ahci sata data overruns

[Wolfgang S. Rupprecht]

Alan Cox <alan at lxorguk.ukuu.org.uk> writes:
> On Thu, 08 Nov 2012 09:43:29 -0800
> "Wolfgang S. Rupprecht" <wolfgang.rupprecht at gmail.com> wrote:
>> Can someone familiar with how the AHCI driver works confirm that data
>> overruns on a SATA link can cause the driver to down-shift the SATA
>> speed 6->3->1.5 Gbits/sec.  I don't see any kprintf's but I notice that
>> a high speed SSD acting as if the SATA were running at 1.5 Gbits/sec.
>
> It has a set of heuristics based upon error rate over time.
>
> I'm not sure what you mean by "data overruns" however

The SSD in question is capable of 550 MByte/sec (Intel 520 SSD), while
the PCIE controller is only an x1 single lane controller which probbly
saturates the single lane at 480 MByte/sec.  I'm assuming (perhaps
incorrectly) that the controller is choking when it receives more data
from the SATA than it can transmit on the PCIE.

The bootup kprintf's show a 6Mbit/sec negotiation, but later when I try
a transfer I see 130 MByte/sec transfers for a raw partition dd, hinting
strongly that it is now talking at 1.5Mbit/sec on the SATA.  I'm
wondering if it is silently downshifting the SATA speed.  In comparison,
a slower SSD rated at 250MBytes/sec (which can stay within the PCIE x1
lane budget) runs at full speed, of slightly over 250MBytes/sec on the
same controller.

-wolfgang
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