drivers: spi: spi_bitbang: reduce the ROM usage by making MISO optional

In the same way as mipi_dbi_spi, check if any of the spi_bitbang
instances define MISO and, if not, omit the supporting code. This is
useful on boards with limited memory with write-only devices such as
many SPI LCDs.

This reduces the ROM usage by 60 B. It didn't reduce as much as I
hoped due to how the half duplex support is implemented.

Signed-off-by: Michael Hope <mlhx@google.com>

drivers/spi/spi_bitbang.c

@@ -14,17 +14,25 @@ LOG_MODULE_REGISTER(spi_bitbang);
 #include <zephyr/drivers/spi.h>
 #include "spi_context.h"
 
+/* Expands to 1 if the node has no MISO GPIOs */
+#define _MISO_PRESENT(n)          (DT_INST_PROP_LEN_OR(n, miso_gpios, 0) != 0) |
+#define SPI_BITBANG_MISO_REQUIRED DT_INST_FOREACH_STATUS_OKAY(_MISO_PRESENT) 0
+
 struct spi_bitbang_data {
 	struct spi_context ctx;
-	int bits;
-	int wait_us;
-	int dfs;
+	int16_t wait_us;
+	/* Number of bits per word. Maximum is 16. */
+	int8_t bits;
+	/* Number of bytes per word. Maximum is 2. */
+	int8_t dfs;
 };
 
 struct spi_bitbang_config {
 	struct gpio_dt_spec clk_gpio;
 	struct gpio_dt_spec mosi_gpio;
+#if SPI_BITBANG_MISO_REQUIRED
 	struct gpio_dt_spec miso_gpio;
+#endif
 };
 
 static int spi_bitbang_configure(const struct spi_bitbang_config *info,
@@ -105,10 +113,11 @@ static int spi_bitbang_transceive(const struct device *dev,
 		if (info->mosi_gpio.port) {
 			mosi = &info->mosi_gpio;
 		}
-
+#if SPI_BITBANG_MISO_REQUIRED
 		if (info->miso_gpio.port) {
 			miso = &info->miso_gpio;
 		}
+#endif
 	}
 
 	if (info->mosi_gpio.port) {
@@ -284,6 +293,7 @@ int spi_bitbang_init(const struct device *dev)
 		}
 	}
 
+#if SPI_BITBANG_MISO_REQUIRED
 	if (config->miso_gpio.port != NULL) {
 		if (!gpio_is_ready_dt(&config->miso_gpio)) {
 			LOG_ERR("GPIO port for miso pin is not ready");
@@ -297,6 +307,7 @@ int spi_bitbang_init(const struct device *dev)
 			return rc;
 		}
 	}
+#endif
 
 	rc = spi_context_cs_configure_all(&data->ctx);
 	if (rc < 0) {
@@ -307,26 +318,25 @@ int spi_bitbang_init(const struct device *dev)
 	return 0;
 }
 
-#define SPI_BITBANG_INIT(inst)						\
-	static struct spi_bitbang_config spi_bitbang_config_##inst = {	\
-		.clk_gpio = GPIO_DT_SPEC_INST_GET(inst, clk_gpios),	\
-		.mosi_gpio = GPIO_DT_SPEC_INST_GET_OR(inst, mosi_gpios, {0}),	\
-		.miso_gpio = GPIO_DT_SPEC_INST_GET_OR(inst, miso_gpios, {0}),	\
-	};								\
-									\
-	static struct spi_bitbang_data spi_bitbang_data_##inst = {	\
-		SPI_CONTEXT_INIT_LOCK(spi_bitbang_data_##inst, ctx),	\
-		SPI_CONTEXT_INIT_SYNC(spi_bitbang_data_##inst, ctx),	\
-		SPI_CONTEXT_CS_GPIOS_INITIALIZE(DT_DRV_INST(inst), ctx)	\
-	};								\
-									\
-	DEVICE_DT_INST_DEFINE(inst,					\
-			    spi_bitbang_init,				\
-			    NULL,					\
-			    &spi_bitbang_data_##inst,			\
-			    &spi_bitbang_config_##inst,			\
-			    POST_KERNEL,				\
-			    CONFIG_SPI_INIT_PRIORITY,			\
-			    &spi_bitbang_api);
+#if SPI_BITBANG_MISO_REQUIRED
+#define SPI_BITBANG_INIT_MISO(inst) .miso_gpio = GPIO_DT_SPEC_INST_GET_OR(inst, miso_gpios, {0}),
+#else
+#define SPI_BITBANG_INIT_MISO(inst)
+#endif
+
+#define SPI_BITBANG_INIT(inst)                                                                     \
+	static const struct spi_bitbang_config spi_bitbang_config_##inst = {                       \
+		.clk_gpio = GPIO_DT_SPEC_INST_GET(inst, clk_gpios),                                \
+		.mosi_gpio = GPIO_DT_SPEC_INST_GET_OR(inst, mosi_gpios, {0}),                      \
+		SPI_BITBANG_INIT_MISO(inst)};                                                      \
+                                                                                                   \
+	static struct spi_bitbang_data spi_bitbang_data_##inst = {                                 \
+		SPI_CONTEXT_INIT_LOCK(spi_bitbang_data_##inst, ctx),                               \
+		SPI_CONTEXT_INIT_SYNC(spi_bitbang_data_##inst, ctx),                               \
+		SPI_CONTEXT_CS_GPIOS_INITIALIZE(DT_DRV_INST(inst), ctx)};                          \
+                                                                                                   \
+	DEVICE_DT_INST_DEFINE(inst, spi_bitbang_init, NULL, &spi_bitbang_data_##inst,              \
+			      &spi_bitbang_config_##inst, POST_KERNEL, CONFIG_SPI_INIT_PRIORITY,   \
+			      &spi_bitbang_api);
 
 DT_INST_FOREACH_STATUS_OKAY(SPI_BITBANG_INIT)