Re: [RFC] PCI: Unassigned Expansion ROM BARs

From: Myron Stowe
Date: Thu Sep 24 2015 - 13:06:59 EST

On Wed, Sep 23, 2015 at 9:21 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Wed, Sep 23, 2015 at 7:47 PM, Myron Stowe <myron.stowe@xxxxxxxxx> wrote:
>> The kernel expects device Expansion ROM BARs to be programmed with valid
>> values - even if the respective Expansion ROM's Enable bit is 0 (i.e. the
>> deviceâs expansion ROM address space is disabled). This seems to be the
>> main contention point with said BIOS engineers. If an Expansion ROM BAR is
>> not programmed, the kernel will attempt to find available resources and, if
>> successful, program it. As this occurs various 'dmesg' entries
>> related to kernel's actions are output.
> ...
>> There is a kernel boot parameter, pci=norom, that is intended to disable the
>> kernel's resource assignment actions for Expansion ROMs that do not already
>> have BIOS assigned address ranges. Note however, if I remember correctly,
>> that this only works if the Expansion ROM BAR is set to "0" by the BIOS
>> before hand-off.
> option rom is used by legacy bios to enable booting from external device.
> usually BIOS call the option rom, so the firmware will be loaded to
> add on cards.
> and firmware get started.
> Also option rom would include tools that is used to configure behavior of cards
> like add/remove raid.

I'm not sure what you are getting at here but yes, there are use cases
where the BIOS
needs to access the Expansion ROM, one of the more common being PXE booting from
a NIC device where the PXE boot content is retrieved from the ROM, but
that has little,
if anything, to do with what I'm after here.

The BIOS engineers are expressing that the kernel should *never* need
to access the Expansion
ROM, and thus should *never* try to allocate resources for these BARs
and program them
to sane address range values.

I know you work with bringing up new hardware. So picture yourself
sitting with some
members from your platform's BIOS team. They tell you: "The OS is
incorrect in thinking
it needs to find, and program, sane resource range values into a
device's Expansion ROM
BAR. We (the BIOS) hand-off the platform with these disabled, thus
whatever values are in
the ROMs BAR should be totally ignored, and the OS should never touch
them." What would you
reply with to them in an attempt to show that your position (i.e. the
kernel finding, and programming
values under these circumstances) is correct and that the BIOS opinion
is in-correct? That is
what I'm after.

> Also there is some use case that kernel driver try to get some parameters from
> BIOS. like intel soft raid ? --- bad practice !

Again, your replies are so terse I have no idea what you are saying; it's
undecipherable! Are you indicating that you agree with the BIOS
engineers views?

> I would like to treat option rom BAR as optional resources during
> resource allocation.
> Subject: PCI: Treat ROM resource as optional during realloc
> Current on realloc path, we just ignore ROM resource if we can not assign
> them in first try.
> Treat ROM resources as optional resources,so try to allocate them together
> with required ones, if can not assign them, could go with other required
> resources only, and try to allocate them second time in expand path.

Yes, while they may have lower priority in obtaining resources, your still
attempting to do so initially. The BIOS engineers seem to believe that this is
incorrect - the OS should not even attempt to allocate them in the first try.

So, which side are you on and can you support your view with some
technical based
argument (and any references from the specifications)?

Please take some time and respond with some thought out explanations
and opinions.
I value your opinion because I have seen your work but your terse
replies are going to
do nothing what so ever in trying to convince BIOS engineers that the
OS should, or needs
to, access such. Otherwise: "Why are we (the kernel) allocating
resources for them?"


> Thanks
> Yinghai
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