Re: [PATCH V7 7/9] PCI/sysfs: Add a 10-Bit Tag sysfs file

On Thu, Aug 05, 2021 at 04:37:39PM +0800, Dongdong Liu wrote:

On 2021/8/5 7:49, Bjorn Helgaas wrote:

quoted

On Wed, Aug 04, 2021 at 09:47:06PM +0800, Dongdong Liu wrote:

quoted

PCIe spec 5.0 r1.0 section 2.2.6.2 says that if an Endpoint supports
sending Requests to other Endpoints (as opposed to host memory), the
Endpoint must not send 10-Bit Tag Requests to another given Endpoint
unless an implementation-specific mechanism determines that the Endpoint
supports 10-Bit Tag Completer capability. Add a 10bit_tag sysfs file,
write 0 to disable 10-Bit Tag Requester when the driver does not bind
the device if the peer device does not support the 10-Bit Tag Completer.
This will make P2P traffic safe. the 10bit_tag file content indicate
current 10-Bit Tag Requester Enable status.

Signed-off-by: Dongdong Liu <redacted>
---
 Documentation/ABI/testing/sysfs-bus-pci | 16 +++++++-
 drivers/pci/pci-sysfs.c                 | 69 +++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index 793cbb7..0e0c97d 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci

@@ -139,7 +139,7 @@ Description:
 		binary file containing the Vital Product Data for the
 		device.  It should follow the VPD format defined in
 		PCI Specification 2.1 or 2.2, but users should consider
-		that some devices may have incorrectly formatted data.
+		that some devices may have incorrectly formatted data.
 		If the underlying VPD has a writable section then the
 		corresponding section of this file will be writable.

@@ -407,3 +407,17 @@ Description:

 		The file is writable if the PF is bound to a driver that
 		implements ->sriov_set_msix_vec_count().
+
+What:		/sys/bus/pci/devices/.../10bit_tag
+Date:		August 2021
+Contact:	Dongdong Liu <liudongdong3@huawei.com>
+Description:
+		If a PCI device support 10-Bit Tag Requester, will create the
+		10bit_tag sysfs file. The file is readable, the value
+		indicate current 10-Bit Tag Requester Enable.
+		1 - enabled, 0 - disabled.
+
+		The file is also writeable, the value only accept by write 0
+		to disable 10-Bit Tag Requester when the driver does not bind
+		the deivce. The typical use case is for p2pdma when the peer
+		device does not support 10-BIT Tag Completer.

quoted

quoted

+static ssize_t pci_10bit_tag_store(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf, size_t count)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+	bool enable;
+
+	if (kstrtobool(buf, &enable) < 0)
+		return -EINVAL;
+
+	if (enable != false )
+		return -EINVAL;

Is this the same as "if (enable)"?

Yes, Will fix.

I actually don't like the one-way nature of this.  When the hierarchy
supports 10-bit tags, we automatically enable them during enumeration.

Then we provide this sysfs file, but it can only *disable* 10-bit
tags.  There's no way to re-enable them except by rebooting (or using
setpci, I guess).

Why can't we allow *enabling* them here if they're supported in this
hierarchy?

quoted

quoted

+	if (pdev->driver)
+		 return -EBUSY;
+
+	pcie_capability_clear_word(pdev, PCI_EXP_DEVCTL2,
+				   PCI_EXP_DEVCTL2_10BIT_TAG_REQ_EN);
+	pci_info(pdev, "disabled 10-Bit Tag Requester\n");
+
+	return count;
+}