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SubjectRe: [PATCH] x86/pci: make pci_mem_start to be aligned only -v4
Ingo Molnar wrote:
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> On Fri, 17 Apr 2009, Ingo Molnar wrote:
>>> Could we perhaps round up to 1MB in this case too?
>> (The below 1MB one).
>>
>> I'd argue against it, at least in this incarnation. I can well
>> imagine somebody wanting to do resource management in the 640k-1M
>> window, so..
>
> ok - indeed - if there's some super-small system with limited
> address lines and all physical addresses tightly packed with RAM?
>

No, much more likely that you're having PCI 2.x or PnP devices which
have 20-bit resources. It's probably worth noting that at least right
now, Linux mishandles 20-bit BARs and treat them like 32-bit BARs. It
turns out to actually work on a majority of the (quite few) known
devices which do have 20-bit BARs.

>
> BIOS-provided physical RAM map:
> BIOS-e820: 0000000000000000 - 000000000009fc00 (usable) 0.639 MB RAM
> BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved) 0.001 MB
> [ hole ] 0.250 MB
> BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved) 0.125 MB
> BIOS-e820: 0000000000100000 - 000000003ed94000 (usable) 1004.5 MB RAM
> BIOS-e820: 000000003ed94000 - 000000003ee4e000 (ACPI NVS) 0.7 MB
> BIOS-e820: 000000003ee4e000 - 000000003fea2000 (usable) 16.3 MB RAM
> BIOS-e820: 000000003fea2000 - 000000003fee9000 (ACPI NVS) 0.3 MB
> BIOS-e820: 000000003fee9000 - 000000003feed000 (usable) 0.15 MB RAM
> BIOS-e820: 000000003feed000 - 000000003feff000 (ACPI data 0.07 MB
> BIOS-e820: 000000003feff000 - 000000003ff00000 (usable) 0.004 MB RAM
> [ hole ] 1.0 MB
> [ hole ] 3072.0 MB
>
> On this map, using your scheme, we'd fill up that small 1MB hole up
> to 1GB [mockup]:
>
> BIOS-e820: 000000003ff00000 - 0000000040000000 (RAM buffer)
>
> I guess that's a good thing not just for robustness: a chipset might
> be faster when DMA or mmio is on some well-isolated physical memory
> range, not too close to real RAM or other devices?
>

Realistically, there probably is RAM there, probably consumed by the SMM
T-seg.

-hpa


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