[PATCH v6 05/15] platform/x86/intel/pmt: Cache the telemetry discovery header

[David E. Box]

pmt_telem_header_decode() only needs the discovery header dwords, but it
currently decodes them by reading directly from entry->disc_table.

Cache the discovery header in intel_pmt_entry when the device is created
and have telemetry decode use the cached values instead of performing MMIO
reads at decode time.

The DVSEC discovery resource for a namespace is sized by its per-entry
entry_size (in dwords), which can be less than the 4-dword cache (e.g.
telemetry uses entry_size = 3, i.e. 12 bytes). Cap the memcpy_fromio()
to resource_size(disc_res) so the new cache does not read past the
mapped region. Any unread dwords stay zero from the zero-initialized
allocation of the containing struct.

This keeps the telemetry header decode path independent of how the
discovery data is backed and avoids baking a direct MMIO assumption into
the feature-specific decode logic.

Assisted-by: GitHub-Copilot:claude-opus-4.7
Signed-off-by: David E. Box <david.e.box@xxxxxxxxxxxxxxx>
---
V6 changes:
  - Added #include <linux/minmax.h> for min_t() macro used in
    memcpy_fromio() size calculation (Ilpo).

V5 changes:
  - Cap memcpy_fromio() of the cached discovery header to
    resource_size(disc_res) so the newly introduced cache does not
    over-read namespaces whose DVSEC entry_size is smaller than the
    cache (e.g. telemetry has entry_size = 3, 12 bytes).

V4 - No changes

V3 changes:
  - New patch split out from PMT header-fetch rework to cache discovery
    header data before downstream decode/population.
  - Added to carry the post-v3 bug fix while preserving the original series
    ordering intent.

 drivers/platform/x86/intel/pmt/class.c     | 12 ++++++++++++
 drivers/platform/x86/intel/pmt/class.h     |  1 +
 drivers/platform/x86/intel/pmt/telemetry.c | 12 ++++++------
 3 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/platform/x86/intel/pmt/class.c b/drivers/platform/x86/intel/pmt/class.c
index 7da8279b54f8..44a0a52014fc 100644
--- a/drivers/platform/x86/intel/pmt/class.c
+++ b/drivers/platform/x86/intel/pmt/class.c
@@ -12,6 +12,7 @@
 #include <linux/log2.h>
 #include <linux/intel_vsec.h>
 #include <linux/io-64-nonatomic-lo-hi.h>
+#include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/mm.h>
 #include <linux/pci.h>
@@ -383,6 +384,17 @@ int intel_pmt_dev_create(struct intel_pmt_entry *entry, struct intel_pmt_namespa
 	if (IS_ERR(entry->disc_table))
 		return PTR_ERR(entry->disc_table);
 
+	/*
+	 * The mapped discovery resource may be smaller than disc_header (its
+	 * size is the namespace's DVSEC entry_size in dwords, which can be
+	 * less than 4). Cap the copy to the actual resource size to avoid
+	 * reading past the mapped region; any unread dwords stay zero from
+	 * the zero-initialized allocation of the containing struct.
+	 */
+	memcpy_fromio(entry->disc_header, entry->disc_table,
+		      min_t(size_t, sizeof(entry->disc_header),
+			    resource_size(disc_res)));
+
 	if (ns->pmt_pre_decode) {
 		ret = ns->pmt_pre_decode(intel_vsec_dev, entry);
 		if (ret)
diff --git a/drivers/platform/x86/intel/pmt/class.h b/drivers/platform/x86/intel/pmt/class.h
index 8a0db0ef58c1..84202fc7920c 100644
--- a/drivers/platform/x86/intel/pmt/class.h
+++ b/drivers/platform/x86/intel/pmt/class.h
@@ -44,6 +44,7 @@ struct intel_pmt_entry {
 	struct telem_endpoint	*ep;
 	struct pci_dev		*pcidev;
 	struct intel_pmt_header	header;
+	u32			disc_header[4];
 	struct bin_attribute	pmt_bin_attr;
 	const struct attribute_group *attr_grp;
 	struct kobject		*kobj;
diff --git a/drivers/platform/x86/intel/pmt/telemetry.c b/drivers/platform/x86/intel/pmt/telemetry.c
index d22f633638be..953f35b6daec 100644
--- a/drivers/platform/x86/intel/pmt/telemetry.c
+++ b/drivers/platform/x86/intel/pmt/telemetry.c
@@ -72,16 +72,16 @@ static bool pmt_telem_region_overlaps(struct device *dev, u32 guid, u32 type)
 static int pmt_telem_header_decode(struct intel_pmt_entry *entry,
 				   struct device *dev)
 {
-	void __iomem *disc_table = entry->disc_table;
 	struct intel_pmt_header *header = &entry->header;
+	u32 *disc_header = entry->disc_header;
 
-	header->access_type = TELEM_ACCESS(readl(disc_table));
-	header->guid = readl(disc_table + TELEM_GUID_OFFSET);
-	header->base_offset = readl(disc_table + TELEM_BASE_OFFSET);
+	header->access_type = TELEM_ACCESS(disc_header[0]);
+	header->guid = disc_header[1];
+	header->base_offset = disc_header[2];
 
 	/* Size is measured in DWORDS, but accessor returns bytes */
-	header->size = TELEM_SIZE(readl(disc_table));
-	header->telem_type = TELEM_TYPE(readl(entry->disc_table));
+	header->size = TELEM_SIZE(disc_header[0]);
+	header->telem_type = TELEM_TYPE(disc_header[0]);
 
 	return 0;
 }
-- 
2.43.0