linux-kernel - Re: [PATCH v2] PCI: Fix no-op wait after secondary bus reset

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Message-ID: <20220519110622.6fd065d2.alex.williamson@redhat.com>
Date:   Thu, 19 May 2022 11:06:22 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Sheng Bi <windy.bi.enflame@...il.com>
Cc:     helgaas@...nel.org, bhelgaas@...gle.com, lukas@...ner.de,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: Fix no-op wait after secondary bus reset

On Wed, 18 May 2022 19:54:32 +0800
Sheng Bi <windy.bi.enflame@...il.com> wrote:

> pci_bridge_secondary_bus_reset() triggers SBR followed by 1 second sleep,
> and then uses pci_dev_wait() for waiting device ready. The dev parameter
> passes to the wait function is currently the bridge itself, but not the
> device been reset.
> 
> If we call pci_bridge_secondary_bus_reset() to trigger SBR to a device,
> there is 1 second sleep but not waiting device ready, since the bridge
> is always ready while resetting downstream devices. pci_dev_wait() here
> is a no-op actually. This would be risky in the case which the device
> becomes ready after more than 1 second, especially while hotplug enabled.
> The late coming hotplug event after 1 second will trigger hotplug module
> to remove/re-insert the device.
> 
> Instead of waiting ready of bridge itself, changing to wait all the
> downstream devices become ready with timeout PCIE_RESET_READY_POLL_MS
> after SBR, considering all downstream devices are affected during SBR.
> Once one of the devices doesn't reappear within the timeout, return
> -ENOTTY to indicate SBR doesn't complete successfully.
> 
> Fixes: 6b2f1351af56 ("PCI: Wait for device to become ready after secondary bus reset")
> Signed-off-by: Sheng Bi <windy.bi.enflame@...il.com>
> ---
>  drivers/pci/pci.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index eb7c0a08ff57..32b7a5c1fa3a 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5049,6 +5049,34 @@ void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev)
>  	}
>  }
>  
> +static int pci_bridge_secondary_bus_wait(struct pci_dev *bridge, int timeout)
> +{
> +	struct pci_dev *dev;
> +	int delay = 0;
> +
> +	if (!bridge->subordinate || list_empty(&bridge->subordinate->devices))
> +		return 0;
> +
> +	list_for_each_entry(dev, &bridge->subordinate->devices, bus_list) {
> +		while (!pci_device_is_present(dev)) {
> +			if (delay > timeout) {
> +				pci_warn(dev, "not ready %dms after secondary bus reset; giving up\n",
> +					delay);
> +				return -ENOTTY;
> +			}
> +
> +			msleep(20);
> +			delay += 20;

Your previous version used the same exponential back-off as used in
pci_dev_wait(), why the change here to poll at 20ms intervals?  Thanks,

Alex

> +		}
> +
> +		if (delay > 1000)
> +			pci_info(dev, "ready %dms after secondary bus reset\n",
> +				delay);
> +	}
> +
> +	return 0;
> +}
> +
>  void pci_reset_secondary_bus(struct pci_dev *dev)
>  {
>  	u16 ctrl;
> @@ -5092,7 +5120,7 @@ int pci_bridge_secondary_bus_reset(struct pci_dev *dev)
>  {
>  	pcibios_reset_secondary_bus(dev);
>  
> -	return pci_dev_wait(dev, "bus reset", PCIE_RESET_READY_POLL_MS);
> +	return pci_bridge_secondary_bus_wait(dev, PCIE_RESET_READY_POLL_MS);
>  }
>  EXPORT_SYMBOL_GPL(pci_bridge_secondary_bus_reset);
>  
> 
> base-commit: 617c8a1e527fadaaec3ba5bafceae7a922ebef7e