[PATCH v4 3/3] PCI: Add CRS handling to pci_dev_wait()

[Stanislav Spassov <stanspas@amazon.com>]

From: Stanislav Spassov <stanspas@amazon.de>

The PCI Express specification dictates minimal amounts of time that the
host needs to wait after triggering different kinds of resets before it
is allowed to attempt accessing the device. After this waiting period,
devices are required to be responsive to Configuration Space reads.
However, if a device needs more time to actually complete the reset
operation internally, it may respond to the read with a Completion
Request Retry Status (CRS), and keep doing so on subsequent reads
for as long as necessary. If the device is broken, it may even keep
responding with CRS indefinitely.

The specification also mandates that any Root Port that supports CRS
and has CRS Software Visibility (CRS SV) enabled will synthesize the
special value 0x0001 for the Vendor ID and set any other bits to 1
upon receiving a CRS Completion for a Configuration Read Request that
includes both bytes of the Vendor ID (offset 0).

If CRS SV is disabled or a different register (not Vendor ID) is being
read, the request is retried autonomously by the Root Port.
Platform-specific configuration registers may exist to limit the number
of or time taken by such retries.

If CRS is not supported, or a device is responding with CA/UR
Completions (rather than CRS), the behavior is platform-dependent, but
generally the Root Port synthesizes ~0 to complete the software read.

Previously, pci_dev_wait() avoided taking advantage of CRS. However,
on platforms where no retry limit/timeout can be configured, a device
responding with CRS for too long (e.g. because it is stuck and cannot
complete its reset) may trigger more severe error conditions (e.g. TOR
timeout, 3-strike CPU CATERR), because the Root Port never reports back
to the lower-level component requesting the transaction.

This patch introduces special handling when CRS is available, and
otherwise falls back to the previous behavior of polling COMMAND.

Signed-off-by: Stanislav Spassov <stanspas@amazon.de>
---
 drivers/pci/pci.c | 55 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 47 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 44f5d4907db6..a028147f4471 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1073,17 +1073,56 @@ static inline int pci_dev_poll_until_not_equal(struct pci_dev *dev, int where,
 
 static int pci_dev_wait(struct pci_dev *dev, char *reset_type, int timeout)
 {
+	int waited = 0;
+	int rc = 0;
+
+
 	/*
 	 * After reset, the device should not silently discard config
 	 * requests, but it may still indicate that it needs more time by
-	 * responding to them with CRS completions.  The Root Port will
-	 * generally synthesize ~0 data to complete the read (except when
-	 * CRS SV is enabled and the read was for the Vendor ID; in that
-	 * case it synthesizes 0x0001 data).
-	 *
-	 * Wait for the device to return a non-CRS completion.  Read the
-	 * Command register instead of Vendor ID so we don't have to
-	 * contend with the CRS SV value.
+	 * responding to them with CRS completions. For such completions:
+	 * - If CRS SV is enabled on the Root Port, and the read request
+	 *   covers both bytes of the Vendor ID register, the Root Port
+	 *   will synthesize the value 0x0001 (and set any extra requested
+	 *   bytes to 0xff)
+	 * - If CRS SV is not enabled on the Root Port, the Root Port must
+	 *   re-issue the Configuration Request as a new Request.
+	 *   Depending on platform-specific Root Complex configurations,
+	 *   the Root Port may stop retrying after a set number of attempts,
+	 *   or a configured timeout is hit, or continue indefinitely
+	 *   (ultimately resulting in non-PCI-specific platform errors, such as
+	 *   a TOR timeout).
+	 */
+	if (dev->crssv_enabled) {
+		u32 id;
+
+		rc = pci_dev_poll_until_not_equal(dev, PCI_VENDOR_ID, 0xffff,
+						  0x0001, reset_type, timeout,
+						  &waited, &id);
+		if (rc)
+			return rc;
+
+		timeout -= waited;
+
+		/*
+		 * If Vendor/Device ID is valid, the device must be ready.
+		 * Note: SR-IOV VFs return ~0 for reads to Vendor/Device
+		 * ID and will not be recognized as ready by this check.
+		 */
+		if (id != 0x0000ffff && id != 0xffff0000 &&
+		    id != 0x00000000 && id != 0xffffffff)
+			return 0;
+	}
+
+	/*
+	 * Root Ports will generally indicate error scenarios (e.g.
+	 * internal timeouts, or received Completion with CA/UR) by
+	 * synthesizing an 'all bits set' value (~0).
+	 * In case CRS is not supported/enabled, as well as for SR-IOV VFs,
+	 * fall back to polling a different register that cannot validly
+	 * contain ~0. As of PCIe 5.0, bits 11-15 of COMMAND are still RsvdP
+	 * and must return 0 when read.
+	 * XXX: These bits might become meaningful in the future
 	 */
 	return pci_dev_poll_until_not_equal(dev, PCI_COMMAND, ~0, ~0,
 					    reset_type, timeout, NULL,
-- 
2.25.1




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