Re: [PATCH v2 2/3] PCI: Enable PCIe Relaxed Ordering if supported

[Alexander Duyck]

On Fri, Jun 2, 2017 at 9:04 PM, Ding Tianhong <dingtianhong@xxxxxxxxxx> 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 platform support relaxed ordering but does not enable it by
> default, enable it in the PCIe configuration. This allows some device
> to send TLPs with the relaxed ordering attributes set, which may
> improve the performance.
>
> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
> ---
>  drivers/pci/pci.c   | 42 ++++++++++++++++++++++++++++++++++++++++++
>  drivers/pci/probe.c | 11 +++++++++++
>  include/linux/pci.h |  3 +++
>  3 files changed, 56 insertions(+)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b01bd5b..f57a374 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4878,6 +4878,48 @@ int pcie_set_mps(struct pci_dev *dev, int mps)
>  EXPORT_SYMBOL(pcie_set_mps);
>
>  /**
> + * pcie_set_relaxed_ordering - set PCI Express relexed ordering bit
> + * @dev: PCI device to query
> + *
> + * If possible sets relaxed ordering
> + */
> +int pcie_set_relaxed_ordering(struct pci_dev *dev)
> +{
> +       return pcie_capability_set_word(dev, PCI_EXP_DEVCTL, PCI_EXP_DEVCTL_RELAX_EN);
> +}
> +EXPORT_SYMBOL(pcie_set_relaxed_ordering);
> +
> +/**
> + * pcie_clear_relaxed_ordering - clear PCI Express relexed 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_get_relaxed_ordering - check PCI Express relexed ordering bit
> + * @dev: PCI device to query
> + *
> + * Returns true if relaxed ordering is been set
> + */
> +int pcie_get_relaxed_ordering(struct pci_dev *dev)
> +{
> +       u16 v;
> +
> +       pcie_capability_read_word(dev, PCI_EXP_DEVCTL, &v);
> +
> +       return (v & PCI_EXP_DEVCTL_RELAX_EN) >> 4;
> +}
> +EXPORT_SYMBOL(pcie_get_relaxed_ordering);
> +
> +/**
> + * pcie_set_mps - set PCI Express maximum payload size
> +/**
>   * 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 19c8950..aeb22b5 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1701,6 +1701,16 @@ static void pci_configure_extended_tags(struct pci_dev *dev)
>                                          PCI_EXP_DEVCTL_EXT_TAG);
>  }
>
> +static void pci_configure_relaxed_ordering(struct pci_dev *dev)
> +{
> +       int ret;
> +
> +       if (dev && (dev->dev_flags & PCI_DEV_FLAGS_NO_RELAXED_ORDERING))

So there is a minor issue here. The problem is this is only trying to
modify relaxed ordering for the device itself. That isn't what we
want. What we want is to modify it on all of the upstream port
interfaces where there is something the path to the root complex that
has an issue. So if the root complex has to set the
NO_RELAXED_ORDERING flag on a root port, all of the interfaces below
it that would be pushing traffic toward it should not have the relaxed
ordering bit set.

Also I am pretty sure this is a PCIe capability, not a PCI capability.
You probably need to make sure you code is making this distinction
which I don't know if it currently is. If you need an example of the
kind of checks I am suggesting just take a look at
pcie_configure_mps(). It is verifying the function is PCIe before
attempting to make any updates. In your case you will probably also
need to make sure there is a bus for you to walk up the chain of.
Otherwise this shouldn't apply.


> +               pcie_set_relaxed_ordering(dev);
> +       else
> +               pcie_clear_relaxed_ordering(dev);
> +}

Also I am not a fan of the way this is handled currently. If you don't
have relaxed ordering set then you don't need to do anything else, if
you do have it set but there is no bus to walk up you shouldn't change
it, and if there is a bus to walk up and you find that the root
complex on that bus has the NO_RELAXED_ORDERING set you should clear
it. Right now this code seems to be enabling relaxed ordering if the
NO_RELAXED_ORDERING flag is set.

> +
>  static void pci_configure_device(struct pci_dev *dev)
>  {
>         struct hotplug_params hpp;
> @@ -1708,6 +1718,7 @@ 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 e1e8428..84bd6af 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1105,6 +1105,9 @@ int __pci_enable_wake(struct pci_dev *dev, pci_power_t state,
>  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_set_relaxed_ordering(struct pci_dev *dev);
> +int pcie_clear_relaxed_ordering(struct pci_dev *dev);
> +int pcie_get_relaxed_ordering(struct pci_dev *dev);
>
>  static inline int pci_enable_wake(struct pci_dev *dev, pci_power_t state,
>                                   bool enable)
> --
> 1.9.0
>
>