linux-kernel - Re: [PATCH V2] PCI: Clear errors logged in Secondary Status Register

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Date: Mon, 22 Jan 2024 17:00:26 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Vidya Sagar <vidyas@...dia.com>
Cc: bhelgaas@...gle.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, treding@...dia.com,
	jonathanh@...dia.com, kthota@...dia.com, mmaddireddy@...dia.com,
	sagar.tv@...il.com
Subject: Re: [PATCH V2] PCI: Clear errors logged in Secondary Status Register

On Tue, Jan 16, 2024 at 08:02:58PM +0530, Vidya Sagar wrote:
> The enumeration process leaves the 'Received Master Abort' bit set in
> the Secondary Status Register of the downstream port in the following
> scenarios.
> 
> (1) The device connected to the downstream port has ARI capability
>     and that makes the kernel set the 'ARI Forwarding Enable' bit in
>     the Device Control 2 Register of the downstream port. This
>     effectively makes the downstream port forward the configuration
>     requests targeting the devices downstream of it, even though they
>     don't exist in reality. It causes the downstream devices return
>     completions with UR set in the status in turn causing 'Received
>     Master Abort' bit set.
> 
>     In contrast, if the downstream device doesn't have ARI capability,
>     the 'ARI Forwarding Enable' bit in the downstream port is not set
>     and any configuration requests targeting the downstream devices
>     that don't exist are terminated (section 6.13 of PCI Express Base
>     6.0 spec) in the downstream port itself resulting in no change of
>     the 'Received Master Abort' bit.
> 
> (2) A PCIe switch is connected to the downstream port and when the
>     enumeration flow tries to explore the presence of devices that
>     don't really exist downstream of the switch, the downstream
>     port receives the completions with UR set causing the 'Received
>     Master Abort' bit set.

Are these the only possible ways this error is logged?  I expected
them to be logged when we enumerate below a Root Port that has nothing
attached, for example.

Does clearing them in pci_scan_bridge_extend() cover all ways this
error might be logged during enumeration?  I can't remember whether
all enumeration goes through this path.

> Clear 'Received Master Abort' bit to keep the bridge device in a clean
> state post enumeration.
> 
> Signed-off-by: Vidya Sagar <vidyas@...dia.com>
> ---
> V2:
> * Changed commit message based on Bjorn's feedback
> 
>  drivers/pci/probe.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 795534589b98..640d2871b061 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1470,6 +1470,9 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>  	}
>  
>  out:
> +	/* Clear errors in the Secondary Status Register */
> +	pci_write_config_word(dev, PCI_SEC_STATUS, 0xffff);
> +
>  	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
>  
>  	pm_runtime_put(&dev->dev);
> -- 
> 2.25.1
>