RE: Responsibilty for enabling bus master enable bit?

["O'Shea, David J" <david.j.oshea@intel.com>]

Richard,

Good question. But the system BIOS are taking "ownership" of the bridge
devices,
because it is not reasonable to assume that a device driver can find the
bridges and
then configure an arbitrary depth of them to allow its device to function.

The assumption is made that if all of the devices behind the bridge are
disabled, then
leaving the BusMaster bit enabled in the bridge itself will not cause any
problems.  I
am sure that someone can find a P2P bridge somewhere by somebody that breaks
this assumption, but that is the assumption made at this point.  So bridges
are left
enabled by the system BIOS.
Regards,
David O'Shea

> -----Original Message-----
> From:	Richard Walter [SMTP:rwalter@corp.auspex.com]
> Sent:	Wednesday, September 02, 1998 10:46 AM
> To:	Mailing List Recipients
> Subject:	Responsibilty for enabling bus master enable bit?
> 
> Folks,
> 
> Following the white paper about warm reset, to prevent outstanding bus
> master
> requests from messing up memory during system boot, system BIOSes can turn
> off
> the bus master enable bit in all devices and the option ROM or drivers for
> the devices must turn them on again.
> 
> Now, suppose that a card vendor makes a card which consists of a
> PCI-to-PCI
> bridge and a device behind that bridge.  Who is resposible for turning on
> the bus master enable bit of the bridge?  Are all bridges considered
> property
> of the system BIOS and so it must do it?  Or, is anything on a plug-in
> card
> considered property of the card and so the option ROM must do it?
> 
> Sincerely,
> -Richard Walter
> rwalter@corp.auspex.com
> Note: I speak for myself, not for Auspex.
>