drivers: spi_context: Add comments for context

Personally I found this file hard to understand at first,
but since now it is clear to me, I decided to put these comments
with my understanding to help anyone else who needs to use these.

Signed-off-by: Declan Snyder <[email protected]>

Author: Declan Snyder

drivers/spi/spi_context.h

@@ -75,17 +75,30 @@ struct spi_context {
 	},										\
 	._ctx_name.num_cs_gpios = DT_PROP_LEN_OR(_node_id, cs_gpios, 0),
 
+/*
+ * Checks if a spi config is the same as the one stored in the spi_context
+ * The intention of this function is to be used to check if a driver can skip
+ * some reconfiguration for a transfer in a fast code path.
+ */
 static inline bool spi_context_configured(struct spi_context *ctx,
 					  const struct spi_config *config)
 {
 	return !!(ctx->config == config);
 }
 
+/* Returns true if the spi configuration stored for this context
+ * specifies a slave mode configuration, returns false otherwise
+ */
 static inline bool spi_context_is_slave(struct spi_context *ctx)
 {
 	return (ctx->config->operation & SPI_OP_MODE_SLAVE);
 }
 
+/*
+ * The purpose of the context lock is to synchronize the usage of the driver/hardware.
+ * The driver should call this function to claim or wait for ownership of the spi resource.
+ * Usually the appropriate time to call this is at the start of the transceive API implementation.
+ */
 static inline void spi_context_lock(struct spi_context *ctx,
 				    bool asynchronous,
 				    spi_callback_t callback,
@@ -108,6 +121,13 @@ static inline void spi_context_lock(struct spi_context *ctx,
 #endif /* CONFIG_SPI_ASYNC */
 }
 
+/*
+ * This function must be called by a driver which has called spi_context_lock in order
+ * to release the ownership of the spi resource.
+ * Usually the appropriate time to call this would be at the end of a transfer that was
+ * initiated by a transceive API call, except in the case that the SPI_LOCK_ON bit was set
+ * in the configuration.
+ */
 static inline void spi_context_release(struct spi_context *ctx, int status)
 {
 #ifdef CONFIG_SPI_SLAVE
@@ -132,6 +152,13 @@ static inline void spi_context_release(struct spi_context *ctx, int status)
 static inline size_t spi_context_total_tx_len(struct spi_context *ctx);
 static inline size_t spi_context_total_rx_len(struct spi_context *ctx);
 
+/* This function essentially is a way for a driver to seamlessly implement both the
+ * synchronous transceive API and the asynchronous transceive_async API in the same way.
+ *
+ * The exact way this function is used may depend on driver implementation, but
+ * essentially this will block waiting for a signal from spi_context_complete,
+ * unless the transfer is asynchronous, in which case it does nothing in master mode.
+ */
 static inline int spi_context_wait_for_completion(struct spi_context *ctx)
 {
 	int status = 0;
@@ -180,6 +207,12 @@ static inline int spi_context_wait_for_completion(struct spi_context *ctx)
 	return status;
 }
 
+/* For synchronous transfers, this will signal to a thread waiting
+ * on spi_context_wait for completion.
+ *
+ * For asynchronous tranfers, this will call the async callback function
+ * with the user data.
+ */
 static inline void spi_context_complete(struct spi_context *ctx,
 					const struct device *dev,
 					int status)
@@ -212,6 +245,14 @@ static inline void spi_context_complete(struct spi_context *ctx,
 #endif /* CONFIG_SPI_ASYNC */
 }
 
+/*
+ * This function initializes all the chip select GPIOs associated with a spi controller.
+ * The context first must be initialized using the SPI_CONTEXT_CS_GPIOS_INITIALIZE macro.
+ * This function should be called during the device init sequence so that
+ * all the CS lines are configured properly before the first transfer begins.
+ * Note: If a controller has native CS control in SPI hardware, they should also be initialized
+ * during device init by the driver with hardware-specific code.
+ */
 static inline int spi_context_cs_configure_all(struct spi_context *ctx)
 {
 	int ret;
@@ -233,6 +274,7 @@ static inline int spi_context_cs_configure_all(struct spi_context *ctx)
 	return 0;
 }
 
+/* Helper function to control the GPIO CS, not meant to be used directly by drivers */
 static inline void _spi_context_cs_control(struct spi_context *ctx,
 					   bool on, bool force_off)
 {
@@ -252,11 +294,22 @@ static inline void _spi_context_cs_control(struct spi_context *ctx,
 	}
 }
 
