linux-kernel - Re: [PATCH v1 06/11] PCI/PM: Write 0 to PMCSR in pci_power

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Date:   Thu, 26 May 2022 14:46:01 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH v1 06/11] PCI/PM: Write 0 to PMCSR in pci_power_up() in
 all cases

On Thu, May 26, 2022 at 11:54:22AM -0500, Bjorn Helgaas wrote:
> On Thu, May 05, 2022 at 08:10:43PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> > Make pci_power_up() write 0 to the device's PCI_PM_CTRL register in
> > order to put it into D0 regardless of the power state returned by
> > the previous read from that register which should not matter.
> > 
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/pci/pci.c |   11 +++--------
> >  1 file changed, 3 insertions(+), 8 deletions(-)
> > 
> > Index: linux-pm/drivers/pci/pci.c
> > ===================================================================
> > --- linux-pm.orig/drivers/pci/pci.c
> > +++ linux-pm/drivers/pci/pci.c
> > @@ -1230,15 +1230,10 @@ int pci_power_up(struct pci_dev *dev)
> >  	}
> >  
> >  	/*
> > -	 * If we're (effectively) in D3, force entire word to 0. This doesn't
> > -	 * affect PME_Status, disables PME_En, and sets PowerState to 0.
> > +	 * Force the entire word to 0. This doesn't affect PME_Status, disables
> > +	 * PME_En, and sets PowerState to 0.
> >  	 */
> > -	if (state == PCI_D3hot)
> > -		pmcsr = 0;
> > -	else
> > -		pmcsr &= ~PCI_PM_CTRL_STATE_MASK;
> > -
> > -	pci_write_config_word(dev, dev->pm_cap + PCI_PM_CTRL, pmcsr);
> > +	pci_write_config_word(dev, dev->pm_cap + PCI_PM_CTRL, 0);
> 
> Can you reassure me why this is safe and useful?
> 
> This is a 16-bit write that includes (PCIe r6.0, sec 7.5.2.2):
> 
>   0x0003 PowerState     RW
>   0x0004                RsvdP
>   0x0008 No_Soft_Reset  RO
>   0x00f0                RsvdP
>   0x0100 PME_En         RW/RWS
>   0x1e00 Data_Select    RW, VF ROZ
>   0x6000 Data_Scale     RO, VF ROZ
>   0x8000 PME_Status     RW1CS
> 
> We intend to set PowerState to 0 (D0), apparently intend to clear
> PME_En, and PME_Status is "write 1 to clear" to writing 0 does
> nothing, so those look OK.
> 
> But the RsvdP fields are reserved for future RW bits and should be
> preserved, and it looks like clearing Data_Select could potentially
> break the Data Register power consumption reporting (which I don't
> think we support today).
> 
> It seems like maybe we should do this instead:
> 
>   pci_write_config_word(dev, dev->pm_cap + PCI_PM_CTRL,
>                         pmcsr & ~PCI_PM_CTRL_STATE_MASK)
> 
> to just unconditionally clear PowerState?

Or I guess this, since we want to clear PME_En as well?

  pci_write_config_word(dev, dev->pm_cap + PCI_PM_CTRL, pmcsr &
                        ~(PCI_PM_CTRL_STATE_MASK | PCI_PM_CTRL_PME_ENABLE));