Re: Permissions denied when enabling DMA
On 03 Aug 2003, Wayne Gemmell wrote:
> Hi
> >
> > The kernel you're using doesn't have support for your IDE chipset. This
> > is why the kernel didn't enable DMA on its own.
> How would you recomend that I get support? apt-get *new-kernel*, or apt-get
> source? Are there maybe modules that I should be looking for?
> >
> > Using hdparm to force DMA is a bad, bad, BAD idea. The kernel usually has
> > very good reasons for not enabling it.
> Do you mean that I shouldn't use hdparm at all? I have a laptop that is set by
> default with DMA and 32bit mode set off, is it also BAD to enable these on
> the laptop because I am able to. How do I make this a default on boot?
>
> Regards
> Wayne
>
>
I don't understand the objection to hdparm. Configuration help for the
kernel (2.4.21) says:
GENERIC PCI BUS-MASTER DMA SUPPORT
If your PCI system uses IDE drive(s) (as opposed to SCSI, say) and
is capable of bus-master DMA operation (most Pentium PCI systems),
you will want to say Y here to reduce CPU overhead. You can then use
the "hdparm" utility to enable DMA for drives for which it was not
enabled automatically. By default, DMA is not enabled automatically
for these drives, but you can change that by saying Y to the
following question "Use DMA by default when available". You can get
the latest version of the hdparm utility from
<ftp://ibiblio.org/pub/Linux/system/hardware/>.
Read the comments at the beginning of drivers/ide/ide-dma.c and the
file Documentation/ide.txt for more information.
Although this seems a little self-contradictory, it does appear to
approve of the use of hdparm if needed to get DMA working.
AC
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