linux-kernel - Re: [PATCH V12 4/5] PCI: Handle CRS ("device not ready") returned by device after FLR

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Message-ID: <20170823213838.GG8498@bhelgaas-glaptop.roam.corp.google.com>
Date:   Wed, 23 Aug 2017 16:38:38 -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, linux-arm-msm@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V12 4/5] PCI: Handle CRS ("device not ready") returned by
 device after FLR

On Wed, Aug 23, 2017 at 12:56:10AM -0400, Sinan Kaya wrote:
> Sporadic reset issues have been observed with Intel 750 NVMe drive while
> assigning the physical function to the guest machine.  The sequence of
> events observed is as follows:
> 
>   - perform a Function Level Reset (FLR)
>   - sleep up to 1000ms total
>   - read ~0 from PCI_COMMAND
>   - warn that the device didn't return from FLR
>   - touch the device before it's ready
>   - device drops config writes when we restore register settings
>   - incomplete register restore leaves device in inconsistent state
>   - device probe fails because device is in inconsistent state
> 
> After reset, an endpoint may respond to config requests with Configuration
> Request Retry Status (CRS) to indicate that it is not ready to accept new
> requests.  See PCIe r3.1, sec 2.3.1 and 6.6.2.
> 
> After an FLR, read the Vendor ID and use pci_bus_wait_crs() to wait for a
> value that indicates the device is ready only if CRS visibility is
> supported and device is responding with 0x0001.
> 
> If pci_bus_wait_crs() fails, i.e., the device has responded with CRS status
> for at least the timeout interval, fall back to the old behavior of reading
> the Command register until it is not ~0.
> 
> Also increase the timeout value from 1 second to 60 seconds to have
> consistent behavior for root ports that do and do not support CRS
> visibility.

So, after all this work, I bet you a nickel that merely increasing the
timeout from 1 sec to 60 sec (while keeping just the original PCI_COMMAND
loop) would be sufficient to fix this problem.

Reading PCI_COMMAND will cause CRS completions just like reading
PCI_VENDOR_ID will.  The only difference is that there's no CRS SV
handling, and the Root Port will synthesize ~0 data when it stops retrying
the read.

If we increase the timeout, is there still value in adding the
pci_bus_wait_crs() stuff?  I'm not sure there is.

> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> ---
>  drivers/pci/pci.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index af0cc34..5980ab3 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3819,19 +3819,35 @@ int pci_wait_for_pending_transaction(struct pci_dev *dev)
>   */
>  static void pci_flr_wait(struct pci_dev *dev)
>  {
> +	int timeout_ms = 60000;
>  	int i = 0;
>  	u32 id;
>  
> +	/*
> +	 * Per PCIe r3.1, sec 6.6.2, the device should finish FLR within
> +	 * 100ms, but even after that, it may respond to config requests
> +	 * with CRS status if it requires more time.
> +	 */
> +	msleep(100);
> +
> +	if (pci_bus_read_config_dword(dev->bus, dev->devfn, PCI_VENDOR_ID, &id))
> +		return;
> +
> +	if (pci_bus_crs_visibility_pending(id) &&
> +		pci_bus_wait_crs(dev->bus, dev->devfn, id, timeout_ms))
> +		return;
> +
> +	timeout_ms -= 100;
>  	do {
>  		msleep(100);
>  		pci_read_config_dword(dev, PCI_COMMAND, &id);
> -	} while (i++ < 10 && id == ~0);
> +	} while (i++ < (timeout_ms / 100) && id == ~0);
>  
>  	if (id == ~0)
>  		dev_warn(&dev->dev, "Failed to return from FLR\n");
>  	else if (i > 1)
>  		dev_info(&dev->dev, "Required additional %dms to return from FLR\n",
> -			 (i - 1) * 100);
> +			 i * 100);
>  }
>  
>  /**
> -- 
> 1.9.1
> 
> 
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
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