Comment 31 for bug 36185

(In reply to comment #30)
> (In reply to comment #26)
>
> > I know that some distributions simply enable it by default, which might be
> > acceptable for their audience, but not for Ubuntu.
>
> Would it be acceptable to have two installation targets say "default" and
> "failsafe". So, by default DMA would be switched on (both during and after the
> install) but if that failed you could install using "failsafe" which would
> disable DMA (not to mention APIC, LAPIC and any others that cause issues) during
> and after install. If it were clearly documented I would say most users would
> be happy enough with this.

This is essentially the approach used by Knoppix for CD DMA ("turn it on unless
the user tells us otherwise"). This causes the boot to fail on systems where
DMA doesn't work properly, requiring the user to a) be persistent and try to
continue installing, despite a catastrophic failure, b) find the appropriate
documentation, and c) interact with the unfriendly isolinux command line. This
seems like more than enough to stop a first-time user in their tracks.

Also, I don't think it would be best to mix this with other hardware-related
parameters; disabling other features like APIC support can cause a new failure
which didn't happen when it was enabled.

I much prefer that we get this right, and enable DMA only where it is known to work.

> If extra stuff could be added to the default where
> it sensed a known bad dvdrom/controller and dropped back that would be better
> again so as to expose as few people as possible to any kind of failure.

See earlier comments; we don't yet know what to look for.

> One thing that seems fairly apparent is that if those distros get away with this
> then relatively few installations fail despite DMA being enabled. Ubuntu
> understandably doesn't want to lock that hardware out. It seems that few would
> need the "failsafe" option but it's important that it's there.

Ubuntu has tried this once already (Warty inadvertently shipped with DMA enabled
on CD-ROMs), and the number of failed installations was significant.