linux-kernel - Re: [PATCH v2] pci/probe: Enable CRS for root port if it is supported

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Date:	Mon, 22 Sep 2014 12:54:18 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Rajat Jain <rajatxjain@...il.com>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rajat Jain <rajatjain@...iper.net>,
	Guenter Roeck <groeck@...iper.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Yang <weiyang@...ux.vnet.ibm.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH v2] pci/probe: Enable CRS for root port if it is supported

On Tue, Sep 02, 2014 at 04:26:00PM -0700, Rajat Jain wrote:
> 
> As per the PCIe spec, an endpoint may return the configuration cycles
> with CRS if it is not yet fully ready to be accessed. If the CRS visibility
> is not enabled at the root port, the spec leaves the retry behaviour open
> to implementation in such a case. The Intel root ports have chosen to retry
> endlessly in this situation. Thus, the root controller may "hang" (repeatedly
> retrying the configuration requests until it gets a status other than CRS) if
> a device returns CRS for a long time. This can cause a broken endpoint to bring
> down the whole PCI hierarchy.
> 
> This was recently known to cause problems on Intel systems and
> was discussed here:
> http://marc.info/?t=140926298500002&r=1&w=2
> 
> Ref1:
> https://www.pcisig.com/specifications/pciexpress/ECN_CRS_Software_Visibility_No27.pdf
> 
> Ref2:
> PCIe spec V3.0, pg119, pg127 for "Configuration Request Retry Status"
> 
> Thus enable the CRS visibility for the root ports that support it. This
> patch reverts the following commit, but enables CRS visibility only
> when the root port supports it:
> 
> ad7edfe04908 ("[PCI] Do not enable CRS Software Visibility by default")
> 
> (Linus' response: http://marc.info/?l=linux-pci&m=140968622520192&w=2)
> 
> Signed-off-by: Rajat Jain <rajatxjain@...il.com>
> Signed-off-by: Rajat Jain <rajatjain@...iper.net>
> Signed-off-by: Guenter Roeck <groeck@...iper.net>

Applied to pci/enumeration for v3.18, thanks!

> ---
> v2: Remove the white list, that was enabling the CRS for certain known Intel systems.
>     Rather now enable it for all systems that support this capability.
> v1: Enable CRS for only some given Intel systems (maintain a whitelist)
> 
>  drivers/pci/probe.c           |   13 +++++++++++++
>  include/uapi/linux/pci_regs.h |    1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index e3cf8a2..3c4c35c 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -740,6 +740,17 @@ struct pci_bus *pci_add_new_bus(struct pci_bus *parent, struct pci_dev *dev,
>  }
>  EXPORT_SYMBOL(pci_add_new_bus);
>  
> +static void pci_enable_crs(struct pci_dev *pdev)
> +{
> +	u16 root_cap = 0;
> +
> +	/* Enable CRS Software visibility if supported */
> +	pcie_capability_read_word(pdev, PCI_EXP_RTCAP, &root_cap);
> +	if (root_cap & PCI_EXP_RTCAP_CRSVIS)
> +		pcie_capability_set_word(pdev, PCI_EXP_RTCTL,
> +					 PCI_EXP_RTCTL_CRSSVE);
> +}
> +
>  /*
>   * If it's a bridge, configure it and scan the bus behind it.
>   * For CardBus bridges, we don't scan behind as the devices will
> @@ -787,6 +798,8 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
>  	pci_write_config_word(dev, PCI_BRIDGE_CONTROL,
>  			      bctl & ~PCI_BRIDGE_CTL_MASTER_ABORT);
>  
> +	pci_enable_crs(dev);
> +
>  	if ((secondary || subordinate) && !pcibios_assign_all_busses() &&
>  	    !is_cardbus && !broken) {
>  		unsigned int cmax;
> diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> index 30db069..a75106d 100644
> --- a/include/uapi/linux/pci_regs.h
> +++ b/include/uapi/linux/pci_regs.h
> @@ -552,6 +552,7 @@
>  #define  PCI_EXP_RTCTL_PMEIE	0x0008	/* PME Interrupt Enable */
>  #define  PCI_EXP_RTCTL_CRSSVE	0x0010	/* CRS Software Visibility Enable */
>  #define PCI_EXP_RTCAP		30	/* Root Capabilities */
> +#define  PCI_EXP_RTCAP_CRSVIS	0x0001	/* CRS software visibility capability */
>  #define PCI_EXP_RTSTA		32	/* Root Status */
>  #define PCI_EXP_RTSTA_PME	0x00010000 /* PME status */
>  #define PCI_EXP_RTSTA_PENDING	0x00020000 /* PME pending */
> -- 
> 1.7.9.5
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