[clue-tech] 2.6.18 (CentOS 5.x) kernel initialize IDE drives in UDMA2 mode?

[David L. Anselmi]

Jim Ockers wrote:
> Hi CLUEbies,
> 
> For various very important reasons we have to run our Linux systems with 
> CentOS 5.x with all IDE drives in UDMA2 (or slower) mode.  Of course the 
> 2.6 kernel IDE device driver, the chipset on our motherboard, and the 
> drives, all support UDMA5 or UDMA6 and autonegotiate the fastest speed 
> they support when the driver initializes the drive.

For such a specific (and rare, as in most people say "who cares?") 
configuration it might be worth standardizing on hardware that works.

It might be worth offering money to a kernel hacker to fix it.  It might 
be worth asking Red Hat to support your needs and then switching to RHEL.

> When we try to use hdparm to change it back to UDMA2 (hdparm -d 1 -X 
> udma2 /dev/hdX) we occasionally get errors.
...
> AND THEN DMA IS DISABLED WHICH IS REALLY BAD FOR OUR SYSTEMS.

Can you use hdparm to enable it again, at least?

If you *have* to run in UDMA2 then you *have* to pitch the systems that 
don't work.

> I think that trying to set the DMA mode after initialization is maybe 
> not a good idea.  
...
> By the way the IDE driver in the CentOS 5.x kernel 
> (2.6.18) is not a module.  My googling has not turned up anything and 
> there is no kernel command line option for the IDE driver that does this.

I don't know which version it's for, but this:

http://www.kernel.org/doc/Documentation/kernel-parameters.txt

says you can use libata.force to set UDMA 2.  Doesn't it?

HTH,
Dave