linux-kernel - Re: [PATCH v2 1/2] PCI/ASPM: Override the ASPM and Clock PM states set by BIOS for devicetree platforms

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Message-ID: <sttsqedadw4cdozfrjkl7jlqk2jtwkaniukczfwm5e7ymdtsh3@jkq3unz2ez2k>
Date: Fri, 31 Oct 2025 11:51:31 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Shawn Lin <shawn.lin@...k-chips.com>, 
	Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>, Niklas Cassel <cassel@...nel.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Krzysztof Wilczyński <kwilczynski@...nel.org>, Rob Herring <robh@...nel.org>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	"David E. Box" <david.e.box@...ux.intel.com>, Kai-Heng Feng <kai.heng.feng@...onical.com>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Heiner Kallweit <hkallweit1@...il.com>, 
	Chia-Lin Kao <acelan.kao@...onical.com>, Dragan Simic <dsimic@...jaro.org>, 
	linux-rockchip@...ts.infradead.org, regressions@...ts.linux.dev, FUKAUMI Naoki <naoki@...xa.com>
Subject: Re: [PATCH v2 1/2] PCI/ASPM: Override the ASPM and Clock PM states
 set by BIOS for devicetree platforms

On Fri, Oct 17, 2025 at 08:45:54AM -0500, Bjorn Helgaas wrote:
> On Fri, Oct 17, 2025 at 06:24:26PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Oct 17, 2025 at 08:19:11PM +0800, Shawn Lin wrote:
> > > 在 2025/10/17 星期五 18:04, Manivannan Sadhasivam 写道:
> > > > On Fri, Oct 17, 2025 at 05:47:44PM +0800, Shawn Lin wrote:
> > ...
> 
> > > > > While we're on the topic of ASPM, may I ask a silly question?
> > > > > I saw the ASPM would only be configured once the function
> > > > > driver calling pci_enable_device. So if the modular driver
> > > > > hasn't been insmoded, the link will be in L0 even though there
> > > > > is no transcation on-going. What is the intention behind it?
> > > > 
> > > > I don't see where ASPM is configured during pci_enable_device().
> > > > It is currently configured for all devices during
> > > > pci_scan_slot().
> > > 
> > > This is the dump_stack() where I observed. If I compile NVMe
> > > driver as a module and never insmod it, the link is always in L0,
> > > namely ASPM Disabled.
> > 
> > I guess this comment answers your question:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pcie/aspm.c?h=v6.18-rc1#n1179
> 
> The comment is:
> 
>    * At this stage drivers haven't had an opportunity to change the
>    * link policy setting. Enabling ASPM on broken hardware can cripple
>    * it even before the driver has had a chance to disable ASPM, so
>    * default to a safe level right now. If we're enabling ASPM beyond
>    * the BIOS's expectation, we'll do so once pci_enable_device() is
>    * called.
> 
> I don't think relying on a driver to disable ASPM to avoid broken
> hardware is the right answer.  If the driver is never loaded, we waste
> power.  And if the user enables ASPM via sysfs, apparently the device
> may be crippled.
> 
> I think it would be better to have an enumeration-time quirk to keep
> us from enabling ASPM.  We might trip over some of this broken
> hardware, but I don't think there are very many drivers that fiddle
> with ASPM, so we should be able to be proactive about it.
> 

There are quite a bit of drivers fiddling with ASPM states:

git grep -l PCI_EXP_LNKCTL_ASPMC drivers/ | wc -l

16

- Mani

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