From 3a4be428b5b06f9882de07985431ad1eaeb1951a Mon Sep 17 00:00:00 2001
From: Thomas Kolb <cfr34k-git@tkolb.de>
Date: Fri, 5 Jan 2024 14:13:11 +0100
Subject: [PATCH] sdr: stop reading packets if the TX buffer is too full

---
 impl/src/layer1/rx.c |  1 -
 impl/src/main.c      | 12 ++++++++++++
 impl/src/sdr/sdr.c   | 20 ++++++++++++++++++++
 impl/src/sdr/sdr.h   |  1 +
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/impl/src/layer1/rx.c b/impl/src/layer1/rx.c
index b8c01a9..293546b 100644
--- a/impl/src/layer1/rx.c
+++ b/impl/src/layer1/rx.c
@@ -1,5 +1,4 @@
 #include <liquid/liquid.h>
-#include <math.h>
 
 #include <assert.h>
 
diff --git a/impl/src/main.c b/impl/src/main.c
index 18a0445..5fc5c33 100644
--- a/impl/src/main.c
+++ b/impl/src/main.c
@@ -262,6 +262,18 @@ int main(void)
 				break;
 			} else if(ret > 0) {
 				// there is a packet to be read.
+
+				// check free buffer space (50 ms required corresponding to 5000 baseband symbols)
+				size_t buffer_free_space_samples = sdr_get_tx_buffer_free_space(&sdr);
+
+				fprintf(stderr, "TX buffer free: %zu\n", buffer_free_space_samples);
+
+				if(buffer_free_space_samples < 100000) { // sample count for 50 ms at 2 MHz
+					// try again after a short delay
+					fsleep(10e-3);
+					continue;
+				}
+
 				uint8_t packetbuf[2048];
 				ret = read(m_tunfd, packetbuf, sizeof(packetbuf));
 				if(ret < 0) {
diff --git a/impl/src/sdr/sdr.c b/impl/src/sdr/sdr.c
index e6d497b..99a5823 100644
--- a/impl/src/sdr/sdr.c
+++ b/impl/src/sdr/sdr.c
@@ -366,6 +366,26 @@ result_t sdr_flush_tx_buffer(sdr_ctx_t *ctx)
 }
 
 
+size_t sdr_get_tx_buffer_free_space(sdr_ctx_t *ctx)
+{
+	size_t free_samples = 0;
+
+	if(sem_wait(&ctx->buf_sem) < 0) {
+		perror("sem_wait");
+		return 0;
+	}
+
+	free_samples = cbuffercf_space_available(ctx->tx_buf);
+
+	if(sem_post(&ctx->buf_sem) < 0) {
+		perror("sem_post");
+		return 0;
+	}
+
+	return free_samples;
+}
+
+
 result_t sdr_rf_to_baseband(sdr_ctx_t *ctx,
 		const float complex *rf_samples, size_t nrf,
 		float complex *bb_samples, size_t *nbb)
diff --git a/impl/src/sdr/sdr.h b/impl/src/sdr/sdr.h
index b6debc2..ae9ba80 100644
--- a/impl/src/sdr/sdr.h
+++ b/impl/src/sdr/sdr.h
@@ -49,6 +49,7 @@ result_t sdr_transmit(sdr_ctx_t *ctx, const float complex *samples, size_t nsamp
 result_t sdr_receive(sdr_ctx_t *ctx, float complex *samples, size_t *nsamples, long timeout_us);
 
 result_t sdr_flush_tx_buffer(sdr_ctx_t *ctx);
+size_t   sdr_get_tx_buffer_free_space(sdr_ctx_t *ctx);
 
 /*!
  * \brief Convert and resample a received signal to baseband.