Re: [RFT] major libata update

[Tejun Heo <htejun@gmail.com>]

Jeff Garzik wrote:
> Jeff Garzik wrote:
>> Tejun Heo wrote:
>>> Hmmm.. The drive is issuing SDB FIS which completes already completed 
>>> tags.  This could be dangerous.  Depending on timing, it might end up 
>>> finishing a command which occupied the slot which hasn't been 
>>> processed yet.  If a drive does this, NCQ shouldn't be enabled for 
>>> it.  Can you post full boot dmesg?
>>
>> I'm not sure the data supports that conclusion?  PORT_IRQ_SDB_FIS is 
>> quite normal and expected during NCQ operation, if that interrupt is 
>> enabled.  Just normal SDB:Entry and SDB:SetIntr states.
> 
> Strike that last part:  PORT_IRQ_SDB_FIS will appear, as with other 
> status bits, even if the enable bit is not set.
> 
> So, you'll see that whenever you get an SDB FIS during normal operation.

The problem is with the second dword.  Here are some of spurious SDB 
FISes Ric's AHCI was receiving.

004040a1:10000000
004040a1:00000020
004040a1:00000080

If the second dword were all zero, it's simply SDB FIS turning on IRQ 
(bit 14 of the first dword) and there's nothing to worry about. 
However, all those spurious SDBs have one bit set in the second dword - 
meaning the SDB completes the corresponding tag, but the tag isn't 
active when those SDBs are received.

This is okay as long as the controller thinks the tags are unoccupied 
when those SDBs are received, but it's not something which can be 
guaranteed.  NCQ command synchronization depends on devices not 
completing the same commands more than once.

The duplicate completions might be okay if the drive guarantees it 
doesn't send it if it loses to command issuance.  e.g.

1. drive sends completion for tag x
2. drive shortly schedules another completion for tag x (spurious)
3. ahci/driver complete tag x
4. ahci/driver issues tag x
5. drive receives command for tag x before sending the spurious 
completion and determines not to send the spurious completion. (not very 
likely)

If above is true, the drive might be okay, but nobody can guarantee how 
  various controllers react.  It depends on how controllers manage 
SActive (when to turn bits on).  At any rate, it's dangerous IMHO.

-- 
tejun