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

[Ahmed Naseef]

On Tue, Mar 17, 2026 at 04:29:08PM -0500, Bjorn Helgaas wrote:
> 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?

Yes, that's exactly right.

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

How about this:

  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, these functions set
  IORESOURCE_IO or IORESOURCE_MEM | IORESOURCE_PREFETCH on
  the bridge resource. This causes the allocator to assign
  space for the window even though the hardware can't
  implement it.

  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, so the
  resource flags remain clear and the allocator does not assign
  space for non-existent windows.


Ahmed Naseef

> > 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>
> > ---
> >  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
> >