[PATCH V3 13/16] platform/x86/intel/pmc/ssram: Refactor memory barrier for reentrant probe

David E. Box
2026-05-01T23:11:18Z

From: Xi Pardee <xi.pardee@linux.intel.com>

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@linux.intel.com>
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
---
V3 - No changes

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..597bfb7ad822 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