linux-kernel - Re: sata_nv + ADMA + Samsung disk problem

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Message-ID: <477D02E0.5040301@rtr.ca>
Date:	Thu, 03 Jan 2008 10:44:32 -0500
From:	Mark Lord <liml@....ca>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Mark Lord <lkml@....ca>, Allen Martin <AMartin@...dia.com>,
	Jeff Garzik <jeff@...zik.org>, Tejun Heo <htejun@...il.com>,
	Gabor Gombas <gombasg@...aki.hu>, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, Kuan Luo <kluo@...dia.com>,
	Peer Chen <pchen@...dia.com>
Subject: Re: sata_nv + ADMA + Samsung disk problem

Robert Hancock wrote:
> Mark Lord wrote:
>> Robert Hancock wrote:
>> ..
>>>  From some of the traces I took previously (posted on LKML as 
>>> "sata_nv ADMA controller lockup investigation" way back in Feb 07), 
>>> what seems to occur is that when the second command is issued very 
>>> rapidly (within less than 20 microseconds, or potentially longer) 
>>> after the previous command's completion, the ADMA status changes from 
>>> 0x500 (STOPPED and IDLE) to 0x400 (just IDLE) as it typically does, 
>>> but then it sticks there, no interrupt is ever raised, and CPB 
>>> response flags remain at 0.
>> ..
>>
>> Assuming that NVidia got their ADMA core logic from Pacific Digital
>> (the inventors), then it may have some of the same bugs as the original.
>>
>> One of those bugs is that the aGO trigger is sampled in a "racey" way,
>> such that it sometimes may miss a recent addition to the ring.
>>
>> The *only* way to guarantee things with the original Pacific Digital core
>> was to (1) always retrigger aGO for a full ring scan with each new 
>> addition,
>> and (2) poll periodically (every half second or so) rather than relying
>> exclusively on the IRQ actually working..
>>
>> Dunno about the NVidia version.
> 
> Theirs works rather differently - the GO bit is there, but there's 
> another append register which is used to tell the controller that a new 
> tag has been added to the CPB list.
..

The PacDigi core uses a "search count" register for that purpose,
but the buggy nature of the core required that it always be set
to "2 * ring_size" to ensure nothing got missed.

Here's some comments from the original ADMA driver.
Maybe something from here might help with the NV stuff, too.

       // There is a chance that the chip will skip over a CPB if a SERVICE interrupt
        // occurs while it's reading the CPB header.  This won't cause us to get
        // stuck anywhere, but it might slow down execution of the new CPB if
        // it has to wait for the next time we hit aGO.  So.. Dxxx/Dxxx suggest
        // that all we need to do is tell the chip to do two passes around the ring
        // from an aGO instead of one pass, so that it will find the "missed" CPB
        // on the second pass.  This isn't as bad as it first looks.
        //
        writew(channel->num_cpbs * 2, &adma_regs->cpb_search_count);

Or again, the NV stuff may be completely different (?).
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