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SubjectRe: [PATCH v7 2/2] PCI/PM: Disable PTM on all devices
Sorry for the delay here.

On Sat, Jun 11, 2022 at 2:12 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Wed, Jun 08, 2022 at 05:10:07PM -0700, Rajvi Jingar wrote:
> > On receiving a PTM Request from a downstream device, if PTM is disabled
> > on the root port, as per PCIe specification, such request would cause
> > an Unsupported Request error. So disable PTM for any downstream devices.
> > PTM state needs to be saved before disabling it to be restored later.
> >
> > Set ptm_enabled from 'struct pci_dev' to 0 in pci_ptm_disable() and
> > it is used in pci_save_state() before saving PTM state to avoid
> > double save.
> >
> > Fixes: a697f072f5da ("PCI: Disable PTM during suspend to save power")
> > Signed-off-by: Rajvi Jingar <rajvi.jingar@linux.intel.com>
> > Suggested-by: David E. Box <david.e.box@linux.intel.com>
> > ---
> > v1 -> v2: add Fixes tag in commit message
> > v2 -> v3: move changelog after "---" marker
> > v3 -> v4: add "---" marker after changelog
> > v4 -> v5: move pci_disable_ptm() out of the pci_dev->state_saved check.
> > disable PTM for all devices, not just root ports.
> > v5 -> v6: move pci_disable_ptm() to pci_pm_suspend()
> > set pci_dev->ptm_enabled to 0 in pci_ptm_disable() and it is
> > used in pci_save_state() before saving PTM state to avoid
> > double save.
> > v6 -> v7: add #ifdef CONFIG_PCIE_PTM in pci_save_state() before saving
> > PTM state
> > ---
> > drivers/pci/pci-driver.c | 21 ++++++++++++++++++++-
> > drivers/pci/pci.c | 28 +++++++++++++---------------
> > drivers/pci/pcie/ptm.c | 1 +
> > 3 files changed, 34 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> > index 1f64de3e5280..db4d7835d7ae 100644
> > --- a/drivers/pci/pci-driver.c
> > +++ b/drivers/pci/pci-driver.c
> > @@ -803,14 +803,33 @@ static int pci_pm_suspend(struct device *dev)
> > pci_dev_adjust_pme(pci_dev);
> > }
> >
> > + /*
> > + * If a PTM Requester is put in a low-power state, a PTM Responder
> > + * upstream from it may also be put in a low-power state. Putting a
> > + * Port in D1, D2, or D3hot does not prohibit it from sending or
> > + * responding to PTM Requests. We want to disable PTM on Responders
> > + * when they are in a low-power state. Per 6.21.3, a PTM Requester
> > + * must not be enabled when the upstream PTM Responder is disabled.
> > + * Therefore, we must disable all PTM on all downstream PTM
> > + * Requesters before disabling it on the PTM Responder, e.g., a Root
> > + * Port.
> > + *
> > + * Also, to restore the PTM state, it needs to be saved before
> > + * disabling it for all devices.
> > + */
> > + pci_save_ptm_state(pci_dev);
> > + pci_disable_ptm(pci_dev);
>
> I think this is a little bit too magical. The PTM disable doesn't
> really fit here in pci_pm_suspend(). It's more like the wakeup
> configuration done by pci_pm_suspend_noirq() in
> pci_prepare_to_sleep().
>
> IIUC, the reason it's here in pci_pm_suspend() is because of the weird
> nvme thing where nvme_suspend() puts the device in a device-specific
> low-power flavor of D0 and subsequent config accesses take it out of
> that low-power situation [1].
>
> I don't think this is a maintainable situation because there's nothing
> about this pci_disable_ptm() that says "this cannot be done after
> pm->suspend()". That's a completely nvme-specific thing that we can't
> deduce from the code and are likely to break in the future.

Well, I'm not sure it is nvme-specific really.

Pretty much the same goes for any driver that wants to do their own
power management (whatever it is) in the ->suspend() callback and
indicate that by calling pci_save_state() by itself.

> We *do* have the rule that if the driver sets pdev->state_saved
> (normally by calling pci_save_state()), it means the driver is
> responsible for *all* the device state, even the standard config space
> that the PCI core would normally handle.
>
> When the driver does set pdev->state_saved, I don't think
> pci_pm_suspend_noirq() actually touches the device itself, and I'm a
> little more comfortable relying on that assumption.

It can be relied on right now which is also why the $subject patch
cannot put the PTM disabling in there and do it regardless of the
state_saved value.

> If this nvme weirdness plays a part here, I think the commit log and
> probably a comment really should mention what's going on because it's
> just feels fragile.

Totally agree on that one.

