linux-kernel - Re: [PATCH] PCI: PM: Do not read power state in pci_enable_device

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Message-ID: <05e3ae3e-1a5f-c413-dae1-4c24e61a8b1b@gmail.com>
Date:   Tue, 16 Mar 2021 23:28:43 +0100
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH] PCI: PM: Do not read power state in
 pci_enable_device_flags()



On 3/16/21 4:51 PM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> It should not be necessary to update the current_state field of
> struct pci_dev in pci_enable_device_flags() before calling
> do_pci_enable_device() for the device, because none of the
> code between that point and the pci_set_power_state() call in
> do_pci_enable_device() invoked later depends on it.
> 
> Moreover, doing that is actively harmful in some cases.  For example,
> if the given PCI device depends on an ACPI power resource whose _STA
> method initially returns 0 ("off"), but the config space of the PCI
> device is accessible and the power state retrieved from the
> PCI_PM_CTRL register is D0, the current_state field in the struct
> pci_dev representing that device will get out of sync with the
> power.state of its ACPI companion object and that will lead to
> power management issues going forward.
> 
> To avoid such issues it is better to leave the current_state value
> as is until it is changed to PCI_D0 by do_pci_enable_device() as
> appropriate.  However, the power state of the device is not changed
> to PCI_D0 if it is already enabled when pci_enable_device_flags()
> gets called for it, so update its current_state in that case, but
> use pci_update_current_state() covering platform PM too for that.
> 
> Link: https://lore.kernel.org/lkml/20210314000439.3138941-1-luzmaximilian@gmail.com/
> Reported-by: Maximilian Luz <luzmaximilian@...il.com>
> Tested-by: Maximilian Luz <luzmaximilian@...il.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> Max, I've added a T-by from you even though the patch is slightly different
> from what you have tested, but the difference shouldn't matter for your case.

Thanks! I've tested this now as well, all looks good.

Regards,
Max

> 
> ---
>   drivers/pci/pci.c |   16 +++-------------
>   1 file changed, 3 insertions(+), 13 deletions(-)
> 
> Index: linux-pm/drivers/pci/pci.c
> ===================================================================
> --- linux-pm.orig/drivers/pci/pci.c
> +++ linux-pm/drivers/pci/pci.c
> @@ -1870,20 +1870,10 @@ static int pci_enable_device_flags(struc
>   	int err;
>   	int i, bars = 0;
>   
> -	/*
> -	 * Power state could be unknown at this point, either due to a fresh
> -	 * boot or a device removal call.  So get the current power state
> -	 * so that things like MSI message writing will behave as expected
> -	 * (e.g. if the device really is in D0 at enable time).
> -	 */
> -	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);
> -	}
> -
> -	if (atomic_inc_return(&dev->enable_cnt) > 1)
> +	if (atomic_inc_return(&dev->enable_cnt) > 1) {
> +		pci_update_current_state(dev, dev->current_state);
>   		return 0;		/* already enabled */
> +	}
>   
>   	bridge = pci_upstream_bridge(dev);
>   	if (bridge)
> 
> 
>