[PATCH v2 3/3] PCI: Skip bridge window reads when window is not supported

[Bjorn Helgaas]

On Mon, Mar 16, 2026 at 03:51:57PM +0000, Caleb James DeLisle wrote:
> pci_read_bridge_io() and pci_read_bridge_mmio_pref() read bridge window
> registers unconditionally. If the registers are hardwired to zero
> (not implemented), both base and limit will be 0. Since (0 <= 0) is
> true, a bogus window [mem 0x00000000-0x000fffff] or [io 0x0000-0x0fff]
> gets created.
> 
> pci_read_bridge_windows() already detects unsupported windows by
> testing register writability and sets io_window/pref_window flags
> accordingly. Check these flags at the start of pci_read_bridge_io()
> and pci_read_bridge_mmio_pref() to skip reading registers when the
> window is not supported.
> 
> Suggested-by: Bjorn Helgaas <helgaas at kernel.org>
> Link: https://lore.kernel.org/all/20260113210259.GA715789@bhelgaas/
> Signed-off-by: Ahmed Naseef <naseefkm at gmail.com>
> Signed-off-by: Caleb James DeLisle <cjd at cjdns.fr>

I applied this to pci/resource for v7.1 with the following commit log:

  PCI: Prevent assignment to unsupported bridge windows

  Previously, pci_read_bridge_io() and pci_read_bridge_mmio_pref()
  unconditionally set resource type flags (IORESOURCE_IO or IORESOURCE_MEM |
  IORESOURCE_PREFETCH) when reading bridge window registers. For windows that
  are not implemented in hardware, this may cause the allocator to assign
  space for a window that doesn't exist.

  For example, the EcoNET EN7528 SoC Root Port doesn't support the
  prefetchable window, but since a downstream device had a prefetchable BAR,
  the allocator mistakenly assigned a prefetchable window:

    pci 0001:00:01.0: [14c3:0811] type 01 class 0x060400 PCIe Root Port
    pci 0001:00:01.0: PCI bridge to [bus 01-ff]
    pci 0001:00:01.0: bridge window [mem 0x28000000-0x280fffff]: assigned
    pci 0001:00:01.0: bridge window [mem 0x28100000-0x282fffff pref]: assigned
    pci 0001:01:00.0: BAR 0 [mem 0x28100000-0x281fffff 64bit pref]: assigned

  pci_read_bridge_windows() already detects unsupported windows by testing
  register writability and sets dev->io_window/pref_window accordingly.

  Check dev->io_window/pref_window so we don't set the resource flags for
  unsupported windows, which prevents the allocator from assigning space to
  them.

  After this commit, the prefetchable BAR is correctly allocated from the
  non-prefetchable window:

    pci 0001:00:01.0: bridge window [mem 0x28000000-0x281fffff]: assigned
    pci 0001:01:00.0: BAR 0 [mem 0x28000000-0x280fffff 64bit pref]: assigned

I also set the author to "Ahmed Naseef <naseefkm at gmail.com>" per
https://lore.kernel.org/all/abRQYM1If/6Vv/tI@DESKTOP-TIT0J8O.localdomain

You can make this work correctly next time by including a
"From: Ahmed Naseef <naseefkm at gmail.com>" line as the very first line
in the message body; see
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=v6.19#n723

> ---
>  drivers/pci/probe.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index bccc7a4bdd79..4eacb741b4ec 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -395,6 +395,9 @@ static void pci_read_bridge_io(struct pci_dev *dev, struct resource *res,
>  	unsigned long io_mask, io_granularity, base, limit;
>  	struct pci_bus_region region;
>  
> +	if (!dev->io_window)
> +		return;
> +
>  	io_mask = PCI_IO_RANGE_MASK;
>  	io_granularity = 0x1000;
>  	if (dev->io_window_1k) {
> @@ -465,6 +468,9 @@ static void pci_read_bridge_mmio_pref(struct pci_dev *dev, struct resource *res,
>  	pci_bus_addr_t base, limit;
>  	struct pci_bus_region region;
>  
> +	if (!dev->pref_window)
> +		return;
> +
>  	pci_read_config_word(dev, PCI_PREF_MEMORY_BASE, &mem_base_lo);
>  	pci_read_config_word(dev, PCI_PREF_MEMORY_LIMIT, &mem_limit_lo);
>  	base64 = (mem_base_lo & PCI_PREF_RANGE_MASK) << 16;
> -- 
> 2.39.5
>