linux-kernel - Re: [PATCH 4/5] PCI / PM: Check for error when reading Power State

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Date:   Mon, 5 Aug 2019 23:09:19 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Linux PCI <linux-pci@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Keith Busch <keith.busch@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 4/5] PCI / PM: Check for error when reading Power State

On Mon, Aug 5, 2019 at 10:52 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> From: Bjorn Helgaas <bhelgaas@...gle.com>
>
> The Power Management Status Register is in config space, and reads while
> the device is in D3cold typically return ~0 data (PCI_ERROR_RESPONSE).  If
> we just look at the PCI_PM_CTRL_STATE_MASK bits, that is 0x3, which looks
> like D3hot, not D3cold.
>
> Check the entire register for PCI_ERROR_RESPONSE so we can distinguish
> D3cold from D3hot.
>
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> ---
>  drivers/pci/pci.c   |  6 +++---
>  include/linux/pci.h | 13 +++++++++++++
>  2 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index af6a97d7012b..d8686e3cd5eb 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -894,7 +894,7 @@ static int pci_raw_set_power_state(struct pci_dev *dev, pci_power_t state)
>                 udelay(PCI_PM_D2_DELAY);
>
>         pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
> -       dev->current_state = (pmcsr & PCI_PM_CTRL_STATE_MASK);
> +       dev->current_state = pci_power_state(pmcsr);

But pci_raw_set_power_state() should not even be called for devices in
D3_cold, so this at best is redundant.

>         if (dev->current_state != state && printk_ratelimit())
>                 pci_info(dev, "Refused to change power state, currently in D%d\n",
>                          dev->current_state);
> @@ -942,7 +942,7 @@ void pci_update_current_state(struct pci_dev *dev, pci_power_t state)
>                 u16 pmcsr;
>
>                 pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
> -               dev->current_state = (pmcsr & PCI_PM_CTRL_STATE_MASK);
> +               dev->current_state = pci_power_state(pmcsr);

The if () branch above should cover the D3cold case, shouldn't it?

>         } else {
>                 dev->current_state = state;
>         }
> @@ -1677,7 +1677,7 @@ static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
>         if (dev->pm_cap) {
>                 u16 pmcsr;
>                 pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
> -               dev->current_state = (pmcsr & PCI_PM_CTRL_STATE_MASK);
> +               dev->current_state = pci_power_state(pmcsr);

So this appears to be only case in which pci_power_state(pmcsr) is
useful at all.

It might be better to use the code from it directly here IMO.

>         }
>
>         if (atomic_inc_return(&dev->enable_cnt) > 1)
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index d64fd3788061..fdfe990e9661 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -152,6 +152,19 @@ static inline const char *pci_power_name(pci_power_t state)
>         return pci_power_names[1 + (__force int) state];
>  }
>
> +/*
> + * Convert a Power Management Status Register value to a pci_power_t.
> + * Note that if we read the register while the device is in D3cold, we
> + * typically get PCI_ERROR_RESPONSE, which looks like D3hot (0x3) if we
> + * only look at the PCI_PM_CTRL_STATE_MASK bits.
> + */
> +static inline pci_power_t pci_power_state(u16 pmcsr)
> +{
> +       if (pmcsr == (u16) PCI_ERROR_RESPONSE)
> +               return PCI_D3cold;
> +       return pmcsr & PCI_PM_CTRL_STATE_MASK;
> +}
> +
>  #define PCI_PM_D2_DELAY                200
>  #define PCI_PM_D3_WAIT         10
>  #define PCI_PM_D3COLD_WAIT     100
> --
> 2.22.0.770.g0f2c4a37fd-goog
>