[PATCH V2 13/17] platform/x86/intel/pmc/ssram: Refactor memory barrier for reentrant probe

[David E. Box]

From: Xi Pardee <xi.pardee@xxxxxxxxxxxxxxx>

Previously, a single global 'device_probed' flag with memory barriers was
used to prevent callers from reading PMC info before probe completion.  The
write barrier in probe ensured all data, devid and base_addr, was visible
before signaling completion, and the read barrier in callers ensured they
checked the flag before reading data.

A following commit will make probe reentrant, requiring that a different
synchronization flag be used since a single global flag cannot coordinate
multiple concurrent probes.

Switch to per-index devid publication. Each probe instance writes base_addr
first, then a write barrier ensures visibility before devid is written as
the completion signal. Callers check devid first, then use a read barrier
before reading base_addr. This per-index approach allows multiple probes
to work independently while maintaining the same memory ordering guarantees.

Signed-off-by: Xi Pardee <xi.pardee@xxxxxxxxxxxxxxx>
Signed-off-by: David E. Box <david.e.box@xxxxxxxxxxxxxxx>
---

V2 changes:
- Expanded commit message to explain synchronization rationale
- Remove unused probe_finish label associated with the old global flag

 .../platform/x86/intel/pmc/ssram_telemetry.c  | 40 ++++++++-----------
 1 file changed, 16 insertions(+), 24 deletions(-)

diff --git a/drivers/platform/x86/intel/pmc/ssram_telemetry.c b/drivers/platform/x86/intel/pmc/ssram_telemetry.c
index 6917a10cbc80..b329e0c0080b 100644
--- a/drivers/platform/x86/intel/pmc/ssram_telemetry.c
+++ b/drivers/platform/x86/intel/pmc/ssram_telemetry.c
@@ -40,7 +40,6 @@ static const struct ssram_type pci_main = {
 };
 
 static struct pmc_ssram_telemetry pmc_ssram_telems[MAX_NUM_PMC];
-static bool device_probed;
 
 static inline u64 get_base(void __iomem *addr, u32 offset)
 {
@@ -55,8 +54,13 @@ static void pmc_ssram_get_devid_pwrmbase(void __iomem *ssram, unsigned int pmc_i
 	pwrm_base = get_base(ssram, SSRAM_PWRM_OFFSET);
 	devid = readw(ssram + SSRAM_DEVID_OFFSET);
 
-	pmc_ssram_telems[pmc_idx].devid = devid;
 	pmc_ssram_telems[pmc_idx].base_addr = pwrm_base;
+	/*
+	 * Memory barrier is used to ensure the correct write order between base_addr
+	 * and devid.
+	 */
+	smp_wmb();
+	pmc_ssram_telems[pmc_idx].devid = devid;
 }
 
 static int
@@ -154,32 +158,28 @@ static int pmc_ssram_telemetry_pci_init(struct pci_dev *pcidev)
  * * 0           - Success
  * * -EAGAIN     - Probe function has not finished yet. Try again.
  * * -EINVAL     - Invalid pmc_idx
- * * -ENODEV     - PMC device is not available
  */
 int pmc_ssram_telemetry_get_pmc_info(unsigned int pmc_idx,
 				     struct pmc_ssram_telemetry *pmc_ssram_telemetry)
 {
+	if (pmc_idx >= MAX_NUM_PMC)
+		return -EINVAL;
+
 	/*
 	 * PMCs are discovered in probe function. If this function is called before
-	 * probe function complete, the result would be invalid. Use device_probed
-	 * variable to avoid this case. Return -EAGAIN to inform the consumer to call
+	 * probe function complete, the result would be invalid. Use devid to avoid
+	 * this case. Return -EAGAIN to inform the consumer to call
 	 * again later.
 	 */
-	if (!device_probed)
+	if (!pmc_ssram_telems[pmc_idx].devid)
 		return -EAGAIN;
 
+	pmc_ssram_telemetry->devid = pmc_ssram_telems[pmc_idx].devid;
 	/*
 	 * Memory barrier is used to ensure the correct read order between
-	 * device_probed variable and PMC info.
+	 * devid variable and base_addr.
 	 */
 	smp_rmb();
-	if (pmc_idx >= MAX_NUM_PMC)
-		return -EINVAL;
-
-	if (!pmc_ssram_telems[pmc_idx].devid)
-		return -ENODEV;
-
-	pmc_ssram_telemetry->devid = pmc_ssram_telems[pmc_idx].devid;
 	pmc_ssram_telemetry->base_addr = pmc_ssram_telems[pmc_idx].base_addr;
 	return 0;
 }
@@ -194,8 +194,7 @@ static int pmc_ssram_telemetry_probe(struct pci_dev *pcidev, const struct pci_de
 	ssram_type = (const struct ssram_type *)id->driver_data;
 	if (!ssram_type) {
 		dev_dbg(&pcidev->dev, "missing driver data\n");
-		ret = -EINVAL;
-		goto probe_finish;
+		return -EINVAL;
 	}
 
 	method = ssram_type->method;
@@ -203,7 +202,7 @@ static int pmc_ssram_telemetry_probe(struct pci_dev *pcidev, const struct pci_de
 	ret = pcim_enable_device(pcidev);
 	if (ret) {
 		dev_dbg(&pcidev->dev, "failed to enable PMC SSRAM device\n");
-		goto probe_finish;
+		return ret;
 	}
 
 	if (method == RES_METHOD_PCI)
@@ -211,13 +210,6 @@ static int pmc_ssram_telemetry_probe(struct pci_dev *pcidev, const struct pci_de
 	else
 		ret = -EINVAL;
 
-probe_finish:
-	/*
-	 * Memory barrier is used to ensure the correct write order between PMC info
-	 * and device_probed variable.
-	 */
-	smp_wmb();
-	device_probed = true;
 	return ret;
 }
 
-- 
2.43.0