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

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, May 23, 2016 at 11:20:48AM +0300, Mika Westerberg wrote:
> When a PCI device is removed through sysfs interface the upstream bridge
> (PCIe port) can be runtime suspended if it was the last device on that bus.
> Now, if the bridge is in D3 we cannot find devices below the bridge
> anymore. For example following fails to find the removed device again:
> 
>    # echo 1 > /sys/bus/pci/devices/0000:00:01.0/0000:01:00.0/remove
>    # echo 1 > /sys/bus/pci/devices/0000:00:01.0/rescan
> 
> Where 0000:00:01.0 is the bridge device.
> 
> In order to be able to rescan devices below the bridge add
> pm_runtime_get_sync()/pm_runtime_put() calls to pci_scan_bridge(). This
> should keep bridges powered on while their children devices are being
> scanned.
> 
> Reported-by: Peter Wu <peter@lekensteyn.nl>
> Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

This looks like basically the same idea as "ACPI / hotplug / PCI:
Runtime resume bridge before rescan".

The hotplug_event() path modified by that patch eventually calls
pci_scan_bridge():

  hotplug_event
    enable_slot
      pci_scan_bridge

so this patch looks a little more general.  Does it make "ACPI /
hotplug / PCI: Runtime resume bridge before rescan" unnecessary?  
Can I just replace that patch with this one?

> ---
>  drivers/pci/probe.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 8004f67c57ec..15e77c92311e 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -16,6 +16,7 @@
>  #include <linux/aer.h>
>  #include <linux/acpi.h>
>  #include <linux/irqdomain.h>
> +#include <linux/pm_runtime.h>
>  #include "pci.h"
>  
>  #define CARDBUS_LATENCY_TIMER	176	/* secondary latency timer */
> @@ -832,6 +833,12 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
>  	u8 primary, secondary, subordinate;
>  	int broken = 0;
>  
> +	/*
> +	 * Make sure the bridge is powered on to be able to access config
> +	 * space of devices below it.
> +	 */
> +	pm_runtime_get_sync(&dev->dev);
> +
>  	pci_read_config_dword(dev, PCI_PRIMARY_BUS, &buses);
>  	primary = buses & 0xFF;
>  	secondary = (buses >> 8) & 0xFF;
> @@ -1012,6 +1019,8 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
>  out:
>  	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
>  
> +	pm_runtime_put(&dev->dev);
> +
>  	return max;
>  }
>  EXPORT_SYMBOL(pci_scan_bridge);
> -- 
> 2.8.1
> 
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