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

[Ding Tianhong]

On 2017/6/4 2:19, Alexander Duyck wrote:
> 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.
> 

Yes, I miss the upstream ports and the pcie/pci capability, will check
the pcie_configure_mps() again and fix it, thanks.

> 
>> +               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.
> 

I'm not very clear about this, I check the PCIe Standard, it said that:

Enable Relaxed Ordering â If this bit is set, the device is
permitted to set the Relaxed Ordering bit in the Attributes field of
transactions it initiates that do not require strong write ordering
(see Sections 2.2.6.4 and 2.4).
Default value of this bit is 1.
This bit may be hardwired to 0 if a device never sets the
Relaxed Ordering attribute in transactions it initiates as a
requester.

Is that means some platform may set the bit to 0 default but it didn't mean the hardware
couldn't support Relaxed Ordering ? if so, I need to set the bit enable if  relaxed
ordering flag is set.

If I didn't understand it correctly, I will fix it follow your opinion, thanks.

Ding


>> +
>>  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
>>
>>
> 
> .
>