drivers: stm32: SPI: SPI nocache buffers can be in CONFIG_NOCACHE_MEMORY

CONFIG_NOCACHE_MEMORY is a valid way of declaring buffers in
nocache regions. Consider them valid in the stm32 SPI driver
nocache check. Also, don't check NULL buffers as the SPI
interface states that such buffers will result in sending
zeroes.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>

drivers/spi/spi_ll_stm32.c

@@ -26,6 +26,10 @@ LOG_MODULE_REGISTER(spi_ll_stm32);
 #include <zephyr/drivers/clock_control.h>
 #include <zephyr/irq.h>
 
+#ifdef CONFIG_NOCACHE_MEMORY
+#include <zephyr/linker/linker-defs.h>
+#endif /* CONFIG_NOCACHE_MEMORY */
+
 #include "spi_ll_stm32.h"
 
 #define WAIT_1US	1U
@@ -756,24 +760,40 @@ static int wait_dma_rx_tx_done(const struct device *dev)
 #ifdef CONFIG_SOC_SERIES_STM32H7X
 static bool buf_in_nocache(uintptr_t buf, size_t len_bytes)
 {
+	bool buf_within_nocache = false;
+
+#ifdef CONFIG_NOCACHE_MEMORY
+	buf_within_nocache = (buf >= ((uintptr_t)_nocache_ram_start)) &&
+		((buf + len_bytes - 1) <= ((uintptr_t)_nocache_ram_end));
+	if (buf_within_nocache) {
+		return true;
+	}
+#endif /* CONFIG_NOCACHE_MEMORY */
+
 	for (size_t i = 0; i < ARRAY_SIZE(nocache_mem_regions); i++) {
 		const struct mem_region *mem_reg = &nocache_mem_regions[i];
 
-		const bool buf_within_bounds =
+		buf_within_nocache =
 			(buf >= mem_reg->start) && ((buf + len_bytes - 1) <= mem_reg->end);
-		if (buf_within_bounds) {
+		if (buf_within_nocache) {
 			return true;
 		}
 	}
 	return false;
 }
 
+static bool is_dummy_buffer(const struct spi_buf *buf)
+{
+	return buf->buf == NULL;
+}
+
 static bool spi_buf_set_in_nocache(const struct spi_buf_set *bufs)
 {
 	for (size_t i = 0; i < bufs->count; i++) {
 		const struct spi_buf *buf = &bufs->buffers[i];
 
-		if (!buf_in_nocache((uintptr_t)buf->buf, buf->len)) {
+		if (!is_dummy_buffer(buf) &&
+				!buf_in_nocache((uintptr_t)buf->buf, buf->len)) {
 			return false;
 		}
 	}

tests/drivers/spi/spi_loopback/boards/nucleo_h753zi.conf

@@ -0,0 +1,6 @@
+#
+# Copyright (c) 2023 Graphcore Ltd, All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+CONFIG_NOCACHE_MEMORY=y

tests/drivers/spi/spi_loopback/boards/nucleo_h753zi.overlay

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2023 Graphcore Ltd, All rights reserved.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&spi1 {
+	dmas = <&dmamux1 0 38 (STM32_DMA_PERIPH_TX | STM32_DMA_PRIORITY_HIGH)
+		&dmamux1 1 37 (STM32_DMA_PERIPH_RX | STM32_DMA_PRIORITY_HIGH)>;
+	dma-names = "tx", "rx";
+	slow@0 {
+		compatible = "test-spi-loopback-slow";
+		reg = <0>;
+		spi-max-frequency = <500000>;
+	};
+	fast@0 {
+		compatible = "test-spi-loopback-fast";
+		reg = <0>;
+		spi-max-frequency = <16000000>;
+	};
+};
+
+&dma1 {
+	status = "okay";
+};
+
+&dma2 {
+	status = "okay";
+};
+
+&dmamux1 {
+	status = "okay";
+};