Re: PCI resource problems caused by improper address rounding

Chuck Ebbert
2007-12-18T17:34:35Z

On 12/17/2007 07:57 PM, Linus Torvalds wrote:
>
> On Mon, 17 Dec 2007, Chuck Ebbert wrote:
>> Looks like a commit that I can't find in git due to the arch merge
>> has broken PCI address assignment. This patch by Richard Henderson
>> against 2.6.23 fixes it for x86_64:
>>
>> --- linux-2.6.23.x86_64/arch/x86_64/kernel/e820.c 2007-10-09 13:31:38.000000000 -0700
>> +++ linux-2.6.23.x86_64-rth/arch/x86_64/kernel/e820.c 2007-12-15 12:37:44.000000000 -0800
>> @@ -718,8 +718,8 @@ __init void e820_setup_gap(void)
>> while ((gapsize >> 4) > round)
>> round += round;
>> /* Fun with two's complement */
>> - pci_mem_start = (gapstart + round) & -round;
>> + pci_mem_start = (gapstart + round - 1) & -round;
>
> No, it's very much meant to be that way.
>
> We do *not* want to have the PCI memory abutthe end of memory exactly. So
> it leaves a gap in between "gapstart" and the actual start of PCI memory
> addressing very much on purpose.
>
> In fact, the very commit (it's f0eca9626c6becb6fc56106b2e4287c6c784af3d in
> the kernel tree) you mention actually explicitly *explains* that, although
> maybe it's a bit indirect: if you start allocating PCI resources directly
> after the end-of-RAM thing, you can easily end up using addresses that are
> actually inside the magic stolen system RAM that is being used for UMA
> video etc.
>
> So you very much want to have a buffer in between the end-of-RAM and the
> actual start of the region we try to allocate in.
>
> So why do you want them to be close, anyway?
>

Because otherwise some video adapters with 256MB of memory end up with their
resources allocated above 4GB, and that doesn't work very well.

https://bugzilla.redhat.com/show_bug.cgi?id=425794#c0

>
> PS. On a different topic: if you do
>
> git log --follow arch/x86/kernel/e820_64.c
>
> you'd see the history past the renames in git. Or just do a "git blame -C"
> which will also follow renames (and copies).

The history in the web interface just ends at the rename.