> [1] https://lore.kernel.org/r/CAJZ5v0iNaAd=yP3DgDVVpffKU6kt+nSpPeqxWJyRddaX5K4FRA@mail.gmail.com
>
> > if (pm->suspend) {
> > pci_power_t prev = pci_dev->current_state;
> > int error;
> >
> > error = pm->suspend(dev);
> > suspend_report_result(dev, pm->suspend, error);
> > - if (error)
> > + if (error) {
> > + pci_restore_ptm_state(pci_dev);
> > return error;
> > + }
> >
> > if (!pci_dev->state_saved && pci_dev->current_state != PCI_D0
> > && pci_dev->current_state != PCI_UNKNOWN) {
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index cfaf40a540a8..3e9dcb1bbffa 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -1669,7 +1669,15 @@ int pci_save_state(struct pci_dev *dev)
> > pci_save_ltr_state(dev);
> > pci_save_dpc_state(dev);
> > pci_save_aer_state(dev);
> > - pci_save_ptm_state(dev);
> > +#ifdef CONFIG_PCIE_PTM
> > + /*
> > + * PCI PM core disables PTM during suspend and saves PTM state before
> > + * that to be able to restore the ptm state restored later. So PCI core
> > + * needs this check to avoid double save.
> > + */
> > + if (dev->ptm_enabled)
> > + pci_save_ptm_state(dev);
> > +#endif
>
> This ptm_enabled check doesn't fit with the rest of the function and
> the semantics are fairly complicated.
>
> > return pci_save_vc_state(dev);
> > }
> > EXPORT_SYMBOL(pci_save_state);
> > @@ -2710,24 +2718,12 @@ int pci_prepare_to_sleep(struct pci_dev *dev)
> > if (target_state == PCI_POWER_ERROR)
> > return -EIO;
> >
> > - /*
> > - * There are systems (for example, Intel mobile chips since Coffee
> > - * Lake) where the power drawn while suspended can be significantly
> > - * reduced by disabling PTM on PCIe root ports as this allows the
> > - * port to enter a lower-power PM state and the SoC to reach a
> > - * lower-power idle state as a whole.
> > - */
> > - if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT)
> > - pci_disable_ptm(dev);
> > -
> > pci_enable_wake(dev, target_state, wakeup);
> >
> > error = pci_set_power_state(dev, target_state);
> >
> > - if (error) {
> > + if (error)
> > pci_enable_wake(dev, target_state, false);
> > - pci_restore_ptm_state(dev);
> > - }
> >
> > return error;
> > }
> > @@ -2775,8 +2771,10 @@ int pci_finish_runtime_suspend(struct pci_dev *dev)
> > * port to enter a lower-power PM state and the SoC to reach a
> > * lower-power idle state as a whole.
> > */
> > - if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT)
> > + if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
> > + pci_save_ptm_state(dev);
> > pci_disable_ptm(dev);
> > + }
> >
> > __pci_enable_wake(dev, target_state, pci_dev_run_wake(dev));
> >
> > diff --git a/drivers/pci/pcie/ptm.c b/drivers/pci/pcie/ptm.c
> > index 368a254e3124..746e29779c27 100644
> > --- a/drivers/pci/pcie/ptm.c
> > +++ b/drivers/pci/pcie/ptm.c
> > @@ -44,6 +44,7 @@ void pci_disable_ptm(struct pci_dev *dev)
> > pci_read_config_word(dev, ptm + PCI_PTM_CTRL, &ctrl);
> > ctrl &= ~(PCI_PTM_CTRL_ENABLE | PCI_PTM_CTRL_ROOT);
> > pci_write_config_word(dev, ptm + PCI_PTM_CTRL, ctrl);
> > + dev->ptm_enabled = 0;
>
> This looks like a bug fix that could be in a separate patch.
>
> > }
> >
> > void pci_save_ptm_state(struct pci_dev *dev)
>
>
> I think something like the sketch below would fit better in the power
> management framework. PTM disable is closely related to device power
> states, so I tried to put it as close as possible to the power state
> transitions. I'm sure there are things missing and things I'm
> overlooking:

There are PCI devices that pci_prepare_to_sleep() is not called for,
so disabling PTM in there may not work in general.

> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index cfaf40a540a8..4dcd0c7381b9 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -2705,28 +2705,21 @@ int pci_prepare_to_sleep(struct pci_dev *dev)
> {
> bool wakeup = device_may_wakeup(&dev->dev);
> pci_power_t target_state = pci_target_state(dev, wakeup);
> + bool ptm = pcie_ptm_enabled(dev);
> int error;
>
> if (target_state == PCI_POWER_ERROR)
> return -EIO;
>
> - /*
> - * There are systems (for example, Intel mobile chips since Coffee
> - * Lake) where the power drawn while suspended can be significantly
> - * reduced by disabling PTM on PCIe root ports as this allows the
> - * port to enter a lower-power PM state and the SoC to reach a
> - * lower-power idle state as a whole.
> - */
> - if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT)
> - pci_disable_ptm(dev);
> -
> + pci_disable_ptm(dev);
> pci_enable_wake(dev, target_state, wakeup);
>
> error = pci_set_power_state(dev, target_state);
>
> if (error) {
> pci_enable_wake(dev, target_state, false);
> - pci_restore_ptm_state(dev);
> + if (ptm)
> + pci_enable_ptm(dev);
> }
>
> return error;
> @@ -2762,6 +2755,7 @@ EXPORT_SYMBOL(pci_back_from_sleep);
> int pci_finish_runtime_suspend(struct pci_dev *dev)
> {
> pci_power_t target_state;
> + bool ptm = pcie_ptm_enabled(dev);
> int error;
>
> target_state = pci_target_state(dev, device_can_wakeup(&dev->dev));
> @@ -2778,13 +2772,15 @@ int pci_finish_runtime_suspend(struct pci_dev *dev)
> if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT)
> pci_disable_ptm(dev);
>
> + pci_disable_ptm(dev);
> __pci_enable_wake(dev, target_state, pci_dev_run_wake(dev));
>
> error = pci_set_power_state(dev, target_state);
>
> if (error) {
> pci_enable_wake(dev, target_state, false);
> - pci_restore_ptm_state(dev);
> + if (ptm)
> + pci_enable_ptm(dev);
> }
>
> return error;

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