linux-kernel - Re: [PATCH v3] PCI: pciehp: Check for valid PCI_BRIDGE_CONTROL in pciehp_unconfigure

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Message-ID: <20171024162822.GA21840@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 24 Oct 2017 11:28:22 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <yehezkel.bernat@...el.com>,
        Mario.Limonciello@...l.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: pciehp: Check for valid PCI_BRIDGE_CONTROL in
 pciehp_unconfigure_device()

On Tue, Oct 24, 2017 at 02:09:14PM +0300, Mika Westerberg wrote:
> During surprise hot-unplug the device is not there anymore. When that
> happens we read 0xffffffff from the registers and pciehp_unconfigure_device()
> might inadvertently think the device is a display device because bridge
> control register returns 0xff refusing to remove it:
> 
>   pciehp 0000:00:1c.0:pcie004: Slot(0): Link Down
>   pciehp 0000:00:1c.0:pcie004: Slot(0): Card present
>   pciehp 0000:00:1c.0:pcie004: Cannot remove display device 0000:01:00.0
> 
> This causes the hotplug functionality to leave the hierarcy untouched
> preventing further hotplug operations.
> 
> Fix this so that we first compare the read bctl value against 0xffff
> (device is not present anymore) before determining whether the device is
> a display or not.
> 
> Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> ---
> Hi Bjorn,
> 
> This is an updated version of the patch 8/8 in my "PCI: Improvements for
> native PCIe hotplug" patch series and applies on top of pci.git/pci/hotplug.
> 
> We now just check bctl against 0xffff before preventing removal of the
> "display" device.
> 
> I'm inclined to remove the whole check of "display" device because I don't
> see any reason for it to be there in the first place. This code is pretty
> much duplicated from shpchp_pci.c where it might make some sense.

Yeah, I don't understand this at all.  If the device is gone, it's
gone, and complaining about it being a display device isn't going to
make it come back.  I don't know the history of that check, but I
agree that it might make sense to remove it altogether.

The message (and consequently your changelog) also seem misleading
because we're removing *dev*, which in this case is a bridge that
might lead to a display device.  But it's not a display device itself.

>  drivers/pci/hotplug/pciehp_pci.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/hotplug/pciehp_pci.c b/drivers/pci/hotplug/pciehp_pci.c
> index 2a1ca020cf5a..5225b12c5a75 100644
> --- a/drivers/pci/hotplug/pciehp_pci.c
> +++ b/drivers/pci/hotplug/pciehp_pci.c
> @@ -79,7 +79,6 @@ int pciehp_configure_device(struct slot *p_slot)
>  int pciehp_unconfigure_device(struct slot *p_slot)
>  {
>  	int rc = 0;
> -	u8 bctl = 0;
>  	u8 presence = 0;
>  	struct pci_dev *dev, *temp;
>  	struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
> @@ -102,8 +101,10 @@ int pciehp_unconfigure_device(struct slot *p_slot)
>  					 bus_list) {
>  		pci_dev_get(dev);
>  		if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
> -			pci_read_config_byte(dev, PCI_BRIDGE_CONTROL, &bctl);
> -			if (bctl & PCI_BRIDGE_CTL_VGA) {
> +			u16 bctl = 0;
> +
> +			pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &bctl);
> +			if (bctl != 0xffff && (bctl & PCI_BRIDGE_CTL_VGA)) {
>  				ctrl_err(ctrl,
>  					 "Cannot remove display device %s\n",
>  					 pci_name(dev));
> -- 
> 2.14.2
>