ahci sata data overruns

Thu Nov 8 17:43:29 UTC 2012

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.

This looks like it could be a bug.  I can see down-shifting as a
response to noise on the SATA link, but downshifting for data overrun
reasons is only going to make the problem worse.

-wolfgang
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