Re: [PATCH 05/23] PCI: Remove old_size limit from bridge window sizing

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jan 26, 2026 at 11:16:01AM -0600, Bjorn Helgaas wrote:
> On Fri, Dec 19, 2025 at 07:40:18PM +0200, Ilpo Järvinen wrote:
> > calculate_memsize() applies lower bound to the resource size before
> > aligning the resource size making it impossible to shrink bridge window
> > resources. I've not found any justification for this lower bound and
> > nothing indicated it was to work around some HW issue.
> > 
> > Prior to the commit 3baeae36039a ("PCI: Use pci_release_resource()
> > instead of release_resource()"), releasing a bridge window during BAR
> > resize resulted in clearing start and end address of the resource.
> > Clearing addresses destroys the resource size as a side-effect,
> > therefore nullifying the effect of the old size lower bound.
> > 
> > After the commit 3baeae36039a ("PCI: Use pci_release_resource() instead
> > of release_resource()"), BAR resize uses the aligned old size, which
> > results in exceeding what fits into the parent window in some cases:
> > 
> > xe 0030:03:00.0: [drm] Attempting to resize bar from 256MiB -> 16384MiB
> > xe 0030:03:00.0: BAR 0 [mem 0x620c000000000-0x620c000ffffff 64bit]: releasing
> > xe 0030:03:00.0: BAR 2 [mem 0x6200000000000-0x620000fffffff 64bit pref]: releasing
> > pci 0030:02:01.0: bridge window [mem 0x6200000000000-0x620001fffffff 64bit pref]: releasing
> > pci 0030:01:00.0: bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref]: releasing
> > pci 0030:00:00.0: bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref]: was not released (still contains assigned resources)
> > pci 0030:00:00.0: Assigned bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref] to [bus 01-04] free space at [mem 0x6200400000000-0x62007ffffffff 64bit pref]
> > pci 0030:00:00.0: Assigned bridge window [mem 0x6200000000000-0x6203fbff0ffff 64bit pref] to [bus 01-04] cannot fit 0x4000000000 required for 0030:01:00.0 bridging to [bus 02-04]
> > 
> > The old size of 0x6200000000000-0x6203fbff0ffff resource was used as
> > the lower bound which results in 0x4000000000 size request due to
> > alignment. That exceed what can fit into the parent window.
> > 
> > Since the lower bound never even was enforced fully because the
> > resource addresses were cleared when the bridge window is released,
> > remove the old_size lower bound entirely and trust the calculated
> > bridge window size is enough.
> > 
> > This same problem may occur on io window side but seems less likely to
> > cause issues due to general difference in alignment. Removing the lower
> > bound may have other unforeseen consequences in case of io window so
> > it's better to do leave as -next material if no problem is reported
> > related to io window sizing (BAR resize shouldn't touch io windows
> > anyway).
> > 
> > Reported-by: Simon Richter <Simon.Richter@hogyros.de>
> 
> I guess this report was
> https://lore.kernel.org/r/f9a8c975-f5d3-4dd2-988e-4371a1433a60@hogyros.de/,
> right?

And this looks like a regression in v6.18 that will persist in v6.19.

Is that the right thing?  I wonder if we should move these first five
patches to pci/for-linus so they land in v6.19?

> > Fixes: 3baeae36039a ("PCI: Use pci_release_resource() instead of release_resource()")
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> > ---
> >  drivers/pci/setup-bus.c | 11 +++--------
> >  1 file changed, 3 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> > index 612288716ba8..8660449f59bd 100644
> > --- a/drivers/pci/setup-bus.c
> > +++ b/drivers/pci/setup-bus.c
> > @@ -1071,16 +1071,13 @@ static resource_size_t calculate_memsize(resource_size_t size,
> >  					 resource_size_t min_size,
> >  					 resource_size_t add_size,
> >  					 resource_size_t children_add_size,
> > -					 resource_size_t old_size,
> >  					 resource_size_t align)
> >  {
> >  	if (size < min_size)
> >  		size = min_size;
> > -	if (old_size == 1)
> > -		old_size = 0;
> >  
> >  	size = max(size, add_size) + children_add_size;
> > -	return ALIGN(max(size, old_size), align);
> > +	return ALIGN(size, align);
> >  }
> >  
> >  resource_size_t __weak pcibios_window_alignment(struct pci_bus *bus,
> > @@ -1298,7 +1295,6 @@ static void pbus_size_mem(struct pci_bus *bus, unsigned long type,
> >  	resource_size_t children_add_size = 0;
> >  	resource_size_t children_add_align = 0;
> >  	resource_size_t add_align = 0;
> > -	resource_size_t old_size;
> >  
> >  	if (!b_res)
> >  		return;
> > @@ -1364,11 +1360,10 @@ static void pbus_size_mem(struct pci_bus *bus, unsigned long type,
> >  		}
> >  	}
> >  
> > -	old_size = resource_size(b_res);
> >  	win_align = window_alignment(bus, b_res->flags);
> >  	min_align = calculate_head_align(aligns, max_order);
> >  	min_align = max(min_align, win_align);
> > -	size0 = calculate_memsize(size, min_size, 0, 0, old_size, win_align);
> > +	size0 = calculate_memsize(size, min_size, 0, 0, win_align);
> >  
> >  	if (size0) {
> >  		resource_set_range(b_res, min_align, size0);
> > @@ -1378,7 +1373,7 @@ static void pbus_size_mem(struct pci_bus *bus, unsigned long type,
> >  	if (realloc_head && (add_size > 0 || children_add_size > 0)) {
> >  		add_align = max(min_align, add_align);
> >  		size1 = calculate_memsize(size, min_size, add_size, children_add_size,
> > -					  old_size, win_align);
> > +					  win_align);
> >  	}
> >  
> >  	if (!size0 && !size1) {
> > -- 
> > 2.39.5
> >