linux-kernel - Re: [PATCH v2 1/3] PCI: Relaxed tail alignment should never increase min

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Message-ID: <3ded51ff-6265-da0a-d8a1-ecf788e99d31@linux.intel.com>
Date: Thu, 21 Aug 2025 18:15:35 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Bjorn Helgaas <helgaas@...nel.org>
cc: linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>, 
    Krzysztof Wilczyński <kw@...ux.com>, 
    Tudor Ambarus <tudor.ambarus@...aro.org>, Rio <rio@....me>, 
    D Scott Phillips <scott@...amperecomputing.com>, 
    LKML <linux-kernel@...r.kernel.org>, 
    Christian König <christian.koenig@....com>, 
    stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] PCI: Relaxed tail alignment should never increase
 min_align

On Thu, 21 Aug 2025, Bjorn Helgaas wrote:

> On Mon, Jun 30, 2025 at 05:26:39PM +0300, Ilpo Järvinen wrote:
> > When using relaxed tail alignment for the bridge window,
> > pbus_size_mem() also tries to minimize min_align, which can under
> > certain scenarios end up increasing min_align from that found by
> > calculate_mem_align().
> > 
> > Ensure min_align is not increased by the relaxed tail alignment.
> > 
> > Eventually, it would be better to add calculate_relaxed_head_align()
> > similar to calculate_mem_align() which finds out what alignment can be
> > used for the head without introducing any gaps into the bridge window
> > to give flexibility on head address too. But that looks relatively
> > complex algorithm so it requires much more testing than fixing the
> > immediate problem causing a regression.
> > 
> > Fixes: 67f9085596ee ("PCI: Allow relaxed bridge window tail sizing for optional resources")
> > Reported-by: Rio <rio@....me>
> 
> Was there a regression report URL we could include here?

There's the Lore thread only:

https://lore.kernel.org/all/o2bL8MtD_40-lf8GlslTw-AZpUPzm8nmfCnJKvS8RQ3NOzOW1uq1dVCEfRpUjJ2i7G2WjfQhk2IWZ7oGp-7G-jXN4qOdtnyOcjRR0PZWK5I=@r26.me/

(It's so far back that if there was something else, I've forgotten them 
by now but looking at the exchanges in the thread, it doesn't look like 
bugzilla entry or so made out of it.)

-- 
 i.

> > Tested-by: Rio <rio@....me>
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> > Cc: <stable@...r.kernel.org>
> > ---
> >  drivers/pci/setup-bus.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> > index 07c3d021a47e..f90d49cd07da 100644
> > --- a/drivers/pci/setup-bus.c
> > +++ b/drivers/pci/setup-bus.c
> > @@ -1169,6 +1169,7 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
> >  	resource_size_t children_add_size = 0;
> >  	resource_size_t children_add_align = 0;
> >  	resource_size_t add_align = 0;
> > +	resource_size_t relaxed_align;
> >  
> >  	if (!b_res)
> >  		return -ENOSPC;
> > @@ -1246,8 +1247,9 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
> >  	if (bus->self && size0 &&
> >  	    !pbus_upstream_space_available(bus, mask | IORESOURCE_PREFETCH, type,
> >  					   size0, min_align)) {
> > -		min_align = 1ULL << (max_order + __ffs(SZ_1M));
> > -		min_align = max(min_align, win_align);
> > +		relaxed_align = 1ULL << (max_order + __ffs(SZ_1M));
> > +		relaxed_align = max(relaxed_align, win_align);
> > +		min_align = min(min_align, relaxed_align);
> >  		size0 = calculate_memsize(size, min_size, 0, 0, resource_size(b_res), win_align);
> >  		pci_info(bus->self, "bridge window %pR to %pR requires relaxed alignment rules\n",
> >  			 b_res, &bus->busn_res);
> > @@ -1261,8 +1263,9 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
> >  		if (bus->self && size1 &&
> >  		    !pbus_upstream_space_available(bus, mask | IORESOURCE_PREFETCH, type,
> >  						   size1, add_align)) {
> > -			min_align = 1ULL << (max_order + __ffs(SZ_1M));
> > -			min_align = max(min_align, win_align);
> > +			relaxed_align = 1ULL << (max_order + __ffs(SZ_1M));
> > +			relaxed_align = max(min_align, win_align);
> > +			min_align = min(min_align, relaxed_align);
> >  			size1 = calculate_memsize(size, min_size, add_size, children_add_size,
> >  						  resource_size(b_res), win_align);
> >  			pci_info(bus->self,
> > -- 
> > 2.39.5
> > 
>