Messages in this thread | | | Date | Fri, 17 Apr 2009 15:04:59 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86/pci: make pci_mem_start to be aligned only -v4 |
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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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