Re: [PATCH v1 06/11] PCI/PM: Write 0 to PMCSR in pci_power_up() in all cases

Bjorn Helgaas
2022-05-26T19:46:08Z

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@intel.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@intel.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));