Date:	Tue, 01 Jan 2008 22:20:02 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Tejun Heo <htejun@...il.com>, Gabor Gombas <gombasg@...aki.hu>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	kluo@...dia.com, AMartin@...dia.com, pchen@...dia.com
Subject: Re: sata_nv + ADMA + Samsung disk problem

Robert Hancock wrote:
> Tejun Heo wrote:
>> [cc'ing Robert Hancock and NVidia people]
>>
>> Whole thread can be read from the following URL.
>>
>>   http://thread.gmane.org/gmane.linux.ide/21710
>>
>> In a nutshell, with ADMA enabled, FLUSH_EXT occasionally times out.  I
>> first suspected faulty disk (reallocation failure on flush) but SMART
>> reports nothing suspicious and w/ ADMA disabled, the drive works just 
>> fine.
>>
>> On a side note, on 2.6.22.1, SMART fails from time to time but the
>> problem went away on 2.6.24-rc6.  This was apparently fixed during that
>> period.  I guess we can ignore this for now.
>>
>> Thanks.
> 
> This is kind of a longstanding problem which has been partially worked 
> around, but it seems not entirely. This is what I had diagnosed some 
> time ago:
> 
> "recently, some issues cropped up with command timeouts when a cache 
> flush command was immediately followed by an NCQ write. In this case, 
> sometimes when the NCQ write was issued, the status register changed 
> from 0x500 (Stopped and Idle) to 0x400 (Stopped) as it normally appears 
> to, however it seems like the controller would get hung in that state, 
> and we would time out with no notifiers set, the gen_ctl register not 
> indicating interrupt status, and the CPB response flags still 0 as we 
> left them, seemingly indicating the controller hasn't done anything with 
> it. Then, when the error handler kicks in we clear the GO bit to put it 
> back into register mode, but the Legacy flag in the status register 
> doesn't get set (or at least it takes longer than 1 microsecond). 
> Finally when we do an ADMA channel reset that seems to get it responding 
> again, until this happens the next time.
> 
>  From some experimentation, I found that when we are issuing a NCQ
> command when the last command was non-NCQ, or vice versa, if I added in
> a delay of 20 microseconds between setting up the CPB and writing to the
> append register, the problem appeared to go away. Problem is I don't
> know if that's because it actually needs this delay, or because it
> changes the timing so that it happens to work even though we're doing
> something wrong, there's some event we're not waiting for, etc.
> 
> I've now verified that no switches between ADMA and register mode occur 
> near the time of these timeouts. Neither are we reading or writing any 
> of the ATA shadow registers while we're in ADMA mode."
> 
> It seems likely that this is what is happening here (a switch from an 
> NCQ command to a non-NCQ command, then the non-NCQ times out). It could 
> be in some cases the 20 microsecond delay is not enough. But it seems 
> bogus that we should need such an arbitrary delay in the first place.
> 
> The question I had for NVIDIA regarding this that I never got answered 
> was, is there any reason why we would need a delay when switching 
> between NCQ and non-NCQ commands on ADMA, and if not, is there any known 
> cause that could cause the controller to get into this seemingly 
> locked-up state?

Well, I guess I did sort of get an answer, but the only theory was that 
the flush and the NCQ commands were being overlapped, which shouldn't be 
possible (the libata core guarantees that, and if it didn't work it 
would affect all controllers).

I'm kind of wondering if there's something funny going on with the 
notifier register stuff, which is supposed to tell us what commands have 
completed. We don't really use it at all (we had some problems with 
missed completions, etc. when I tried using it, also it doesn't work if 
ATAPI is enabled on the other port on the controller, apparently). I 
know these controllers will do strange things like not signalling 
interrupts for later events if you don't clear the notifiers in just the 
right way (that being mostly determined by trial and error).

Or, maybe somehow the flush is getting issued before the controller is 
really "ready" for it somehow (it's not finished cleaning up after 
preceding NCQ command).

It's pretty hard for me to figure out which of the above might be the 
case, especially without access to the detailed controller documentation..
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