linux-kernel - Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend

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Message-ID: <20220516200927.GA1046452@bhelgaas>
Date:   Mon, 16 May 2022 15:09:27 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Jingar, Rajvi" <rajvi.jingar@...el.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        David Box <david.e.box@...ux.intel.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM

On Fri, May 13, 2022 at 10:00:48PM +0000, Jingar, Rajvi wrote:
> 
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@...nel.org>
> > Sent: Thursday, May 12, 2022 11:36 AM
> > To: Rafael J. Wysocki <rafael@...nel.org>
> > Cc: Jingar, Rajvi <rajvi.jingar@...el.com>; Wysocki, Rafael J
> > <rafael.j.wysocki@...el.com>; Bjorn Helgaas <bhelgaas@...gle.com>; David Box
> > <david.e.box@...ux.intel.com>; Linux PCI <linux-pci@...r.kernel.org>; Linux
> > Kernel Mailing List <linux-kernel@...r.kernel.org>; Linux PM <linux-
> > pm@...r.kernel.org>
> > Subject: Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM
> > 
> > On Thu, May 12, 2022 at 07:52:36PM +0200, Rafael J. Wysocki wrote:
> > > On Thu, May 12, 2022 at 7:42 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > On Thu, May 12, 2022 at 03:49:18PM +0200, Rafael J. Wysocki wrote:
> > 
> > > > > Something like this should suffice IMV:
> > > > >
> > > > > if (!dev_state_saved || pci_dev->current_state != PCI_D3cold)
> > > > >
> > > > >         pci_disable_ptm(pci_dev);
> > > >
> > > > It makes sense to me that we needn't disable PTM if the device is in
> > > > D3cold.  But the "!dev_state_saved" condition depends on what the
> > > > driver did.  Why is that important?  Why should we not do the
> > > > following?
> > > >
> > > >   if (pci_dev->current_state != PCI_D3cold)
> > > >     pci_disable_ptm(pci_dev);
> > >
> > > We can do this too.  I thought we could skip the power state
> > > check if dev_state_saved was unset, because then we would know
> > > that the power state was not D3cold.  It probably isn't worth
> > > the hassle though.
> 
> We see issue with certain platforms where only checking if device
> power state in D3Cold is not enough and the !dev_state_saved check
> is needed when disabling PTM. Device like nvme is relying on ASPM,
> it stays in D0 but state is saved. Touching the config space wakes
> up the device which prevents the system from entering into low power
> state.

Correct me if I'm wrong: for NVMe devices, nvme_suspend() has already
saved state and put the device in some low-power state.  Disabling PTM
here is functionally OK but prevents a system low power state, so you
want to leave PTM enabled.

But I must be missing something because pci_prepare_to_sleep()
currently disables PTM for Root Ports.  If we leave PTM enabled on
NVMe but disable it on the Root Port above it, any PTM Request from
NVMe will cause an Unsupported Request error.

Disabling PTM must be coordinated across PTM Requesters and PTM
Responders.  That means the decision to disable cannot depend on
driver-specific things like whether the driver has saved state.

> Following would fix the issue:
> 
>  if (!pci_dev->state_save) {
>                 pci_save_state(pci_dev);
> 
>                pci_disable_ptm(pci_dev);
> 
>                 if (!pci_dev->skip_bus_pm && pci_power_manageable(pci_dev))
>                         pci_prepare_to_sleep(pci_dev);
> }
> 
> > Ah, thanks.  IMHO it's easier to analyze for correctness if we only
> > check the power state.
> > 
> > Bjorn