ACPI / CPPC: Fix invalid PCC channel status errors

Replace the faulty PCC status register polling code with a iopoll.h
macro to fix incorrect reporting of PCC check errors ("PCC check
channel failed").

There were potential codepaths where we could incorrectly return
PCC channel status as busy even without checking the PCC status
register once or not checking the status register before breaking
out of the polling loop. For example, if the thread polling PCC
status register was preempted and scheduled back after we have
crossed the deadline then we can report that the channel is busy
even without checking the status register.

Signed-off-by: Prashanth Prakash <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Prakash, Prashanth

drivers/acpi/cppc_acpi.c

@@ -39,6 +39,7 @@
 
 #include <linux/cpufreq.h>
 #include <linux/delay.h>
+#include <linux/iopoll.h>
 #include <linux/ktime.h>
 #include <linux/rwsem.h>
 #include <linux/wait.h>
@@ -49,7 +50,7 @@ struct cppc_pcc_data {
 	struct mbox_chan *pcc_channel;
 	void __iomem *pcc_comm_addr;
 	bool pcc_channel_acquired;
-	ktime_t deadline;
+	unsigned int deadline_us;
 	unsigned int pcc_mpar, pcc_mrtt, pcc_nominal;
 
 	bool pending_pcc_write_cmd;	/* Any pending/batched PCC write cmds? */
@@ -198,42 +199,31 @@ static struct kobj_type cppc_ktype = {
 
 static int check_pcc_chan(int pcc_ss_id, bool chk_err_bit)
 {
-	int ret = -EIO, status = 0;
+	int ret, status;
 	struct cppc_pcc_data *pcc_ss_data = pcc_data[pcc_ss_id];
 	struct acpi_pcct_shared_memory __iomem *generic_comm_base =
 		pcc_ss_data->pcc_comm_addr;
-	ktime_t next_deadline = ktime_add(ktime_get(),
-					  pcc_ss_data->deadline);
 
 	if (!pcc_ss_data->platform_owns_pcc)
 		return 0;
 
-	/* Retry in case the remote processor was too slow to catch up. */
-	while (!ktime_after(ktime_get(), next_deadline)) {
-		/*
-		 * Per spec, prior to boot the PCC space wil be initialized by
-		 * platform and should have set the command completion bit when
-		 * PCC can be used by OSPM
-		 */
-		status = readw_relaxed(&generic_comm_base->status);
-		if (status & PCC_CMD_COMPLETE_MASK) {
-			ret = 0;
-			if (chk_err_bit && (status & PCC_ERROR_MASK))
-				ret = -EIO;
-			break;
-		}
-		/*
-		 * Reducing the bus traffic in case this loop takes longer than
-		 * a few retries.
-		 */
-		udelay(3);
-	}
+	/*
+	 * Poll PCC status register every 3us(delay_us) for maximum of
+	 * deadline_us(timeout_us) until PCC command complete bit is set(cond)
+	 */
+	ret = readw_relaxed_poll_timeout(&generic_comm_base->status, status,
+					status & PCC_CMD_COMPLETE_MASK, 3,
+					pcc_ss_data->deadline_us);
 
-	if (likely(!ret))
+	if (likely(!ret)) {
 		pcc_ss_data->platform_owns_pcc = false;
-	else
-		pr_err("PCC check channel failed for ss: %d. Status=%x\n",
-		       pcc_ss_id, status);
+		if (chk_err_bit && (status & PCC_ERROR_MASK))
+			ret = -EIO;
+	}
+
+	if (unlikely(ret))
+		pr_err("PCC check channel failed for ss: %d. ret=%d\n",
+		       pcc_ss_id, ret);
 
 	return ret;
 }
@@ -585,7 +575,7 @@ static int register_pcc_channel(int pcc_ss_idx)
 		 * So add an arbitrary amount of wait on top of Nominal.
 		 */
 		usecs_lat = NUM_RETRIES * cppc_ss->latency;
-		pcc_data[pcc_ss_idx]->deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);
+		pcc_data[pcc_ss_idx]->deadline_us = usecs_lat;
 		pcc_data[pcc_ss_idx]->pcc_mrtt = cppc_ss->min_turnaround_time;
 		pcc_data[pcc_ss_idx]->pcc_mpar = cppc_ss->max_access_rate;
 		pcc_data[pcc_ss_idx]->pcc_nominal = cppc_ss->latency;