linux-kernel - Re: [PATCH] PCI/PM: Put devices to low power state on shutdown

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Message-ID: <20241009222403.GA507767@bhelgaas>
Date: Wed, 9 Oct 2024 17:24:03 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: Kai-Heng Feng <kaihengfeng@...il.com>, bhelgaas@...gle.com,
	mario.limonciello@....com, linux-pci@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] PCI/PM: Put devices to low power state on shutdown

On Fri, Sep 13, 2024 at 11:01:23AM +0300, Mika Westerberg wrote:
> On Fri, Sep 13, 2024 at 02:00:58PM +0800, Kai-Heng Feng wrote:
> > On Fri, Sep 13, 2024 at 12:57 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > On Thu, Sep 12, 2024 at 11:00:43AM +0800, Kai-Heng Feng wrote:
> > > > On Thu, Sep 12, 2024 at 3:05 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > > On Fri, Jul 12, 2024 at 02:24:11PM +0800, Kai-Heng Feng wrote:
> > > > > > Some laptops wake up after poweroff when HP Thunderbolt
> > > > > > Dock G4 is connected.
> > > > > >
> > > > > > The following error message can be found during shutdown:
> > > > > > pcieport 0000:00:1d.0: AER: Correctable error message received from 0000:09:04.0
> > > > > > pcieport 0000:09:04.0: PCIe Bus Error: severity=Correctable, type=Data Link Layer, (Receiver ID)
> > > > > > pcieport 0000:09:04.0:   device [8086:0b26] error status/mask=00000080/00002000
> > > > > > pcieport 0000:09:04.0:    [ 7] BadDLLP
> > > > > >
> > > > > > Calling aer_remove() during shutdown can quiesce the error
> > > > > > message, however the spurious wakeup still happens.
> > > > > >
> > > > > > The issue won't happen if the device is in D3 before
> > > > > > system shutdown, so putting device to low power state
> > > > > > before shutdown to solve the issue.
> > > > > >
> > > > > > I don't have a sniffer so this is purely guesswork,
> > > > > > however I believe putting device to low power state it's
> > > > > > the right thing to do.
> > > > >
> > > > > My objection here is that we don't have an explanation of
> > > > > why this should matter or a pointer to any spec language
> > > > > about this situation, so it feels a little bit random.
> ...

> I don't mean to confuse you guys but with this one too, I wonder if you
> tried to "disable" the device instead of putting it into D3? On another
> thread (Mario at least is aware of this) I mentioned that our PCIe SV
> folks identified a couple issues in Linux implementation around power
> management and one thing that we are missing is to disable I/O and MMIO
> upon entering D3.
> ...

This is really interesting -- did they discover a functional problem,
or did they just notice that we don't follow the PCI PM spec?

> +++ b/drivers/pci/pci.c
> @@ -2218,6 +2218,13 @@ static void do_pci_disable_device(struct pci_dev *dev)
>  		pci_command &= ~PCI_COMMAND_MASTER;
>  		pci_write_config_word(dev, PCI_COMMAND, pci_command);
>  	}
> +	/*
> +	 * PCI PM 1.2 sec 8.2.2 says that when a function is put into D3
> +	 * the OS needs to disable I/O and MMIO space in addition to bus
> +	 * mastering so do that here.
> +	 */
> +	pci_command &= ~(PCI_COMMAND_IO | PCI_COMMAND_MEMORY);
> +	pci_write_config_word(dev, PCI_COMMAND, pci_command);
>  
>  	pcibios_disable_device(dev);
>  }

This do_pci_disable_device() proposal is interesting.

pci_enable_device() turns on PCI_COMMAND_MEMORY and PCI_COMMAND_IO,
which enables the device to respond to MMIO and I/O port accesses to
its BARs from the driver.  It also makes sure the device is in D0,
because BAR access only works in D0.

pci_set_master() turns on PCI_COMMAND_MASTER, which enables the device
to perform DMA (including generating MSIs).

pci_disable_device() *sounds* like it should be the opposite of
pci_enable_device(), but it's currently basically the same as
pci_clear_master(), which clears PCI_COMMAND_MASTER to prevent DMA.
I didn't know about this text in 8.2.2, and I wish I knew why we
don't currently clear PCI_COMMAND_MEMORY and PCI_COMMAND_IO.

If we want to pursue this, I think it should be split to its own patch
and moved out of pci_disable_device() because I don't think this path
necessary implies putting the device in D3, so I think it would fit
better with the spec if we cleared PCI_COMMAND_MEMORY and
PCI_COMMAND_IO in a path that explicitly does put it in D3.

I think there's a significant chance of breaking something because
drivers are currently able to access BARs after pci_disable_device(),
and there are a LOT of callers.  But if there's a problem it would
fix, we should definitely explore it.

Bjorn