linux-kernel - Re: [PATCH 1/4] PCI/PTM: Preserve PTM Root Select

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Date:   Fri, 2 Sep 2022 10:24:05 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Rajvi Jingar <rajvi.jingar@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>
Cc:     Koba Ko <koba.ko@...onical.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "David E . Box" <david.e.box@...ux.intel.com>,
        linux-pci@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 1/4] PCI/PTM: Preserve PTM Root Select



On 9/2/22 7:58 AM, Bjorn Helgaas wrote:
> From: Bjorn Helgaas <bhelgaas@...gle.com>
> 
> When disabling PTM, there's no need to clear the Root Select bit.  We
> disable PTM during suspend, and we want to re-enable it during resume.
> Clearing Root Select here makes re-enabling more complicated.

Currently, it looks like we disable PCI_PTM_CTRL_ROOT in pci_disable_ptm(),
but not enable it in pci_enable_ptm(). Do you know this did not trigger an
issue?

Also, you mentioned that it is complicated to enable it, can you add some
details?


> 
> Per PCIe r6.0, sec 7.9.15.3, "When set, if the PTM Enable bit is also Set,
> this Time Source is the PTM Root," so if PTM Enable is cleared, the value
> of Root Select should be irrelevant.
> 
> Preserve Root Select to simplify re-enabling PTM.
> 
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: David E. Box <david.e.box@...ux.intel.com>
> ---
>  drivers/pci/pcie/ptm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pcie/ptm.c b/drivers/pci/pcie/ptm.c
> index 368a254e3124..b6a417247ce3 100644
> --- a/drivers/pci/pcie/ptm.c
> +++ b/drivers/pci/pcie/ptm.c
> @@ -42,7 +42,7 @@ void pci_disable_ptm(struct pci_dev *dev)
>  		return;
>  
>  	pci_read_config_word(dev, ptm + PCI_PTM_CTRL, &ctrl);
> -	ctrl &= ~(PCI_PTM_CTRL_ENABLE | PCI_PTM_CTRL_ROOT);
> +	ctrl &= ~PCI_PTM_CTRL_ENABLE;
>  	pci_write_config_word(dev, ptm + PCI_PTM_CTRL, ctrl);
>  }
>  

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer