ice: do not busy-wait to read GNSS data

The ice-gnss-<dev_name> kernel thread, which reads data from the u-blox
GNSS module, keep a CPU core almost 100% busy. The main reason is that
it busy-waits for data to become available.

A simple improvement would be to replace the "mdelay(10);" in
ice_gnss_read() with sleeping. A better fix is to not do any waiting
directly in the function and just requeue this delayed work as needed.
The advantage is that canceling the work from ice_gnss_exit() becomes
immediate, rather than taking up to ~2.5 seconds (ICE_MAX_UBX_READ_TRIES
* 10 ms).

This lowers the CPU usage of the ice-gnss-<dev_name> thread on my system
from ~90 % to ~8 %.

I am not sure if the larger 0.1 s pause after inserting data into the
gnss subsystem is really necessary, but I'm keeping that as it was.

Of course, ideally the driver would not have to poll at all, but I don't
know if the E810 can watch for GNSS data availability over the i2c bus
by itself and notify the driver.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: michich

drivers/net/ethernet/intel/ice/ice_gnss.c

@@ -117,6 +117,7 @@ static void ice_gnss_read(struct kthread_work *work)
 {
 	struct gnss_serial *gnss = container_of(work, struct gnss_serial,
 						read_work.work);
+	unsigned long delay = ICE_GNSS_POLL_DATA_DELAY_TIME;
 	unsigned int i, bytes_read, data_len, count;
 	struct ice_aqc_link_topo_addr link_topo;
 	struct ice_pf *pf;
@@ -136,11 +137,6 @@ static void ice_gnss_read(struct kthread_work *work)
 		return;
 
 	hw = &pf->hw;
-	buf = (char *)get_zeroed_page(GFP_KERNEL);
-	if (!buf) {
-		err = -ENOMEM;
-		goto exit;
-	}
 
 	memset(&link_topo, 0, sizeof(struct ice_aqc_link_topo_addr));
 	link_topo.topo_params.index = ICE_E810T_GNSS_I2C_BUS;
@@ -151,25 +147,24 @@ static void ice_gnss_read(struct kthread_work *work)
 	i2c_params = ICE_GNSS_UBX_DATA_LEN_WIDTH |
 		     ICE_AQC_I2C_USE_REPEATED_START;
 
-	/* Read data length in a loop, when it's not 0 the data is ready */
-	for (i = 0; i < ICE_MAX_UBX_READ_TRIES; i++) {
-		err = ice_aq_read_i2c(hw, link_topo, ICE_GNSS_UBX_I2C_BUS_ADDR,
-				      cpu_to_le16(ICE_GNSS_UBX_DATA_LEN_H),
-				      i2c_params, (u8 *)&data_len_b, NULL);
-		if (err)
-			goto exit_buf;
+	err = ice_aq_read_i2c(hw, link_topo, ICE_GNSS_UBX_I2C_BUS_ADDR,
+			      cpu_to_le16(ICE_GNSS_UBX_DATA_LEN_H),
+			      i2c_params, (u8 *)&data_len_b, NULL);
+	if (err)
+		goto requeue;
 
-		data_len = be16_to_cpu(data_len_b);
-		if (data_len != 0 && data_len != U16_MAX)
-			break;
+	data_len = be16_to_cpu(data_len_b);
+	if (data_len == 0 || data_len == U16_MAX)
+		goto requeue;
 
-		mdelay(10);
-	}
+	/* The u-blox has data_len bytes for us to read */
 
 	data_len = min_t(typeof(data_len), data_len, PAGE_SIZE);
-	if (!data_len) {
+
+	buf = (char *)get_zeroed_page(GFP_KERNEL);
+	if (!buf) {
 		err = -ENOMEM;
-		goto exit_buf;
+		goto requeue;
 	}
 
 	/* Read received data */
@@ -183,18 +178,19 @@ static void ice_gnss_read(struct kthread_work *work)
 				      cpu_to_le16(ICE_GNSS_UBX_EMPTY_DATA),
 				      bytes_read, &buf[i], NULL);
 		if (err)
-			goto exit_buf;
+			goto free_buf;
 	}
 
 	count = gnss_insert_raw(pf->gnss_dev, buf, i);
 	if (count != i)
 		dev_warn(ice_pf_to_dev(pf),
 			 "gnss_insert_raw ret=%d size=%d\n",
 			 count, i);
-exit_buf:
+	delay = ICE_GNSS_TIMER_DELAY_TIME;
+free_buf:
 	free_page((unsigned long)buf);
-	kthread_queue_delayed_work(gnss->kworker, &gnss->read_work,
-				   ICE_GNSS_TIMER_DELAY_TIME);
+requeue:
+	kthread_queue_delayed_work(gnss->kworker, &gnss->read_work, delay);
 exit:
 	if (err)
 		dev_dbg(ice_pf_to_dev(pf), "GNSS failed to read err=%d\n", err);

drivers/net/ethernet/intel/ice/ice_gnss.h

@@ -5,6 +5,7 @@
 #define _ICE_GNSS_H_
 
 #define ICE_E810T_GNSS_I2C_BUS		0x2
+#define ICE_GNSS_POLL_DATA_DELAY_TIME	(HZ / 100) /* poll every 10 ms */
 #define ICE_GNSS_TIMER_DELAY_TIME	(HZ / 10) /* 0.1 second per message */
 #define ICE_GNSS_TTY_WRITE_BUF		250
 #define ICE_MAX_I2C_DATA_SIZE		FIELD_MAX(ICE_AQC_I2C_DATA_SIZE_M)
@@ -20,8 +21,6 @@
  * passed as I2C addr parameter.
  */
 #define ICE_GNSS_UBX_WRITE_BYTES	(ICE_MAX_I2C_WRITE_BYTES + 1)
-#define ICE_MAX_UBX_READ_TRIES		255
-#define ICE_MAX_UBX_ACK_READ_TRIES	4095
 
 struct gnss_write_buf {
 	struct list_head queue;