linux-kernel - Re: [PATCH V4] PCI: handle CRS returned by device after FLR

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Date:   Thu, 13 Jul 2017 07:17:58 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        alex.williamson@...hat.com, vikrams@...eaurora.org,
        Lorenzo.Pieralisi@....com, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V4] PCI: handle CRS returned by device after FLR

On Thu, Jul 06, 2017 at 05:07:14PM -0400, Sinan Kaya wrote:
> An endpoint is allowed to issue Configuration Request Retry Status (CRS)
> following a Function Level Reset (FLR) request to indicate that it is not
> ready to accept new requests.
> 
> Seen a timeout message with Intel 750 NVMe drive and FLR reset.
> 
> Kernel enables CRS visibility in pci_enable_crs() function for each bridge
> it discovers. The OS observes a special vendor ID read value of 0xFFFF0001
> in this case. We need to keep polling until this special read value
> disappears. pci_bus_read_dev_vendor_id() takes care of CRS handling for a
> given vendor id read request under the covers.
> 
> Adding a vendor ID read if this is a physical function before attempting
> to read any other registers on the endpoint. A CRS indication will only
> be given if the address to be read is vendor ID register.
> 
> Note that virtual functions report their vendor ID through another
> mechanism.
> 
> The spec is calling to wait up to 1 seconds if the device is sending CRS.
> The NVMe device seems to be requiring more. Relax this up to 60 seconds.

Can you add a pointer to the "1 second" requirement in the spec here?
We use 60 seconds in pci_scan_device() and acpiphp_add_context().  Is
there a basis in the spec for the 60 second timeout?

What's the NVMe excuse for requiring more time than the spec allows?
Is this a hardware erratum?  Is there some PCIe ECN pending to address
this?

I try to avoid adding generic changes based on one specific piece of
hardware because it can penalize everybody else who actually bothered
to follow the spec.  For example, if FLR fails because a non-NVMe
device is broken, it will now take 60 seconds to notice that instead
of 1 second.

> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> ---
>  drivers/pci/pci.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index aab9d51..83a9784 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3723,10 +3723,16 @@ static void pci_flr_wait(struct pci_dev *dev)
>  	int i = 0;
>  	u32 id;
>  
> -	do {
> -		msleep(100);
> -		pci_read_config_dword(dev, PCI_COMMAND, &id);
> -	} while (i++ < 10 && id == ~0);
> +	if (dev->is_virtfn) {
> +		do {
> +			msleep(100);
> +			pci_read_config_dword(dev, PCI_COMMAND, &id);
> +		} while (i++ < 10 && id == ~0);
> +	} else {
> +		if (!pci_bus_read_dev_vendor_id(dev->bus, dev->devfn, &id,
> +						60*1000))
> +			id = ~0;
> +	}
>  
>  	if (id == ~0)
>  		dev_warn(&dev->dev, "Failed to return from FLR\n");
> -- 
> 1.9.1
> 
> 
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
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