Re: [PATCH V7 6/9] PCI: Enable 10-Bit Tag support for PCIe RP devices

[Dongdong Liu <liudongdong3@huawei.com>]

On 2021/8/10 1:26, Bjorn Helgaas wrote:
> On Thu, Aug 05, 2021 at 04:25:23PM +0800, Dongdong Liu wrote:
>> On 2021/8/5 7:38, Bjorn Helgaas wrote:
>>> On Wed, Aug 04, 2021 at 09:47:05PM +0800, Dongdong Liu wrote:
>>>> PCIe spec 5.0r1.0 section 2.2.6.2 implementation note, In configurations
>>>> where a Requester with 10-Bit Tag Requester capability needs to target
>>>> multiple Completers, one needs to ensure that the Requester sends 10-Bit
>>>> Tag Requests only to Completers that have 10-Bit Tag Completer capability.
>>>> So we enable 10-Bit Tag Requester for root port only when the devices
>>>> under the root port support 10-Bit Tag Completer.
>>>
>>> Fix quoting.  I can't tell what is from the spec and what you wrote.
>> Will fix.
>>>
>>>> Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
>>>> ---
>>>>  drivers/pci/pcie/portdrv_pci.c | 69 ++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 69 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
>>>> index c7ff1ee..2382cd2 100644
>>>> --- a/drivers/pci/pcie/portdrv_pci.c
>>>> +++ b/drivers/pci/pcie/portdrv_pci.c
>>>> @@ -90,6 +90,72 @@ static const struct dev_pm_ops pcie_portdrv_pm_ops = {
>>>>  #define PCIE_PORTDRV_PM_OPS	NULL
>>>>  #endif /* !PM */
>>>>
>>>> +static int pci_10bit_tag_comp_support(struct pci_dev *dev, void *data)
>>>> +{
>>>> +	bool *support = (bool *)data;
>>>> +
>>>> +	if (!pci_is_pcie(dev)) {
>>>> +		*support = false;
>>>> +		return 1;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * PCIe spec 5.0r1.0 section 2.2.6.2 implementation note.
>>>> +	 * For configurations where a Requester with 10-Bit Tag Requester
>>>> +	 * capability targets Completers where some do and some do not have
>>>> +	 * 10-Bit Tag Completer capability, how the Requester determines which
>>>> +	 * NPRs include 10-Bit Tags is outside the scope of this specification.
>>>> +	 * So we do not consider hotplug scenario.
>>>> +	 */
>>>> +	if (dev->is_hotplug_bridge) {
>>>> +		*support = false;
>>>> +		return 1;
>>>> +	}
>>>> +
>>>> +	if (!(dev->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_COMP)) {
>>>> +		*support = false;
>>>> +		return 1;
>>>> +	}
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static void pci_configure_rp_10bit_tag(struct pci_dev *dev)
>>>> +{
>>>> +	bool support = true;
>>>> +
>>>> +	if (dev->subordinate == NULL)
>>>> +		return;
>>>> +
>>>> +	/* If no devices under the root port, no need to enable 10-Bit Tag. */
>>>> +	if (list_empty(&dev->subordinate->devices))
>>>> +		return;
>>>> +
>>>> +	pci_10bit_tag_comp_support(dev, &support);
>>>> +	if (!support)
>>>> +		return;
>>>> +
>>>> +	/*
>>>> +	 * PCIe spec 5.0r1.0 section 2.2.6.2 implementation note.
>>>> +	 * In configurations where a Requester with 10-Bit Tag Requester
>>>> +	 * capability needs to target multiple Completers, one needs to ensure
>>>> +	 * that the Requester sends 10-Bit Tag Requests only to Completers
>>>> +	 * that have 10-Bit Tag Completer capability. So we enable 10-Bit Tag
>>>> +	 * Requester for root port only when the devices under the root port
>>>> +	 * support 10-Bit Tag Completer.
>>>> +	 */
>>>> +	pci_walk_bus(dev->subordinate, pci_10bit_tag_comp_support, &support);
>>>> +	if (!support)
>>>> +		return;
>>>> +
>>>> +	if (!(dev->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_REQ))
>>>> +		return;
>>>> +
>>>> +	pci_dbg(dev, "enabling 10-Bit Tag Requester\n");
>>>> +	pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
>>>> +				 PCI_EXP_DEVCTL2_10BIT_TAG_REQ_EN);
>>>> +}
>>>> +
>>>>  /*
>>>>   * pcie_portdrv_probe - Probe PCI-Express port devices
>>>>   * @dev: PCI-Express port device being probed
>>>> @@ -111,6 +177,9 @@ static int pcie_portdrv_probe(struct pci_dev *dev,
>>>>  	     (type != PCI_EXP_TYPE_RC_EC)))
>>>>  		return -ENODEV;
>>>>
>>>> +	if (type == PCI_EXP_TYPE_ROOT_PORT)
>>>> +		pci_configure_rp_10bit_tag(dev);
>>>
>>> I don't think this has anything to do with the portdrv, so all this
>>> should go somewhere else.
>>
>> Yes, any suggestion where to put the code?
>
> It seems similar to pci_configure_ltr(), pci_configure_eetlp_prefix(),
> and other things in drivers/pci/probe.c, so maybe there?
Seems similar to pcie_bus_configure_settings().
Enable RP 10-bit tag requester need to know all the EP devices 10-bit 
tag completer capability under the RP.

>
> Or, if this is more of a theoretical advantage than a demonstrated
> performance improvement, we could just hold off on doing it until it
> becomes important.  I can't tell if you have a scenario that actually
> benefits from this yet.

Ok, I will remove this patch from the patchset.
We will do this later when get performance improvement data.

Thanks,
Dongdong
>
>>> Out of curiosity, IIUC this enables 10-bit tags for MMIO transactions
>>> from the root port toward the device, i.e., traffic that originates
>>> from a CPU.  Is that a significant benefit?  I would expect high-speed
>>> devices would primarily operate via DMA with relatively little MMIO
>>> traffic.
>>
>> The benefits of 10-Bit Tag for EP are obvious.
>> There are few RP scenarios. Unless there are two:
>> 1. RC has its own DMA.
>> 2. The P2P tag is replaced at the RP when the P2PDMA go through RP.
>
> .
>