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

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Message-ID: <20250821154745.GA677624@bhelgaas>
Date: Thu, 21 Aug 2025 10:47:45 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
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, Aug 21, 2025 at 06:24:12PM +0300, Ilpo Järvinen wrote:
> On Thu, 21 Aug 2025, Ilpo Järvinen wrote:
> > 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/

The email thread is fine and contains good information about how the
reporter tripped over it.

> > (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.)
> 
> Making it "official" tag in case that's easier for you to handle 
> automatically...
> 
> Link: https://lore.kernel.org/all/o2bL8MtD_40-lf8GlslTw-AZpUPzm8nmfCnJKvS8RQ3NOzOW1uq1dVCEfRpUjJ2i7G2WjfQhk2IWZ7oGp-7G-jXN4qOdtnyOcjRR0PZWK5I=@r26.me/

Thanks for all of these, I added them to the commit logs.

Bjorn