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List:       linux-ide
Subject:    Re: ide-io.c, ide_do_request -- race condition?
From:       Jens Axboe <axboe () suse ! de>
Date:       2004-07-16 17:57:30
Message-ID: 20040716175729.GI11255 () suse ! de
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On Fri, Jul 16 2004, Max T. Woodbury wrote:
> Jens Axboe wrote:
> > 
> > On Fri, Jul 16 2004, Max T. Woodbury wrote:
> > > "Eric D. Mudama" wrote:
> > > >
> > > > On Mon, Jul 12 at 13:52, Max T. Woodbury wrote:
> > > > >Still, why would PIO mode be unsafe?  (I can see slower, but I don't
> > > > >expect speed from this beast.  Oh well.  Thanks for the pointer.)
> > > >
> > > > PIO has no data integrity check, so bogus cables that glitch the data
> > > > will not be detected.  Not sure if that is what he was talking about,
> > > > but is definitely a problem for PIO.
> > >
> > > Huh? Unless something major has changed since the last time I looked at
> > > DMA hardware (and it has been a few years), DMA uses the same transfer
> > > sequence from the devices point of view as PIO.  The fact that the
> > > transfer is under the control of another device rather than a program
> > > should be transparent to the target device.  Impedance mismatches,
> > > reflections and constructive and destructive interference caused by
> > > cable problems don't care about who's in control of the busses.
> > >
> > > I can see a possible problem with cache consistency causing problems
> > > with PIO, but there are similar (abet in some sense inverted or
> > > reversed) problems with DMA.
> > 
> > Yes that's very clever of you. But read what Eric writes - PIO has no
> > data integrity check. DMA transfers are crc'ed so you know if something
> > goes bad between device and host in the data phase, with PIO you do not.
> 
> Sorry, NO.  From the device point of view, DMA and PIO are
> indistinguishable.

That's nonsense. Even the commands are different.

> Both have CRCs on some busses and neither have CRCs on others.  There
> are ALWAYS CRCs on transfers across the drive interface cables.  This
> is controlled by the IDE/CPU interface chip and not by the DMA
> hardware.  The transfer of the CRC is triggered by the termination of
> data transfer which happens with both DMA and PIO.  These are design
> issues that go back at least thirty years and are generally well
> understood.

So the icrc bit is where on the non-dma read commands? I repeat, if you
are in pio mode, you will not notice crc differences in data transferred
between host and device.

-- 
Jens Axboe

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