From 4a9740668803529602a92a1218ba25224dc9b448 Mon Sep 17 00:00:00 2001
From: Michael Stoops <spam@michaelstoops.com>
Date: Tue, 13 Apr 2021 14:36:57 -0700
Subject: [PATCH] Fixing arithmetic underflow
 https://github.com/raspberrypi/pico-sdk/issues/337

---
 src/rp2_common/hardware_spi/spi.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/rp2_common/hardware_spi/spi.c b/src/rp2_common/hardware_spi/spi.c
index 175d1998f..bc598452a 100644
--- a/src/rp2_common/hardware_spi/spi.c
+++ b/src/rp2_common/hardware_spi/spi.c
@@ -76,7 +76,7 @@ int __not_in_flash_func(spi_write_read_blocking)(spi_inst_t *spi, const uint8_t
     size_t rx_remaining = len, tx_remaining = len;
 
     while (rx_remaining || tx_remaining) {
-        if (tx_remaining && spi_is_writable(spi) && rx_remaining - tx_remaining < fifo_depth) {
+        if (tx_remaining && spi_is_writable(spi) && rx_remaining < tx_remaining + fifo_depth) {
             spi_get_hw(spi)->dr = (uint32_t) *src++;
             --tx_remaining;
         }
@@ -125,7 +125,7 @@ int __not_in_flash_func(spi_read_blocking)(spi_inst_t *spi, uint8_t repeated_tx_
     size_t rx_remaining = len, tx_remaining = len;
 
     while (rx_remaining || tx_remaining) {
-        if (tx_remaining && spi_is_writable(spi) && rx_remaining - tx_remaining < fifo_depth) {
+        if (tx_remaining && spi_is_writable(spi) && rx_remaining < tx_remaining + fifo_depth) {
             spi_get_hw(spi)->dr = (uint32_t) repeated_tx_data;
             --tx_remaining;
         }
@@ -147,7 +147,7 @@ int __not_in_flash_func(spi_write16_read16_blocking)(spi_inst_t *spi, const uint
     size_t rx_remaining = len, tx_remaining = len;
 
     while (rx_remaining || tx_remaining) {
-        if (tx_remaining && spi_is_writable(spi) && rx_remaining - tx_remaining < fifo_depth) {
+        if (tx_remaining && spi_is_writable(spi) && rx_remaining < tx_remaining + fifo_depth) {
             spi_get_hw(spi)->dr = (uint32_t) *src++;
             --tx_remaining;
         }
@@ -192,7 +192,7 @@ int __not_in_flash_func(spi_read16_blocking)(spi_inst_t *spi, uint16_t repeated_
     size_t rx_remaining = len, tx_remaining = len;
 
     while (rx_remaining || tx_remaining) {
-        if (tx_remaining && spi_is_writable(spi) && rx_remaining - tx_remaining < fifo_depth) {
+        if (tx_remaining && spi_is_writable(spi) && rx_remaining < tx_remaining + fifo_depth) {
             spi_get_hw(spi)->dr = (uint32_t) repeated_tx_data;
             --tx_remaining;
         }