This would be legacy behaviour from the days of buggy ATA33/UDMA
implementations, where falling back to PIO mode would allow a device with a
buggy UDMA implementation (Unfortunately rather common at the time) to
----- Original Message -----
From: "Bruce Campbell" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: <[EMAIL PROTECTED]>
Sent: Sunday, January 05, 2003 10:01 PM
Subject: Re: ata "fallback to PIO mode" on dual processor AMD systems
> Quoting Bruce Campbell <[EMAIL PROTECTED]>:
> > Quoting Matthew Emmerton <[EMAIL PROTECTED]>:
> > > [ cc'ing Soren since he's the ATA guru ]
> > >
> > > > Dec 30 23:27:00 ecserv13 /kernel: ad0: trying fallback to PIO mode
> > > > Dec 30 23:27:00 ecserv13 /kernel: ata0: resetting devices .. done
> > > >
> > > > The test continues to run with the ata controller in PIO mode, with
> > > > slower performance, and higher load average.
> > > >
> > > > Once the master drops to PIO, attempts to access the slave then
> > > > it to drop to PIO.
> > >
> > > Are you using 80-conductor cables on all your drives? These are
> > to
> > > get consistent high throughput, and running without them may cause the
> > > problems you're seeing.
> > Thanks for the information about the design of IDE etc, and the
> > about the cables. I was about to shuffle things to get the disks
> > onto separate channels, but I now see that would be a mistake as my
> > CD drive would share a cable with a disk.
> ps. As an aside, I have since determined that putting a PIO device and
> a UDMA device on the same channel does not affect the performance
> of the UDMA device, unless the PIO device is in use. So, sharing
> a low use CD rom drive with a disk wouldn't be so bad.
> I am puzzled about the fallback to PIO concept. If a disk has
> gives some sort of timeout error or whatever, why would trying
> PIO correct the problem ? That seems equivalent to asking the
> disk to do the same thing, just more slowly.
> In my case, some sort of timeout error occurs on ad0, so
> it falls back to PIO, and works. A later access to ad1
> also yields a timeout error, and then it drops to PIO,
> and works too. I'm fairly confident both disks did not
> experience media errors at the same time, which suggests
> a problem with the onboard IDE controller, or a driver bug.
> Tests continue...
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