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

[Bjorn Helgaas <helgaas@kernel.org>]

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.

The fundamental problem here is that assigned space to a bridge window
that isn't implemented.  I wish we understood the connection between
this "read window" path and the assignment path.

Maybe this patch fixes it because we enter pci_read_bridge_mmio_pref()
with res->flags being NULL, and we set IORESOURCE_MEM |
IORESOURCE_PREFETCH again, which makes it look like we can assign
space for it?

If that's the case, I think it would improve the commit log to mention
the actual mechanism by which we avoid assigning space.

> Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
> Link: https://lore.kernel.org/all/20260113210259.GA715789@bhelgaas/
> Signed-off-by: Ahmed Naseef <naseefkm@gmail.com>
> Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
> ---
>  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
>