Re: [PATCH] PCI: Power on bridges before scanning new devices

[Lukas Wunner <lukas@wunner.de>]

Hi Mika,

On Tue, May 24, 2016 at 03:53:23PM +0300, Mika Westerberg wrote:
> One thing I noticed, though. When a bridge is transitioned to D0 we only
> wait for 10ms which is requirement for PCI functions. However, PCI PM
> specification 1.2 (chapter 4.2) requires that for buses to transition
> from B2 to B0 we need to wait minimum of 50ms before accessing a
> function on that bus.
> 
> We even have PCI_PM_BUS_WAIT defined in include/linux/pci.h but it is
> not used anywhere. Maybe it was not needed originally because we never
> powered down bridges anyway but now when we do, I think it is good idea
> to do what the spec requires.

The macro was introduced with

    commit aa8c6c93747f7b55fa11e1624fec8ca33763a805
    Author: Rafael J. Wysocki <rjw@sisk.pl>
    Date:   Fri Jan 16 21:54:43 2009 +0100
    PCI PM: Restore standard config registers of all devices early

but the only usage of it was removed with

    commit 476e7faefc43f106a90b5c96166c59b75de19d30
    Author: Rafael J. Wysocki <rjw@sisk.pl>
    Date:   Thu Jan 22 23:39:57 2009 +0100
    PCI PM: Do not wait for buses in B2 or B3 during resume

Best regards,

Lukas

>
> What do you think? We could add a separate patch doing something like
> below to make sure the spec is followed.
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e785dc260e72..b3b794caa380 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -2361,7 +2361,12 @@ void pci_pm_init(struct pci_dev *dev)
>  	}
>  
>  	dev->pm_cap = pm;
> -	dev->d3_delay = PCI_PM_D3_WAIT;
> +	/*
> +	 * PCI PM 1.2 specification requires minimum of 50ms before any
> +	 * function on the bus is accessed after the bus is transitioned
> +	 * from B2 to B0.
> +	 */
> +	dev->d3_delay = pci_is_bridge(dev) ? PCI_PM_BUS_WAIT : PCI_PM_D3_WAIT;
>  	dev->d3cold_delay = PCI_PM_D3COLD_WAIT;
>  	dev->bridge_d3 = pci_bridge_d3_possible(dev);
>  	dev->d3cold_allowed = true;