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 11:54:22 -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 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?