E T wrote:
> > Is it absolutely sure in this case, that the CDROM supports
> > _UDMA_ ??? (It may only support _DMA_ which is not the
> > same thing).
>
> This is what BIOS used to say when booting...
Okay, but it would be better to check the CD-ROM documentation
(the BIOS auto-probe could be mistaken, only the CD-ROM doc
can tell you for sure).
> What is the difference between DMA and UDMA, if you don't mind?
As far as I remember, UDMA is directly driven by the PCI clock
(with UDMA/33 using one edge and UDMA/66 using both), whereas
normal DMA (which existed long before EIDE) doesn't have to be,
and can support slow devices.
Also, UDMA is specifically related to IDE controllers, whereas
DMA is not (it has its specific controller, used among other
things by lots of sound cards).
To activate _DMA_ in linux, you set UDMA off in the BIOS,
and set "Generic DMA support" and "use DMA by default"
(or something like this) in your kernel configuration,
or use hdparm with proper parameters.
> > As for myself I can choose in the BIOS (AWARD 4.51PG)
> > either PIO or UDMA (or AUTO) for each _device_, not
> > each _controller_. So I disabled UDMA for the CDROM on hdd,
> > but it is still active for both disks on hda and hdc.
>
> Same for me. The problem is, I cannot choose between UDMA/66 and
> UDMA/33 - so I have to keep UDMA off.
Do you mean that the BIOS auto-probe finds that the CD-ROM supports
UDMA/66 whereas it doesn't ???
This might well mean that the device does _not_ support UDMA at all
(it might support DMA). Please check with cd-rom doc.
--
Jean-Louis Debert [EMAIL PROTECTED]
74 Annemasse France
old Linux fan
