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Subject[PATCH] x86/pci: make pci_mem_start to be aligned only -v2

Impact: make more big space below 4g for assigning to unassigned pci devices

don't need to reserved one round after the gapstart.

v2: Linus said: "
We've definitely seen ACPI code or integrated graphics stuff
that steals a lot of memory at the end, which means that end-of-RAM might
be not at 2GB, but at 2GB-16MB-1MB, for example (1MB of "ACPI data", and
16MB of "stolen video ram").

At a minimum, if we do this, I'd like to make sure we round up to a big
boundary (eg 32MB or something - exactly because a missing 16MB can easily
be some integrated stolen video memory).

Sure, we do that whole

while ((gapsize >> 4) > round)
round += round;

thing, so that if the gap is large, then we'll certainly get to 32MB too,
but I think your patch matters the most exactly when the gap is small.
Maybe we could just raise the initial minimum rounding from 1MB to 32MB?
Alternatively, maybe we can make sure that we round up to at least X bytes
from the end of RAM, and to at least Y bytes from the end of some RESERVED

Reported-and-tested-by: Yannick <>
Signed-off-by: Yinghai Lu <>

arch/x86/kernel/e820.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)

Index: linux-2.6/arch/x86/kernel/e820.c
--- linux-2.6.orig/arch/x86/kernel/e820.c
+++ linux-2.6/arch/x86/kernel/e820.c
@@ -619,6 +619,7 @@ __init void e820_setup_gap(void)
unsigned long gapstart, gapsize, round;
int found;
+ unsigned long low_top_ram;

gapstart = 0x10000000;
gapsize = 0x400000;
@@ -636,14 +637,32 @@ __init void e820_setup_gap(void)

* See how much we want to round up: start off with
- * rounding to the next 1MB area.
+ * rounding to the next 32MB area.
- round = 0x100000;
+ round = 0x2000000;
while ((gapsize >> 4) > round)
round += round;
+ pci_mem_start = roundup(gapstart, round);
+ low_top_ram = e820_end_of_low_ram_pfn() << PAGE_SHIFT;
+ /* try to check if there is gap between last RAM below 4g to that start */
+ if (pci_mem_start > low_top_ram) {
+ if (e820_any_mapped(low_top_ram, pci_mem_start, E820_RESERVED))
+ goto out;
+ if (e820_any_mapped(low_top_ram, pci_mem_start, E820_ACPI))
+ goto out;
+ if (e820_any_mapped(low_top_ram, pci_mem_start, E820_NVS))
+ goto out;
+ if ((pci_mem_start - low_top_ram) > round)
+ goto out;
+ }
/* Fun with two's complement */
pci_mem_start = (gapstart + round) & -round;

"Allocating PCI resources starting at %lx (gap: %lx:%lx)\n",
pci_mem_start, gapstart, gapsize);

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