linux-kernel - Re: [PATCH 02/23] PCI: Rewrite bridge window head alignment function

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Message-ID: <20260126221723.GA318550@bhelgaas>
Date: Mon, 26 Jan 2026 16:17:23 -0600
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>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	linux-kernel@...r.kernel.org,
	Malte Schröder <malte+lkml@...ip.de>,
	stable@...r.kernel.org
Subject: Re: [PATCH 02/23] PCI: Rewrite bridge window head alignment function

On Fri, Dec 19, 2025 at 07:40:15PM +0200, Ilpo Järvinen wrote:
> The calculation of bridge window head alignment is done by
> calculate_mem_align() [*]. With the default bridge window alignment, it
> is used for both head and tail alignment.
> 
> The selected head alignment does not always result in tight-fitting
> resources (gap at d4f00000-d4ffffff):
> 
>     d4800000-dbffffff : PCI Bus 0000:06
>       d4800000-d48fffff : PCI Bus 0000:07
>         d4800000-d4803fff : 0000:07:00.0
>           d4800000-d4803fff : nvme
>       d4900000-d49fffff : PCI Bus 0000:0a
>         d4900000-d490ffff : 0000:0a:00.0
>           d4900000-d490ffff : r8169
>         d4910000-d4913fff : 0000:0a:00.0
>       d4a00000-d4cfffff : PCI Bus 0000:0b
>         d4a00000-d4bfffff : 0000:0b:00.0
>           d4a00000-d4bfffff : 0000:0b:00.0
>         d4c00000-d4c07fff : 0000:0b:00.0
>       d4d00000-d4dfffff : PCI Bus 0000:15
>         d4d00000-d4d07fff : 0000:15:00.0
>           d4d00000-d4d07fff : xhci-hcd
>       d4e00000-d4efffff : PCI Bus 0000:16
>         d4e00000-d4e7ffff : 0000:16:00.0
>         d4e80000-d4e803ff : 0000:16:00.0
>           d4e80000-d4e803ff : ahci
>       d5000000-dbffffff : PCI Bus 0000:0c
> 
> This has not been caused problems (for years) with the default bridge
> window tail alignment that grossly over-estimates the required tail
> alignment leaving more tail room than necessary. With the introduction
> of relaxed tail alignment that leaves no extra tail room whatsoever,
> any gaps will immediately turn into assignment failures.
> 
> Introduce head alignment calculation that ensures no gaps are left and
> apply the new approach when using relaxed alignment. We may want to
> consider using it for the normal alignment eventually, but as the first
> step, solve only the problem with the relaxed tail alignment.
> 
> ([*] I don't understand the algorithm in calculate_mem_align().)
> 
> Fixes: 5d0a8965aea9 ("[PATCH] 2.5.14: New PCI allocation code (alpha, arm, parisc) [2/2]")

check_commits complains that this SHA1 doesn't exist:

  In commit

    a21a27a0e893 ("PCI: Rewrite bridge window head alignment function")

  Fixes tag

    Fixes: 5d0a8965aea9 ("[PATCH] 2.5.14: New PCI allocation code (alpha, arm, parisc) [2/2]")

  has these problem(s):

    - Target SHA1 does not exist

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d0a8965aea9
does find it, but says it's not reachable.

It's so old (2002) that I'm not sure it's worth including it as a
Fixes: tag.

Bjorn