linux-kernel - Re: [PATCH v3] PCI: Make sure the bus bridge powered on when scanning bus

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Message-ID: <20220420163249.GA1305194@bhelgaas>
Date:   Wed, 20 Apr 2022 11:32:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Yicong Yang <yangyicong@...ilicon.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        prime.zeng@...wei.com,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] PCI: Make sure the bus bridge powered on when
 scanning bus

[+cc Rafael, linux-pm, since I'd really like his ack/review]

On Thu, Apr 14, 2022 at 08:37:36PM +0800, Yicong Yang wrote:
> When the bus bridge is runtime suspended, we'll fail to rescan
> the devices through sysfs as we cannot access the configuration
> space correctly when the bridge is in D3hot.
> It can be reproduced like:
> 
> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/0000:81:00.1/remove
> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/pci_bus/0000:81/rescan
> 
> 0000:80:00.0 is root port and is runtime suspended and we cannot
> get 0000:81:00.1 after rescan.
> 
> Make bridge powered on when scanning the child bus, by adding
> pm_runtime_get_sync()/pm_runtime_put() in pci_scan_child_bus_extend().
> 
> A similar issue is met and solved by
> d963f6512e15 ("PCI: Power on bridges before scanning new devices")
> which rescan the devices through /sys/bus/pci/devices/0000:80:00.0/rescan.
> The callstack is like:
> 
> dev_rescan_restore()
>   pci_rescan_bus()
>     pci_scan_bridge_extend()
>       pci_scan_child_bus_extend() /* will wake up the bridge with this patch */
> 
> With this patch the issue is also resolved, so let's remove the calls of
> pm_runtime_*() in pci_scan_bridge_extend().
> 
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> ---
> Change since v2:
> - just rebase it on v5.18-rc2
> Link: https://lore.kernel.org/linux-pci/1601029386-4928-1-git-send-email-yangyicong@hisilicon.com/
> 
> Change since v1:
> - use an intermediate variable *bridge as suggested
> - remove the pm_runtime_*() calls in pci_scan_bridge_extend()
> Link: https://lore.kernel.org/linux-pci/1596022223-4765-1-git-send-email-yangyicong@hisilicon.com/
> 
>  drivers/pci/probe.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 17a969942d37..2ca6b4b708e3 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1257,12 +1257,6 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>  	u8 fixed_sec, fixed_sub;
>  	int next_busnr;
>  
> -	/*
> -	 * 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;
> @@ -1464,8 +1458,6 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>  out:
>  	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
>  
> -	pm_runtime_put(&dev->dev);
> -
>  	return max;
>  }
>  
> @@ -2859,11 +2851,19 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>  	unsigned int used_buses, normal_bridges = 0, hotplug_bridges = 0;
>  	unsigned int start = bus->busn_res.start;
>  	unsigned int devfn, fn, cmax, max = start;
> -	struct pci_dev *dev;
> +	struct pci_dev *dev, *bridge = bus->self;
>  	int nr_devs;
>  
>  	dev_dbg(&bus->dev, "scanning bus\n");
>  
> +	/*
> +	 * Make sure the bus bridge is powered on, otherwise we may not be
> +	 * able to scan the devices as we may fail to access the configuration
> +	 * space of subordinates.
> +	 */
> +	if (bridge)
> +		pm_runtime_get_sync(&bridge->dev);
> +
>  	/* Go find them, Rover! */
>  	for (devfn = 0; devfn < 256; devfn += 8) {
>  		nr_devs = pci_scan_slot(bus, devfn);
> @@ -2976,6 +2976,9 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>  		}
>  	}
>  
> +	if (bridge)
> +		pm_runtime_put(&bridge->dev);
> +
>  	/*
>  	 * We've scanned the bus and so we know all about what's on
>  	 * the other side of any bridges that may be on this bus plus
> -- 
> 2.24.0
>