Re: [patch] Make MMCONFIG space (extended PCI config space) a driver opt-in issue

Robert Hancock
2007-12-22T16:22:50Z

Arjan van de Ven wrote:
> Hi,
>
> Linus really wants the extended (4Kb) PCI configuration space (using MCFG acpi table etc) to be opt-in, since there's many issues with it and most drivers don't even use/need it. The idea behind opt-in is that if you don't use it, you don't get to suffer the bugs...
>
> Booted on my 64 bit test machine; sadly it has a defunct BIOS that doesn't have a working MCFG.
>
>
> From: Arjan van de Ven <arjan@linux.intel.com>
> Subject: Make MMCONFIG space a driver opt-in
>
> There are many issues with using the extended PCI configuration space
> (CPU, Chipset and most of all BIOS bugs). This while the vast majority of drivers
> and devices don't even use/need to use the memory mapped access methods since they
> don't use the config space beyond the traditional 256 bytes.
>
> This patch makes accessing the extended config space a driver choice, via the
>
> pci_enable_ext_config(pdev)
>
> API function; drivers that want/need the extended configuration space should call this.
> (a separate patch will be posted to add this function call to the driver that uses this)

I don't really like this approach. Whether MMCONFIG works or not has
nothing to do with the device itself, it's an attribute of the machine,
and possibly the bus it's been plugged into. This patch might prevent
problems in some cases, but it's equally likely to just delay problems
until somebody plugs in a device that tries to use extended config
space. Neither do I really like the approach of limiting MMCONFIG
accesses to ones beyond a certain address in the config space, for a
similar reason.

The detection of whether MMCONFIG works or not has to work properly (and
I think we're pretty close, or at least we know what we need to do to
get there, like fixing the stupid MMCONFIG/PCI bar sizing overlap
problem, and likely Tony Camuso's patch or something like it, to disable
MMCONFIG accesses to devices behind certain broken host bridges). Once
that works, then this patch really serves no purpose.

--
Robert Hancock Saskatoon, SK, Canada
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