Re: [PATCH v7 2/3] PCI: Enable PCIe Relaxed Ordering if supported

[Raj, Ashok]

Hi Ding

patch looks good, except would reword the patch description for clarity

here is my crack at it, feel free to use.

On Thu, Jul 13, 2017 at 10:21:31PM +0800, Ding Tianhong wrote:
> The PCIe Device Control Register use the bit 4 to indicate that
> whether the device is permitted to enable relaxed ordering or not.
> But relaxed ordering is not safe for some platform which could only
> use strong write ordering, so devices are allowed (but not required)
> to enable relaxed ordering bit by default.
> 
> If a PCIe device didn't enable the relaxed ordering attribute default,
> we should not do anything in the PCIe configuration, otherwise we
> should check if any of the devices above us do not support relaxed
> ordering by the PCI_DEV_FLAGS_NO_RELAXED_ORDERING flag, then base on
> the result if we get a return that indicate that the relaxed ordering
> is not supported we should update our device to disable relaxed ordering
> in configuration space. If the device above us doesn't exist or isn't
> the PCIe device, we shouldn't do anything and skip updating relaxed ordering
> because we are probably running in a guest machine.

When bit4 is set in the PCIe Device Control register, it indicates
whether the device is permitted to use relaxed ordering.
On some platforms using relaxed ordering can have performance issues or
due to erratum can cause data-corruption. In such cases devices must avoid
using relaxed ordering.

This patch checks if there is any node in the hierarchy that indicates that
using relaxed ordering is not safe. In such cases the patch turns off the
relaxed ordering by clearing the eapability for this device.

> 
> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
> ---
>  drivers/pci/pci.c   | 29 +++++++++++++++++++++++++++++
>  drivers/pci/probe.c | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/pci.h |  2 ++
>  3 files changed, 68 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d88edf5..7a6b32f 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4854,6 +4854,35 @@ int pcie_set_mps(struct pci_dev *dev, int mps)
>  EXPORT_SYMBOL(pcie_set_mps);
>  
>  /**
> + * pcie_clear_relaxed_ordering - clear PCI Express relaxed ordering bit
> + * @dev: PCI device to query
> + *
> + * If possible clear relaxed ordering
> + */
> +int pcie_clear_relaxed_ordering(struct pci_dev *dev)
> +{
> +	return pcie_capability_clear_word(dev, PCI_EXP_DEVCTL,
> +					  PCI_EXP_DEVCTL_RELAX_EN);
> +}
> +EXPORT_SYMBOL(pcie_clear_relaxed_ordering);
> +
> +/**
> + * pcie_relaxed_ordering_supported - Probe for PCIe relexed ordering support
> + * @dev: PCI device to query
> + *
> + * Returns true if the device support relaxed ordering attribute.
> + */
> +bool pcie_relaxed_ordering_supported(struct pci_dev *dev)
> +{
> +	u16 v;
> +
> +	pcie_capability_read_word(dev, PCI_EXP_DEVCTL, &v);
> +
> +	return !!(v & PCI_EXP_DEVCTL_RELAX_EN);
> +}
> +EXPORT_SYMBOL(pcie_relaxed_ordering_supported);
> +
> +/**
>   * pcie_get_minimum_link - determine minimum link settings of a PCI device
>   * @dev: PCI device to query
>   * @speed: storage for minimum speed
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index c31310d..48df012 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1762,6 +1762,42 @@ static void pci_configure_extended_tags(struct pci_dev *dev)
>  					 PCI_EXP_DEVCTL_EXT_TAG);
>  }
>  
> +/**
> + * pci_dev_should_disable_relaxed_ordering - check if the PCI device
> + * should disable the relaxed ordering attribute.
> + * @dev: PCI device
> + *
> + * Return true if any of the PCI devices above us do not support
> + * relaxed ordering.
> + */
> +static bool pci_dev_should_disable_relaxed_ordering(struct pci_dev *dev)
> +{
> +	while (dev) {
> +		if (dev->dev_flags & PCI_DEV_FLAGS_NO_RELAXED_ORDERING)
> +			return true;
> +
> +		dev = dev->bus->self;
> +	}
> +
> +	return false;
> +}
> +
> +static void pci_configure_relaxed_ordering(struct pci_dev *dev)
> +{
> +	/* We should not alter the relaxed ordering bit for the VF */
> +	if (dev->is_virtfn)
> +		return;
> +
> +	/* If the releaxed ordering enable bit is not set, do nothing. */
> +	if (!pcie_relaxed_ordering_supported(dev))
> +		return;
> +
> +	if (pci_dev_should_disable_relaxed_ordering(dev)) {
> +		pcie_clear_relaxed_ordering(dev);
> +		dev_info(&dev->dev, "Disable Relaxed Ordering\n");
> +	}
> +}
> +
>  static void pci_configure_device(struct pci_dev *dev)
>  {
>  	struct hotplug_params hpp;
> @@ -1769,6 +1805,7 @@ static void pci_configure_device(struct pci_dev *dev)
>  
>  	pci_configure_mps(dev);
>  	pci_configure_extended_tags(dev);
> +	pci_configure_relaxed_ordering(dev);
>  
>  	memset(&hpp, 0, sizeof(hpp));
>  	ret = pci_get_hp_params(dev, &hpp);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 412ec1c..3aa23a2 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1127,6 +1127,8 @@ int pci_add_ext_cap_save_buffer(struct pci_dev *dev,
>  void pci_pme_wakeup_bus(struct pci_bus *bus);
>  void pci_d3cold_enable(struct pci_dev *dev);
>  void pci_d3cold_disable(struct pci_dev *dev);
> +int pcie_clear_relaxed_ordering(struct pci_dev *dev);
> +bool pcie_relaxed_ordering_supported(struct pci_dev *dev);
>  
>  /* PCI Virtual Channel */
>  int pci_save_vc_state(struct pci_dev *dev);
> -- 
> 1.8.3.1
> 
>