Re: [PATCH 02/11] PCI: Try to allocate mem64 above 4G at first

Bjorn Helgaas
2012-06-05T04:51:09Z

On Mon, Jun 4, 2012 at 7:37 PM, Yinghai Lu <yinghai@kernel.org> wrote:
> On Sun, Jun 3, 2012 at 6:05 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
>> On Fri, Jun 1, 2012 at 4:30 PM, Yinghai Lu <yinghai@kernel.org> wrote:
>>> On Tue, May 29, 2012 at 1:50 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>>>>
>>>> The bus-side address space should not be more than 32 bits no matter
>>>> what.  As Bjorn indicates, you seem to be mixing up bus and cpu
>>>> addresses all over the place.
>>>
>>> please check update patches that is using converted pci bus address
>>> for boundary checking.
>>
>> What problem does this fix?  There's significant risk that this
>> allocation change  will make us trip over something, so it must fix
>> something to make it worth considering.
>
> If we do not enable that, we would not find the problem.

Sorry, that didn't make any sense to me. I'm hoping you will point us
to a bug report that is fixed by this patch.

> On one my test setup that _CRS does state 64bit resource range,
> but when I clear some device resource manually and let kernel allocate
> high, just then find out those devices does not work with drivers.
> It turns out _CRS have more big range than what the chipset setting states.
> with fixing in BIOS, allocate high is working now on that platform.

I didn't understand this either, sorry. Are you saying that this
patch helps us work around a BIOS defect?

>> Steve's problem doesn't count because that's a "pci=nocrs" case that
>> will always require special handling.
>
> but pci=nocrs is still supported, even some systems does not work with
> pci=use_crs
>
>> A general solution is not
>> possible without a BIOS change (to describe >4GB apertures) or a
>> native host bridge driver (to discover >4GB apertures from the
>> hardware).  These patches only make Steve's machine work by accident
>> -- they make us put the video device above 4GB, and we're just lucky
>> that the host bridge claims that region.
>
> Some bios looks enabling the non-stated range default to legacy chain.
> Some bios does not do that. only stated range count.
> So with pci=nocrs we still have some chance to get allocate high working.

The patch as proposed changes behavior for all systems, whether we're
using _CRS or not (in fact, it even changes the behavior for non-x86
systems). The only case we know of where it fixes something is
Steve's system, where he already has to use "pci=nocrs" in order for
it to help. My point is that it would be safer to leave things as
they are for everybody, and merely ask Steve to use "pci=nocrs
pci=alloc_high" or something similar.

>> One possibility is some sort of boot-time option to force a PCI device
>> to a specified address.  That would be useful for debugging as well as
>> for Steve's machine.
>
> yeah, how about
>
> pci=alloc_high
>
> and default to disabled ?

I was actually thinking of something more specific, e.g., a way to
place one device at an exact address. I've implemented that a couple
times already for testing various things. But maybe a more general
option like "pci=alloc_high" would make sense, too.

Linux has a long history of allocating bottom-up. Windows has a long
history of allocating top-down. You're proposing a third alternative,
allocating bottom-up starting at 4GB for 64-bit BARs. If we change
this area, I would prefer something that follows Windows because I
think it will be closer to what's been tested by Windows. Do you
think your alternative is better?