enabling DMA on hard drives

[Dave Stubbs]

Peter L. Peres wrote:

>
>
>
>
> On Wed, 19 Jan 2005, Lennart Sorensen wrote:
>
>> When asked to the kernel tends to do a good job, but you can build
>> kernels that don't default to turning on DMA.
>
>
> Thanks for the reality check ;-)
>
> I thought all kernels are built without DMA tuning built in in case a 
> disk is changed and wnats a DMA mode lower than atoselected. If that 
> happens, there will be trouble. In all systems I have seen so far the 
> dma was turned on by a script run at boot time. If you are aware of 
> kernel tuning parameters that allow the kernel itself to turn on DMA 
> (why would one want that ?), please share them.

Sure.

In the 2.4 Kernel, just set the CONFIG_IDEDMA_PCI_AUTO flag to yes.

Here's the help message in the "menuconfig" help screen for this option:
-------------------------------------
Prior to kernel version 2.1.112, Linux used to automatically use DMA for 
IDE drives and chipsets which support it. Due to concerns about a couple 
of cases where buggy hardware may have caused damage, the default is now 
to NOT use DMA automatically. To revert to the previous behaviour, say Y 
to this question. 

If you suspect your hardware is at all flakey, say N here.  Do NOT email 
the IDE kernel people regarding this issue! 
It is normally safe to answer Y to this question unless your motherboard 
uses a VIA VP2 chipset, in which case you should say N.   
--------------------------------------

I just checked and the 2.6 Kernel config says the same thing.

Best to let the kernel do what it does best.  Heck - even Windows seems 
to get THIS one right most of the time...

Dave
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