci.c around line 502 for the device_printf()
> call and what the arguments are (specifically AHCI_CAP_ISS and
> AHCI_CAP_NPMASK)).

Just a technical follow-up:

I spent some time this evening looking at AHCI specification 1.30.  I'll
try to explain the situation.

First, at the HBA level (meaning the entire AHCI controller):

Bits 23-30 of CAP (reg. offset 0x00): Interface Speed Support (ISS).
This indicates, quote, "the maximum speed the HBA can support on its
ports".

Next, on a per-port basis, there are two registers available relating to
speed: one indicates speed, the other controls/limits speed:

1) Bits 7-4 of PxSSTS (reg. offset 0x28): SPD: Port x Serial ATA Status
(SCR0: SStatus).  This indicates, quote, "the negotiated interface
speed".

2) Bits 7-4 of PxSCTL (reg. offset 0x2c): SPD: Port x Serial ATA Control
(SCR2: SControl).  The register controls, quote, "the highest allowable
speed of the interface".  The bit definitions indicate a way to limit
the speed of a port and do not indicate capability.

The actual 1.30 specification even has a section (10.5) on this whole
ordeal, which states clearly, quote:

====
10.5 Interface Speed Support

The HBA indicates the maximum speed it can support via the CAP.ISS
register. Software can further limit the speed of a port by manipulating
each port's PxSCTL.SPD field to a lower value.
====

AHCI spec "proposal" 1.31 also does not address/cover this (all that
adds is per-port sleep capabilities).

I will point out that SATA600 is not officially mentioned in any spec at
this time (that I can get my hands on), so what all the OSes run off of
are educated assumptions.  :-)  But theoretically, a newer AHCI spec
could support per-port maximum speed indication.

It's not easy to phrase all this tersely in a single device_printf(),
and there has already been opposition to adding printing of more lines
to the existing drivers/in dmesg (meaning, printing 6 lines, one for
each port, indicating active speed + maximum speed, would probably be
looked down upon outside of verbose booting).  The best I can come up
with is this:

ahci0: AHCI v1.30, 6 ports, maximum 6Gbps, Port Multiplier not supported

...which is better, but could still be interpreted as "6 ports
with a maximum of 6Gbps per port".

Hope this sheds light in some way or another.

-- 
| Jeremy Chadwick                                   jdc at koitsu.org |
| UNIX Systems Administrator                http://jdc.koitsu.org/ |
| Making life hard for others since 1977.             PGP 4BD6C0CB |