platform/x86/amd/pmc: Handle overflow cases where the num_samples range is higher

In amd_pmc_stb_debugfs_open_v2(), the stb buffer is created based on the
num_samples and the read/write pointer offset. This holds good when the
num_samples reported by PMFW is less than S2D_TELEMETRY_BYTES_MAX; where
the stb buffer gets filled from 0th position until
S2D_TELEMETRY_BYTES_MAX - 1 based on the read/write pointer offset.

But when the num_samples exceeds the S2D_TELEMETRY_BYTES_MAX, the current
code does not handle it well as it does not account for the cases where
the stb buffer has to filled up as a circular buffer.

Handle this scenario into two cases, where first memcpy will have the
samples from location:
(num_samples % S2D_TELEMETRY_BYTES_MAX) - (S2D_TELEMETRY_BYTES_MAX - 1)
and next memcpy will have the newest ones i.e.
0 - (num_samples % S2D_TELEMETRY_BYTES_MAX - 1)

Suggested-by: Hans de Goede <[email protected]>
Reviewed-by: Hans de Goede <[email protected]>
Signed-off-by: Sanket Goswami <[email protected]>
Signed-off-by: Shyam Sundar S K <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ij: renamed flex_arr -> stb_data_arr]
Reviewed-by: Ilpo Järvinen <[email protected]>
Signed-off-by: Ilpo Järvinen <[email protected]>

Author: Shyam Sundar S K

drivers/platform/x86/amd/pmc/pmc.c

@@ -276,16 +276,23 @@ static int amd_pmc_stb_debugfs_open_v2(struct inode *inode, struct file *filp)
 
 	stb_data_arr->size = fsize;
 
-	/* Start capturing data from the last push location */
+	/*
+	 * Start capturing data from the last push location.
+	 * This is for general cases, where the stb limits
+	 * are meant for standard usage.
+	 */
 	if (num_samples > S2D_TELEMETRY_BYTES_MAX) {
-		fsize  = S2D_TELEMETRY_BYTES_MAX;
-		stb_rdptr_offset = num_samples - fsize;
+		/* First read oldest data starting 1 behind last write till end of ringbuffer */
+		stb_rdptr_offset = num_samples % S2D_TELEMETRY_BYTES_MAX;
+		fsize = S2D_TELEMETRY_BYTES_MAX - stb_rdptr_offset;
+
+		memcpy_fromio(stb_data_arr->data, dev->stb_virt_addr + stb_rdptr_offset, fsize);
+		/* Second copy the newer samples from offset 0 - last write */
+		memcpy_fromio(stb_data_arr->data + fsize, dev->stb_virt_addr, stb_rdptr_offset);
 	} else {
-		fsize = num_samples;
-		stb_rdptr_offset = 0;
+		memcpy_fromio(stb_data_arr->data, dev->stb_virt_addr, fsize);
 	}
 
-	memcpy_fromio(stb_data_arr->data, dev->stb_virt_addr + stb_rdptr_offset, fsize);
 	filp->private_data = stb_data_arr;
 
 	return 0;