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

Bjorn Helgaas
2022-05-26T16:54:29Z

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?