platform/x86/amd: pmc: Write dummy postcode into the STB DRAM

Based on the recommendation from the PMFW team in order to get the
recent telemetry data present on the STB DRAM the driver is required
to send one dummy write to the STB buffer, so it internally triggers
the PMFW to emit the latest telemetry data in the STB DRAM region.
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20230206150855.1938810-3-Shyam-sundar.S-k@amd.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/amd/pmc.c

@@ -43,6 +43,7 @@
 #define AMD_PMC_STB_S2IDLE_PREPARE	0xC6000001
 #define AMD_PMC_STB_S2IDLE_RESTORE	0xC6000002
 #define AMD_PMC_STB_S2IDLE_CHECK	0xC6000003
+#define AMD_PMC_STB_DUMMY_PC		0xC6000007
 
 /* STB S2D(Spill to DRAM) has different message port offset */
 #define STB_SPILL_TO_DRAM		0xBE
@@ -250,6 +251,11 @@ static int amd_pmc_stb_debugfs_open_v2(struct inode *inode, struct file *filp)
 	u32 *buf, fsize, num_samples, stb_rdptr_offset = 0;
 	int ret;
 
+	/* Write dummy postcode while reading the STB buffer */
+	ret = amd_pmc_write_stb(dev, AMD_PMC_STB_DUMMY_PC);
+	if (ret)
+		dev_err(dev->dev, "error writing to STB: %d\n", ret);
+
 	buf = kzalloc(S2D_TELEMETRY_BYTES_MAX, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;