linux-kernel - Re: [PATCH] ACPI / hotplug / PCI: Avoid parent bus rescans on spurious device checks

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Date:	Mon, 09 Sep 2013 10:32:57 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>, Jiang Liu <liuj97@...il.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] ACPI / hotplug / PCI: Avoid parent bus rescans on
 spurious device checks

On Sun, 2013-09-08 at 00:16 +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> In the current ACPIPHP notify handler we always go directly for a
> rescan of the parent bus if we get a device check notification for
> a device that is not a bridge.  However, this obviously is
> overzealous if nothing really changes, because this way we may rescan
> the whole PCI hierarchy pretty much in vain.
> 
> That happens on Alex Williamson's machine whose ACPI tables contain
> device objects that are supposed to coresspond to PCIe root ports,
> but those ports aren't physically present (or at least they aren't
> visible in the PCI config space to us).  The BIOS generates multiple
> device check notifies for those objects during boot and for each of
> them we go straight for the parent bus rescan, but the parent bus is
> the root bus in this particular case.  In consequence, we rescan the
> whole PCI bus from the top several times in a row, which is
> completely unnecessary, increases boot time by 50% (after previous
> fixes) and generates excess dmesg output from the PCI subsystem.
> 
> Fix the problem by checking if we can find anything new in the
> slot corresponding to the device we've got a device check notify
> for and doing nothig if that's not the case.
> 
> The spec (ACPI 5.0, Section 5.6.6) appears to mandate this behavior,
> as it says:
> 
>   Device Check. Used to notify OSPM that the device either appeared
>   or disappeared. If the device has appeared, OSPM will re-enumerate
>   from the parent. If the device has disappeared, OSPM will
>   invalidate the state of the device. OSPM may optimize out
>   re-enumeration.
> 
> Therefore, according to the spec, we are free to do nothing if
> nothing changes.
> 
> References: https://bugzilla.kernel.org/show_bug.cgi?id=60865
> Reported-by: Alex Williamson <alex.williamson@...hat.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---

Works for me.  Thanks!

Tested-by: Alex Williamson <alex.williamson@...hat.com>

> On top of linux-pm.git/linux-next.
> 
> Thanks,
> Rafael
> 
> ---
>  drivers/pci/hotplug/acpiphp_glue.c |   32 +++++++++++++++++++++++++-------
>  1 file changed, 25 insertions(+), 7 deletions(-)
> 
> Index: linux-pm/drivers/pci/hotplug/acpiphp_glue.c
> ===================================================================
> --- linux-pm.orig/drivers/pci/hotplug/acpiphp_glue.c
> +++ linux-pm/drivers/pci/hotplug/acpiphp_glue.c
> @@ -528,6 +528,16 @@ static void check_hotplug_bridge(struct
>  	}
>  }
>  
> +static int acpiphp_rescan_slot(struct acpiphp_slot *slot)
> +{
> +	struct acpiphp_func *func;
> +
> +	list_for_each_entry(func, &slot->funcs, sibling)
> +		acpiphp_bus_add(func_to_handle(func));
> +
> +	return pci_scan_slot(slot->bus, PCI_DEVFN(slot->device, 0));
> +}
> +
>  /**
>   * enable_slot - enable, configure a slot
>   * @slot: slot to be enabled
> @@ -544,10 +554,7 @@ static void __ref enable_slot(struct acp
>  	LIST_HEAD(add_list);
>  	int nr_found;
>  
> -	list_for_each_entry(func, &slot->funcs, sibling)
> -		acpiphp_bus_add(func_to_handle(func));
> -
> -	nr_found = pci_scan_slot(bus, PCI_DEVFN(slot->device, 0));
> +	nr_found = acpiphp_rescan_slot(slot);
>  	max = acpiphp_max_busnr(bus);
>  	for (pass = 0; pass < 2; pass++) {
>  		list_for_each_entry(dev, &bus->devices, bus_list) {
> @@ -840,11 +847,22 @@ static void hotplug_event(acpi_handle ha
>  	case ACPI_NOTIFY_DEVICE_CHECK:
>  		/* device check */
>  		dbg("%s: Device check notify on %s\n", __func__, objname);
> -		if (bridge)
> +		if (bridge) {
>  			acpiphp_check_bridge(bridge);
> -		else
> -			acpiphp_check_bridge(func->parent);
> +		} else {
> +			struct acpiphp_slot *slot = func->slot;
> +			int ret;
>  
> +			/*
> +			 * Check if anything has changed in the slot and rescan
> +			 * from the parent if that's the case.
> +			 */
> +			mutex_lock(&slot->crit_sect);
> +			ret = acpiphp_rescan_slot(slot);
> +			mutex_unlock(&slot->crit_sect);
> +			if (ret)
> +				acpiphp_check_bridge(func->parent);
> +		}
>  		break;
>  
>  	case ACPI_NOTIFY_EJECT_REQUEST:


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