Re: [PATCH 09/10] PCI: Align head space better

Ilpo Järvinen
2026-03-25T12:04:16Z

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On Tue, 24 Mar 2026, Ilpo Järvinen wrote:

> When a bridge window contains big and small resource(s), the small
> resource(s) may not amount to the half of the size of the big resource
> which would allow calculate_head_align() to shrink the head alignment.
> This results in always placing the small resource(s) after the big
> resource.
>
> In general, it would be good to be able to place the small resource(s)
> before the big resource to achieve better utilization of the address
> space. In the cases where the large resource can only fit at the end
> of the window, it is even required.
>
> However, carrying the information over from pbus_size_mem() and
> calculate_head_align() to __pci_assign_resource() and
> pcibios_align_resource() is not easy with the current data structures.
>
> A somewhat hacky way to move the non-aligning tail part to the head is
> possible within pcibios_align_resource(). The free space between the
> start of the free space span and the aligned start address can be
> compared with the non-aligning remainder of the size. If the free space
> is larger than the remainder, placing the remainder before the start
> address is possible. This relocation should generally work, because PCI
> resources consist only power-of-2 atoms.
>
> Various arch requirements may still need to override the relocation, so
> the relocation is only applied selectively in such cases.
>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221205
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> ---
> arch/arm/kernel/bios32.c | 3 +++
> arch/m68k/kernel/pcibios.c | 4 ++++
> arch/mips/pci/pci-generic.c | 3 +++
> arch/mips/pci/pci-legacy.c | 2 ++
> arch/parisc/kernel/pci.c | 3 +++
> arch/powerpc/kernel/pci-common.c | 2 ++
> arch/sh/drivers/pci/pci.c | 2 ++
> arch/x86/pci/i386.c | 2 ++
> arch/xtensa/kernel/pci.c | 2 ++
> drivers/pci/setup-res.c | 39 +++++++++++++++++++++++++++++++-
> include/linux/pci.h | 5 ++++
> kernel/resource.c | 2 +-
> 12 files changed, 67 insertions(+), 2 deletions(-)

> diff --git a/arch/sh/drivers/pci/pci.c b/arch/sh/drivers/pci/pci.c
> index 7a0522316ee3..994c3bd36ef2 100644
> --- a/arch/sh/drivers/pci/pci.c
> +++ b/arch/sh/drivers/pci/pci.c
> @@ -185,6 +185,8 @@ resource_size_t pcibios_align_resource(void *data, const struct resource *res,
> */
> if (start & 0x300)
> start = (start + 0x3ff) & ~0x3ff;
> + } else (res->flags & IORESOURCE_MEM) {

I'll be adding the missing if into this in v2 (found by sashiko). It seems
lkp didn't test this so it was not caught earlier.

> + start = pci_align_resource(dev, res, empty_res, size, align);
> }
>
> return start;

--
i.