enabling DMA on hard drives

[Lennart Sorensen]

On Wed, Jan 19, 2005 at 10:59:04PM +0200, Peter L. Peres 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.

Well this one for example:
CONFIG_IDEDMA_PCI_AUTO=y
in .config seems to do it on debian.

Why would someone want to have to add all their disks to a script that
runs later in the boot sequence to get dma enabled on their drives when
the kernel can run code that looks at the controller abilities, the
drive abilities, blacklists, etc, and makes a determination about what
mode it should use.

The guy that writes the Linux ide code knows a whole lot more about ide
than I do.  I will let his knowledge and code decide if my drives can do
DMA or not (and all my drives can and do use DMA, since I guess I have
avoided the few bad drives made somehow).

Lennart Sorensen
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