linux-kernel - Re: PCI/PM: Add comments for PME poll support for PCIe

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Date:	Fri, 26 Oct 2012 13:18:04 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-pm@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: PCI/PM: Add comments for PME poll support for PCIe

On Friday, October 26, 2012 01:07:51 PM Huang Ying wrote:
> There are comments on why PME poll support is necessary for PCI
> devices, but not for PCIe devices.  That may lead to misunderstanding
> that PME poll is only necessary for PCI devices.  So add comments
> related to PCIe PME poll to make it more clear.
> 
> The content of comments comes from the changelog of commit:
> 
> 379021d5c0899fcf9410cae4ca7a59a5a94ca769
> 
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: Huang Ying <ying.huang@...el.com>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
>  drivers/pci/pci.c |   28 +++++++++++++++++++---------
>  1 file changed, 19 insertions(+), 9 deletions(-)
> 
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1578,15 +1578,25 @@ void pci_pme_active(struct pci_dev *dev,
>  
>  	pci_write_config_word(dev, dev->pm_cap + PCI_PM_CTRL, pmcsr);
>  
> -	/* PCI (as opposed to PCIe) PME requires that the device have
> -	   its PME# line hooked up correctly. Not all hardware vendors
> -	   do this, so the PME never gets delivered and the device
> -	   remains asleep. The easiest way around this is to
> -	   periodically walk the list of suspended devices and check
> -	   whether any have their PME flag set. The assumption is that
> -	   we'll wake up often enough anyway that this won't be a huge
> -	   hit, and the power savings from the devices will still be a
> -	   win. */
> +	/*
> +	 * PCI (as opposed to PCIe) PME requires that the device have
> +	 * its PME# line hooked up correctly. Not all hardware vendors
> +	 * do this, so the PME never gets delivered and the device
> +	 * remains asleep. The easiest way around this is to
> +	 * periodically walk the list of suspended devices and check
> +	 * whether any have their PME flag set. The assumption is that
> +	 * we'll wake up often enough anyway that this won't be a huge
> +	 * hit, and the power savings from the devices will still be a
> +	 * win.
> +	 *
> +	 * Although PCIe uses in-band PME message instead of PME# line
> +	 * to report PME, PME does not work for some PCIe devices in
> +	 * reality.  For example, there are devices that set their PME
> +	 * status bits, but don't really bother to send a PME message;
> +	 * there are PCI Express Root Ports that don't bother to
> +	 * trigger interrupts when they receive PME messages from the
> +	 * devices below.  So PME poll is used for PCIe devices too.
> +	 */
>  
>  	if (dev->pme_poll) {
>  		struct pci_pme_device *pme_dev;
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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