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

[Rafael J. Wysocki]

On Sat, May 28, 2022 at 1:09 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Fri, May 27, 2022 at 05:51:48PM -0500, Bjorn Helgaas wrote:
> > On Fri, May 27, 2022 at 08:52:17PM +0200, Rafael J. Wysocki wrote:
> > > On Thu, May 26, 2022 at 9:46 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > >
> > > > 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@xxxxxxxxx>
> > > > > >
> > > > > > 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@xxxxxxxxx>
> > > > > > ---
> > > > > >  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));
> > >
> > > Yes.
> > >
> > > Also, this patch actually only makes a difference if the device is
> > > going into D0 from D1 or D2, because we have always written 0 to the
> > > PMCSR during transitions from D3hot.
> > >
> > > It is inconsistent and confusing to do different things depending on
> > > the initial power state here and the code is simpler when 0 is written
> > > regardless.
> >
> > I agree that depending on the initial power state is confusing (it
> > confused me :)).
> >
> > What would you think of replacing this patch with the one below?
>
> Well, I don't know why I sent this, since I had already sent the pull
> request.  Not thinking clearly, I guess.  Anyway, your original patch
> is now upstream.  Sorry for the distraction.

No biggie.

If it turns out to be problematic, it can be changed to preserving the
reserved bits easily enough.