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SubjectRe: PCI memory allocation bug with CONFIG_HIGHMEM

On Mon, 5 Jan 2004, David Hinds wrote:
> In arch/i386/kernel/setup.c we have:
> /* Tell the PCI layer not to allocate too close to the RAM area.. */
> low_mem_size = ((max_low_pfn << PAGE_SHIFT) + 0xfffff) & ~0xfffff;
> if (low_mem_size > pci_mem_start)
> pci_mem_start = low_mem_size;
> which is meant to round up pci_mem_start to the nearest 1 MB boundary
> past the top of physical RAM. However this does not consider highmem.
> Should this just be using max_pfn rather than max_low_pfn?

Yes and no. That doesn't really work either, for any machine with more
than 4GB of RAM.

We want to find the memory hole (in the low 4GB region), and usually the
e820 memory map should make that all happen properly. What does that
report on this laptop?

This is why we put the memory resources in /proc/iomem, and mark them
busy: so that the PCI subsystem won't try to allocate PCI memory in the
RAM (or ACPI reserved) area. The "pci_mem_start" thing is just a point to
_start_ the allocation, the PCI subsystem still should honor the fact that
we have memory above it. That's the whole point of doing proper resource
allocation, after all.

Does this not work, or have you disabled e820 for some reason?

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