From 4a722a134d8dddc1253f8e04f88af842c2be6b8d Mon Sep 17 00:00:00 2001
From: David Lechner <david@pybricks.com>
Date: Mon, 27 May 2019 17:45:10 -0500
Subject: [PATCH] pbio/util: Introduce PBIO_CONTAINER_OF macro

This macro lets us get the container of a nested structure. Make use of it in the debug UART driver so we don't have to be so careful about the offset of the handle field.
---
 lib/pbio/drv/debug/uart.c    | 11 ++++++-----
 lib/pbio/include/pbio/util.h |  8 ++++++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/lib/pbio/drv/debug/uart.c b/lib/pbio/drv/debug/uart.c
index 939dbc7d7..7a6c1943a 100644
--- a/lib/pbio/drv/debug/uart.c
+++ b/lib/pbio/drv/debug/uart.c
@@ -10,6 +10,7 @@
 #include "pbio/event.h"
 #include "pbio/port.h"
 #include "pbio/uartdev.h"
+#include "pbio/util.h"
 #include "pbsys/sys.h"
 #include "sys/process.h"
 
@@ -20,7 +21,7 @@
 #define RX_BUF_SIZE 64  // must be power of 2!
 
 typedef struct {
-    UART_HandleTypeDef handle;      // must be first in struct!
+    UART_HandleTypeDef handle;
     const uint8_t irq;
     uint8_t tx_buf[TX_BUF_SIZE];
     uint8_t rx_buf[RX_BUF_SIZE];
@@ -50,7 +51,7 @@ PROCESS(pbdrv_uart_process, "UART");
 
 // overrides weak function in stm32f4xx_hal_uart.c
 void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
-    pbdrv_uart_t *uart = (void *)huart;
+    pbdrv_uart_t *uart = PBIO_CONTAINER_OF(huart, pbdrv_uart_t, handle);
     uint8_t new_head;
 
     new_head = (uart->rx_buf_head + 1) & (RX_BUF_SIZE - 1);
@@ -100,7 +101,7 @@ pbio_error_t pbdrv_uart_get_char(pbio_port_t port, uint8_t *c) {
 
 // overrides weak function in stm32f4xx_hal_uart.c
 void HAL_UART_TxCpltCallback(UART_HandleTypeDef *huart) {
-    pbdrv_uart_t *uart = (void *)huart;
+    pbdrv_uart_t *uart = PBIO_CONTAINER_OF(huart, pbdrv_uart_t, handle);
 
     if (uart->tx_buf_tail != uart->tx_buf_head) {
         // there is still more data to send
@@ -160,7 +161,7 @@ pbio_error_t pbdrv_uart_set_baud_rate(pbio_port_t port, uint32_t baud) {
 
 // overrides weak function in stm32f4xx_hal_uart.c
 void HAL_UART_MspInit(UART_HandleTypeDef *huart) {
-    pbdrv_uart_t *uart = (void *)huart;
+    pbdrv_uart_t *uart = PBIO_CONTAINER_OF(huart, pbdrv_uart_t, handle);
 
     // clocks are enabled in sys.c
     // pin mux is handled in ioport.c
@@ -171,7 +172,7 @@ void HAL_UART_MspInit(UART_HandleTypeDef *huart) {
 
 // overrides weak function in stm32f4xx_hal_uart.c
 void HAL_UART_MspDeInit(UART_HandleTypeDef *huart) {
-    pbdrv_uart_t *uart = (void *)huart;
+    pbdrv_uart_t *uart = PBIO_CONTAINER_OF(huart, pbdrv_uart_t, handle);
 
     HAL_NVIC_DisableIRQ(uart->irq);
 }
diff --git a/lib/pbio/include/pbio/util.h b/lib/pbio/include/pbio/util.h
index 5a9ebc6f1..48ba8c3d6 100644
--- a/lib/pbio/include/pbio/util.h
+++ b/lib/pbio/include/pbio/util.h
@@ -10,4 +10,12 @@
  */
 #define PBIO_ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))
 
+/**
+ * Get a pointer to the structure that contains another structure.
+ * @param ptr       Pointer to the child structure
+ * @param type      The type of the parent structure
+ * @param member    The name of the field containing ptr
+ */
+#define PBIO_CONTAINER_OF(ptr, type, member) ((type *)((char *)(ptr) - offsetof(type, member)))
+
 #endif // _PBIO_UTIL_H_