On Thu, Aug 05, 2021 at 04:25:23PM +0800, Dongdong Liu wrote:

On 2021/8/5 7:38, Bjorn Helgaas wrote:

quoted

On Wed, Aug 04, 2021 at 09:47:05PM +0800, Dongdong Liu wrote:

quoted

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.

quoted

quoted

Signed-off-by: Dongdong Liu <redacted>
---
 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?

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.

quoted

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.