linux-kernel - Re: [PATCH] drivers/pci: wait downstream hierarchy ready instead of slot itself ready, after secondary bus reset

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Message-ID: <20220516202825.GA1047972@bhelgaas>
Date:   Mon, 16 May 2022 15:28:25 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "windy.bi.enflame" <windy.bi.enflame@...il.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] drivers/pci: wait downstream hierarchy ready instead of
 slot itself ready, after secondary bus reset

[+cc Lukas, pciehp expert; Alex, reset person]

Thanks for the testing, analysis, and patch!

Run "git log --oneline drivers/pci/pci.c" and make your subject line
similar.

On Tue, May 17, 2022 at 01:30:47AM +0800, windy.bi.enflame wrote:
> While I do reset test of a PCIe endpoint device on a server, I find that
> the EP device always been removed and re-inserted again by hotplug module,
>  after secondary bus reset.
> 
> After checking I find:
> 1> "pciehp_reset_slot()" always disable slot's DLLSC interrupt before
>    doing reset and restore after reset, to try to filter the hotplug
>    event happened during reset.
> 2> "pci_bridge_secondary_bus_reset()" sleep 1 seconad and "pci_dev_wait()"
>    until device ready with "PCIE_RESET_READY_POLL_MS" timeout.
> 3> There is a PCIe switch between CPU and the EP devicem the topology as:
>    CPU <-> Switch <-> EP.
> 4> While trigger sbr reset at the switch's downstream port, it needs 1.5
>    seconds for internal enumeration.

s/seconad/second/
s/devicem/device/
s/sbr/SBR/
s/"pciehp_reset_slot()"/pciehp_reset_slot()/ also for other functions

> About why 1.5 seconds ready time is not filtered by "pci_dev_wait()" with
> "PCIE_RESET_READY_POLL_MS" timeout, I find it is because in
> "pci_bridge_secondary_bus_reset()", the function is operating slot's
> config space to trigger sbr and also wait slot itself ready by input same
> "dev" parameter. Different from other resets like FLR which is triggered
> by operating the config space of EP device itself, sbr is triggered by
> up slot but need to wait downstream devices' ready, so I think function
> "pci_dev_wait()" works for resets like FLR but not for sbr.
> 
> In this proposed patch, I'm changing the waiting function used in sbr to
> "pci_bridge_secondary_bus_wait()" which will wait all the downstream
> hierarchy ready with the same timeout setting "PCIE_RESET_READY_POLL_MS".
> In "pci_bridge_secondary_bus_wait()" the "subordinate" and
> "subordinate->devices" will be checked firstly, and then downstream
> devices' present state.
> 
> Signed-off-by: windy.bi.enflame <windy.bi.enflame@...il.com>

See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=v5.17#n407
regarding pseudonyms.

> ---
>  drivers/pci/pci.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 9ecce435fb3f..d7ec3859268b 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5002,6 +5002,29 @@ void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev)
>  	}
>  }
>  
> +int pci_bridge_secondary_bus_wait(struct pci_dev *bridge, int timeout)
> +{
> +	struct pci_dev *dev;
> +	int delay = 1;
> +
> +	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, "secondary bus not ready after %dms\n", delay);
> +				return -ENOTTY;
> +			}
> +
> +			msleep(delay);
> +			delay *= 2;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
>  void pci_reset_secondary_bus(struct pci_dev *dev)
>  {
>  	u16 ctrl;
> @@ -5045,7 +5068,7 @@ 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);
>  
> -- 
> 2.36.1
>