linux-kernel - Re: Linux 2.6.25-rc4

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Message-ID: <20080316203109.50e8c526@core>
Date:	Sun, 16 Mar 2008 20:31:09 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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

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.

Alan
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