linux-kernel - Re: Linux 2.6.25-rc4

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Message-ID: <47E3CE44.4000000@rtr.ca>
Date:	Fri, 21 Mar 2008 11:03:32 -0400
From:	Mark Lord <lkml@....ca>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Anders Eriksson <aeriksson@...tmail.fm>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.25-rc4

Alan Cox wrote:
> On Sun, 16 Mar 2008 12:39:43 -0700 (PDT)
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>>
>> On Sun, 16 Mar 2008, Alan Cox wrote:
>>> No it doesn't. DRQ simply means "drive has more data for the controller
>>> if you want it". Interrupts are controlled via IEN and the interrupt line.
>> A _lot_ of chips require you to clear the DRQ by taking the data they 
>> have.
> 
> Almost none and mostly very old ones. I'm not saying we shouldn't do it
> (except where it hangs the hardware - hence the FIFO flag) but for the
> traces presented and hardware reported it appears to be a bit of a red
> herring.
> 
>>> If the drive wants to give us data and we end the transaction that is
>>> fine. In practice a tiny few devices crap themselves if we don't.
>> More than a few tiny devices from what I remember. It tends to be the 
>> other way around - most devices do *not* want to get new commands until 
>> you've finished the previous one by draining the queues.
> 
> Not in my experience having maintained a lot of ATA drivers for a very
> long time. In fact the changes for draining the DRQ went into libata only
> very recently because it was only when we had a distro sized userbase
> with PATA devices that it became apparent that a few corner case problems
> remained.
..

Err.. I have a fairly modern PATA drive (160GB seagate 2.5")
that gets pissed if we leave DRQ hanging.  So it's not just
something for old/obsolete drives.

Or maybe it's the also-modern-ish ICH chipset that gets stuck.  Whatever.

-ml
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