Fix CSV time by avoiding accmulating errors

As reported in https://sigrok.org/bugzilla/show_bug.cgi?id=1027 , the
CSV reported time in ns can have an error that increases over time when
the sample frequency does not divide into an integer number of
nanoseconds (i.e. the common 24MHz USB analyzers).

Fix the error by calculating the time as a function of the sample at
each step.

src/output/csv.c

@@ -93,7 +93,8 @@ struct context {
 	uint32_t num_samples;
 	uint32_t channel_count, logic_channel_count;
 	uint32_t channels_seen;
-	uint64_t period;
+	uint64_t sample_rate;
+	uint64_t sample_scale;
 	uint64_t sample_time;
 	uint8_t *previous_sample;
 	float *analog_samples;
@@ -218,31 +219,28 @@ static GString *gen_header(const struct sr_output *o,
 	GString *header;
 	GSList *channels, *l;
 	unsigned int num_channels, i;
-	uint64_t samplerate = 0, sr;
 	char *samplerate_s;
 
 	ctx = o->priv;
 	header = g_string_sized_new(512);
 
-	if (ctx->period == 0) {
+	if (ctx->sample_rate == 0) {
 		if (sr_config_get(o->sdi->driver, o->sdi, NULL,
 				  SR_CONF_SAMPLERATE, &gvar) == SR_OK) {
-			samplerate = g_variant_get_uint64(gvar);
+			ctx->sample_rate = g_variant_get_uint64(gvar);
 			g_variant_unref(gvar);
 		}
 
 		i = 0;
-		sr = 1;
-		while (sr < samplerate) {
+		ctx->sample_scale = 1;
+		while (ctx->sample_scale < ctx->sample_rate) {
 			i++;
-			sr *= 1000;
+			ctx->sample_scale *= 1000;
 		}
-		if (samplerate)
-			ctx->period = sr / samplerate;
 		if (i < ARRAY_SIZE(xlabels))
 			ctx->xlabel = xlabels[i];
-		sr_info("Set sample period to %" PRIu64 " %s",
-			ctx->period, ctx->xlabel);
+		sr_info("Set sample rate, scale to %" PRIu64 ", %" PRIu64 " %s",
+			ctx->sample_rate, ctx->sample_scale, ctx->xlabel);
 	}
 	ctx->title = (o->sdi && o->sdi->driver) ? o->sdi->driver->longname : "unknown";
 
@@ -271,8 +269,8 @@ static GString *gen_header(const struct sr_output *o,
 			g_string_truncate(header, header->len - 1);
 		}
 		g_string_append_printf(header, "\n");
-		if (samplerate != 0) {
-			samplerate_s = sr_samplerate_string(samplerate);
+		if (ctx->sample_rate != 0) {
+			samplerate_s = sr_samplerate_string(ctx->sample_rate);
 			g_string_append_printf(header, "%s Samplerate: %s\n",
 					       ctx->comment, samplerate_s);
 			g_free(samplerate_s);
@@ -442,7 +440,7 @@ static void dump_saved_values(struct context *ctx, GString **out)
 			ctx->previous_sample = g_malloc0(analog_size + ctx->num_logic_channels);
 
 		for (i = 0; i < ctx->num_samples; i++) {
-			ctx->sample_time += ctx->period;
+			ctx->sample_time = (ctx->sample_scale * i) / ctx->sample_rate;
 			analog_sample =
 			    &ctx->analog_samples[i * ctx->num_analog_channels];
 			logic_sample =