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Date: Tue, 29 Sep 2009 09:54:16 +0200
From: "Lennart Baruschka" <FunFlyer@gmx.net>
In-Reply-To: <20090928234025.6dc4e3f7@lxorguk.ukuu.org.uk>
Message-ID: <20090929075416.77960@gmx.net>
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References: <1254173481.4454.32.camel@goodbyte.homelinux.com>
 <20090928234025.6dc4e3f7@lxorguk.ukuu.org.uk>
Subject: Re: Disabling DMA with ICH10?
To: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
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Hi, 

On Mon, 2009-09-28 at 23:40 +0100, Alan Cox wrote:
> > 2. My system uses an ICH10 chipset, the harddrive is connected to the
> > Intel SATA controller. Is there a way to disable DMA and switch back to
> > PIO? I tried compiling with libata support and without SCSI support, but
> > the kernel is unable to mount root (no matter if /dev/sda1
> > or /dev/hda1), then.
> 
> If you disable DMA you will make your performance and latency worse not
> better as the PIO transfers will stall the bus and thus the processor.

I thought that PIO transfers (which I understand to be
write32()/read32()'s) unlike DMA transfers could be interrupted by an
high-priority interrupt. Is that wrong?


> If you really are that latency sensitive then the more normal approach
> would be to lock one core for real time use, load the critical code into
> that core CPU cache and run from cache. If you are utterly pushing the
> limit you might even do crazy stuff like use on thread on the core to
> execute RT stuff and the other to issue any I/O accesses that might stall.

Actually, that's what I do - except for locking the page, yet. I do need
to access the PCI bus in real time, though. So I wonder what happens
when the RT CPU is getting data from the PCI device, doing some
calculations on it and then writing back some data to the device,
__while at the same time__ another (non-RT) CPU starts a DMA transfer. I
figured the DMA would block the PCI bus, having my interrupt wait for it
to finish. That's why I'm trying to avoid DMA. 

Please let me know at what point I am mistaken.


Cheers
Lennart

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