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Wysocki" Cc: Bjorn Helgaas , Ilpo =?iso-8859-1?Q?J=E4rvinen?= , linux-pm@vger.kernel.org, linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, Lukas Wunner Subject: Re: [PATCH v3] PCI/PM: Prevent runtime suspend before devices are fully initialized Message-ID: References: <20251022141434.v3.1.I60a53c170a8596661883bd2b4ef475155c7aa72b@changeid> Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hi again, On Thu, Oct 23, 2025 at 11:34:57AM -0700, Brian Norris wrote: > On Thu, Oct 23, 2025 at 11:41:38AM +0200, Rafael J. Wysocki wrote: > > On Wed, Oct 22, 2025 at 11:15 PM Brian Norris wrote: > > > > > > Today, it's possible for a PCI device to be created and > > > runtime-suspended before it is fully initialized. When that happens, the > > > device will remain in D0, but the suspend process may save an > > > intermediate version of that device's state -- for example, without > > > appropriate BAR configuration. When the device later resumes, we'll > > > restore invalid PCI state and the device may not function. > > > > > > Prevent runtime suspend for PCI devices by deferring pm_runtime_enable() > > > until we've fully initialized the device. > > > > > > More details on how exactly this may occur: > > > > > > 1. PCI device is created by pci_scan_slot() or similar > > > 2. As part of pci_scan_slot(), pci_pm_init() enables runtime PM; the > > > device starts "active" and we initially prevent (pm_runtime_forbid()) > > > suspend -- but see [*] footnote > > > 3. Underlying 'struct device' is added to the system (device_add()); > > > runtime PM can now be configured by user space > > > 4. PCI device receives BAR configuration > > > (pci_assign_unassigned_bus_resources(), etc.) > > > 5. PCI device is added to the system in pci_bus_add_device() > > > > > > The device may potentially suspend between #3 and #4. > > > > > > [*] By default, pm_runtime_forbid() prevents suspending a device; but by > > > design [**], this can be overridden by user space policy via > > > > > > echo auto > /sys/bus/pci/devices/.../power/control > > > > > > Thus, the above #3/#4 sequence is racy with user space (udev or > > > similar). > > > > > > Notably, many PCI devices are enumerated at subsys_initcall time and so > > > will not race with user space. However, there are several scenarios > > > where PCI devices are created later on, such as with hotplug or when > > > drivers (pwrctrl or controller drivers) are built as modules. > > > > > > [**] The relationship between pm_runtime_forbid(), pm_runtime_allow(), > > > /sys/.../power/control, and the runtime PM usage counter can be subtle. > > > It appears that the intention of pm_runtime_forbid() / > > > pm_runtime_allow() is twofold: > > > > > > 1. Allow the user to disable runtime_pm (force device to always be > > > powered on) through sysfs. > > > 2. Allow the driver to start with runtime_pm disabled (device forced > > > on) and user space could later enable runtime_pm. > > > > > > This conclusion comes from reading `Documentation/power/runtime_pm.rst`, > > > specifically the section starting "The user space can effectively > > > disallow". > > > > > > This means that while pm_runtime_forbid() does technically increase the > > > runtime PM usage counter, this usage counter is not a guarantee of > > > functional correctness, because sysfs can decrease that count again. > > > > > > Link: https://lore.kernel.org/all/20251016155335.1.I60a53c170a8596661883bd2b4ef475155c7aa72b@changeid/ > > > Signed-off-by: Brian Norris > > > Cc: > > > Reviewed-by: Rafael J. Wysocki (Intel) > > > --- > > > > > > Changes in v3: > > > * Add Link to initial discussion > > > * Add Rafael's Reviewed-by > > > * Add lengthier footnotes about forbid vs allow vs sysfs > > > > > > Changes in v2: > > > * Update CC list > > > * Rework problem description > > > * Update solution: defer pm_runtime_enable(), instead of trying to > > > get()/put() > > > > > > drivers/pci/bus.c | 3 +++ > > > drivers/pci/pci.c | 1 - > > > 2 files changed, 3 insertions(+), 1 deletion(-) > > > > > > diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c > > > index f26aec6ff588..fc66b6cb3a54 100644 > > > --- a/drivers/pci/bus.c > > > +++ b/drivers/pci/bus.c > > > @@ -14,6 +14,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > > > > @@ -375,6 +376,8 @@ void pci_bus_add_device(struct pci_dev *dev) > > > put_device(&pdev->dev); > > > } > > > > > > + pm_runtime_enable(&dev->dev); > > > + > > > if (!dn || of_device_is_available(dn)) > > > pci_dev_allow_binding(dev); > > > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c > > > index b14dd064006c..f792164fa297 100644 > > > --- a/drivers/pci/pci.c > > > +++ b/drivers/pci/pci.c > > > @@ -3226,7 +3226,6 @@ void pci_pm_init(struct pci_dev *dev) > > > pci_pm_power_up_and_verify_state(dev); > > > pm_runtime_forbid(&dev->dev); > > > pm_runtime_set_active(&dev->dev); > > > > Actually, I think that the two statements above can be moved too. > > > > The pm_runtime_forbid() call doesn't matter until runtime PM is > > enabled and it is better to do pm_runtime_set_active() right before > > enabling runtime PM. > > Ehh, sure, I can do that I suppose. That signs me up for fixing some > excessive Documentation/ that spells out exactly what goes on in > pci_pm_init() for some reason, but I can do that too :) Hmm, never mind, I think I agreed with you too soon. It should be ok to move pm_runtime_set_active(), but I believe moving pm_runtime_forbid() would introduce a similar race condition problem to the bug I'm resolving -- pci_bus_add_device() is run after device_add(), and so user space might already see the device and try to configure power/control. If we forbid() after that point, user space would be confused. Documentation/power/runtime_pm.rst even calls this out: "It should be noted, however, that if the user space has already intentionally changed the value of /sys/devices/.../power/control to "auto" to allow the driver to power manage the device at run time, the driver may confuse it by using pm_runtime_forbid() this way." So I'd rather not make this change. Brian > > > - pm_runtime_enable(&dev->dev); > > > } > > > > > > static unsigned long pci_ea_flags(struct pci_dev *dev, u8 prop) > > > -- >