RE: BIOS Bus Master Option

["Olaf Birkeland" <Olaf.Birkeland@fast.no>]

> -----Original Message-----
> From: Lloyd Bircher [mailto:nicad2@yahoo.com]
> Sent: 7 December, 1999 17:22
> To: Mailing List Recipients
> Subject: BIOS Bus Master Option
>
>
> I am having an issue with a legacy PCI configuration
> in an x86 PC. There is an option in many BIOS setups
> for disabling Bus Mastering for ALL PCI devices in a
> system. The original purpose of this option was to
> allow broken Bus Masters to function. However, this
> setting does not selectively disable problem adapters,
> it disables Bus Mastering on ALL PCI devices.
>
> Does anyone know a good reason why this option should
> exist in a modern PCI system? Are there many cards
> that have broken BM capabilty?
>

I don't quite understand the issue on broken bus masters, but this setting
might be related to a PCI SIG white paper on "soft reset" problems in PCI
systems.

Since soft reset does not trigger a PCI reset in all systems, bus master
disabling is required to prevent a bus master with a pending access from
before the reset to start an unintended operation after the reset (since the
soft reset was not observed by the bus master device). If the BIOS assert
PCIRST# also on soft reset, then explicit bus master disabling is not
required as this is part of the PCI reset. In both cases, all PCI bus
masters will and should be disabled when BIOS POST completes.

Likewise, expansion ROM/device driver/operating system should always assume
that bus mastering is initially disabled, and enable each through the PCI
header after proper initialization.

Regards,
- Olaf