baylibre-acme: Optimize reading of values from sysfs.

Opening a file has a cost (security, allocation, syscalls). The
read_sample() function always does an open/read/close sequence.

In order to optimize that, let's open the file at the moment the
acquisition starts, close it when the acquisition stops and make
read_sample() only lseek() to the beginning of the file and read
the value.

Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

src/hardware/baylibre-acme/api.c

@@ -318,6 +318,34 @@ static int config_list(uint32_t key, GVariant **data,
 	return ret;
 }
 
+static void dev_acquisition_close(const struct sr_dev_inst *sdi)
+{
+	GSList *chl;
+	struct sr_channel *ch;
+
+	for (chl = sdi->channels; chl; chl = chl->next) {
+		ch = chl->data;
+		bl_acme_close_channel(ch);
+	}
+}
+
+static int dev_acquisition_open(const struct sr_dev_inst *sdi)
+{
+	GSList *chl;
+	struct sr_channel *ch;
+
+	for (chl = sdi->channels; chl; chl = chl->next) {
+		ch = chl->data;
+		if (bl_acme_open_channel(ch)) {
+			sr_err("Error opening channel %s", ch->name);
+			dev_acquisition_close(sdi);
+			return SR_ERR;
+		}
+	}
+
+	return 0;
+}
+
 static int dev_acquisition_start(const struct sr_dev_inst *sdi, void *cb_data)
 {
 	struct dev_context *devc;
@@ -327,6 +355,9 @@ static int dev_acquisition_start(const struct sr_dev_inst *sdi, void *cb_data)
 	if (sdi->status != SR_ST_ACTIVE)
 		return SR_ERR_DEV_CLOSED;
 
+	if (dev_acquisition_open(sdi))
+		return SR_ERR;
+
 	devc = sdi->priv;
 	devc->samples_read = 0;
 

src/hardware/baylibre-acme/protocol.c

@@ -33,6 +33,7 @@ struct channel_group_priv {
 
 struct channel_priv {
 	int ch_type;
+	int fd;
 	struct channel_group_priv *probe;
 };
 
@@ -494,10 +495,32 @@ static float adjust_data(int val, int type)
 static float read_sample(struct sr_channel *ch)
 {
 	struct channel_priv *chp;
-	char path[64], *file, buf[16];
+	char buf[16];
 	ssize_t len;
 	int fd;
 
+	chp = ch->priv;
+	fd = chp->fd;
+
+	lseek(fd, 0, SEEK_SET);
+
+	len = read(fd, buf, sizeof(buf));
+	if (len < 0) {
+		sr_err("Error reading from channel %s (hwmon: %s): %s",
+			ch->name, chp->probe->hwmon_num, strerror(errno));
+		ch->enabled = FALSE;
+		return -1.0;
+	}
+
+	return adjust_data(strtol(buf, NULL, 10), chp->ch_type);
+}
+
+SR_PRIV int bl_acme_open_channel(struct sr_channel *ch)
+{
+	struct channel_priv *chp;
+	char path[64], *file;
+	int fd;
+
 	chp = ch->priv;
 
 	switch (chp->ch_type) {
@@ -508,27 +531,31 @@ static float read_sample(struct sr_channel *ch)
 	case TEMP_OUT:	file = "temp2_input";	break;
 	default:
 		sr_err("Invalid channel type: %d.", chp->ch_type);
-		return -1.0;
+		return SR_ERR;
 	}
 
 	snprintf(path, sizeof(path), "/sys/class/hwmon/hwmon%d/%s",
 		 chp->probe->hwmon_num, file);
+
 	fd = open(path, O_RDONLY);
 	if (fd < 0) {
 		sr_err("Error opening %s: %s", path, strerror(errno));
 		ch->enabled = FALSE;
-		return -1.0;
+		return SR_ERR;
 	}
 
-	len = read(fd, buf, sizeof(buf));
-	close(fd);
-	if (len < 0) {
-		sr_err("Error reading from %s: %s", path, strerror(errno));
-		ch->enabled = FALSE;
-		return -1.0;
-	}
+	chp->fd = fd;
 
-	return adjust_data(strtol(buf, NULL, 10), chp->ch_type);
+	return 0;
+}
+
+SR_PRIV void bl_acme_close_channel(struct sr_channel *ch)
+{
+	struct channel_priv *chp;
+
+	chp = ch->priv;
+	close(chp->fd);
+	chp->fd = -1;
 }
 
 SR_PRIV int bl_acme_receive_data(int fd, int revents, void *cb_data)

src/hardware/baylibre-acme/protocol.h

@@ -98,4 +98,7 @@ SR_PRIV int bl_acme_set_power_off(const struct sr_channel_group *cg,
 
 SR_PRIV int bl_acme_receive_data(int fd, int revents, void *cb_data);
 
+SR_PRIV int bl_acme_open_channel(struct sr_channel *ch);
+
+SR_PRIV void bl_acme_close_channel(struct sr_channel *ch);
 #endif