+/* This function should be called by drivers to control the chip select line in master mode
+ * in the case of the CS being a GPIO. The de facto usage of the zephyr SPI API expects that the
+ * chip select be asserted throughout the entire transfer specified by a transceive call,
+ * ie all buffers in a spi_buf_set should be finished before deasserting CS. And usually
+ * the deassertion is at the end of the transfer, except in the case that the
+ * SPI_HOLD_ON_CS bit was set in the configuration.
+ */
 static inline void spi_context_cs_control(struct spi_context *ctx, bool on)
 {
 	_spi_context_cs_control(ctx, on, false);
 }
 
+/* Forcefully releases the spi context and removes the owner, allowing taking the lock
+ * with spi_context_lock without the previous owner releasing the lock.
+ * This is usually used to aid in implementation of the spi_release driver API.
+ */
 static inline void spi_context_unlock_unconditionally(struct spi_context *ctx)
 {
 	/* Forcing CS to go to inactive status */
@@ -268,6 +321,11 @@ static inline void spi_context_unlock_unconditionally(struct spi_context *ctx)
 	}
 }
 
+/*
+ * Helper function for incrementing buffer pointer.
+ * Generally not needed to be used directly by drivers.
+ * Use spi_context_update_(tx/rx) instead.
+ */
 static inline void *spi_context_get_next_buf(const struct spi_buf **current,
 					     size_t *count,
 					     size_t *buf_len,
@@ -287,6 +345,17 @@ static inline void *spi_context_get_next_buf(const struct spi_buf **current,
 	return NULL;
 }
 
+/*
+ * The spi context private api works with the driver by providing code to
+ * keep track of how much of the transfer has been completed. The driver
+ * calls functions to report when some tx or rx has finished, and the driver
+ * then can use the spi context to keep track of how much is left to do.
+ */
+
+/*
+ * This function must be called at the start of a transfer by the driver
+ * to initialize the spi context fields for tracking the progress.
+ */
 static inline
 void spi_context_buffers_setup(struct spi_context *ctx,
 			       const struct spi_buf_set *tx_bufs,
@@ -322,8 +391,10 @@ void spi_context_buffers_setup(struct spi_context *ctx,
 }
 
 /*
- * Note: dfs is the number of bytes needed to store a data frame,
- * while len is the number of data frames sent.
+ * Should be called to update the tracking of TX being completed.
+ *
+ * Parameter "dfs" is the number of bytes needed to store a data frame.
+ * Parameter "len" is the number of data frames of TX that were sent.
  */
 static ALWAYS_INLINE
 void spi_context_update_tx(struct spi_context *ctx, uint8_t dfs, uint32_t len)
@@ -353,21 +424,29 @@ void spi_context_update_tx(struct spi_context *ctx, uint8_t dfs, uint32_t len)
 	LOG_DBG("tx buf/len %p/%zu", (void *)ctx->tx_buf, ctx->tx_len);
 }
 
+/* Returns true if there is still TX buffers left in the spi_buf_set
+ * even if they are "null" (nop) buffers.
+ */
 static ALWAYS_INLINE
 bool spi_context_tx_on(struct spi_context *ctx)
 {
 	return !!(ctx->tx_len);
 }
 
+/* Similar to spi_context_tx_on, but only returns true if the current buffer is
+ * not a null/NOP placeholder.
+ */
 static ALWAYS_INLINE
 bool spi_context_tx_buf_on(struct spi_context *ctx)
 {
 	return !!(ctx->tx_buf && ctx->tx_len);
 }
 
 /*
- * Note: dfs is the number of bytes needed to store a data frame,
- * while len is the number of data frames received.
+ * Should be called to update the tracking of RX being completed.
+ *
+ * @param dfs is the number of bytes needed to store a data frame.
+ * @param len is the number of data frames of RX that were received.
  */
 static ALWAYS_INLINE
 void spi_context_update_rx(struct spi_context *ctx, uint8_t dfs, uint32_t len)
@@ -404,12 +483,18 @@ void spi_context_update_rx(struct spi_context *ctx, uint8_t dfs, uint32_t len)
 	LOG_DBG("rx buf/len %p/%zu", (void *)ctx->rx_buf, ctx->rx_len);
 }
 
+/* Returns true if there is still RX buffers left in the spi_buf_set
+ * even if they are "null" (nop) buffers.
+ */
 static ALWAYS_INLINE
 bool spi_context_rx_on(struct spi_context *ctx)
 {
 	return !!(ctx->rx_len);
 }
 
+/* Similar to spi_context_rx_on, but only returns true if the current buffer is
+ * not a null/NOP placeholder.
+ */
 static ALWAYS_INLINE
 bool spi_context_rx_buf_on(struct spi_context *ctx)
 {
@@ -420,6 +505,10 @@ bool spi_context_rx_buf_on(struct spi_context *ctx)
  * Returns the maximum length of a transfer for which all currently active
  * directions have a continuous buffer, i.e. the maximum SPI transfer that
  * can be done with DMA that handles only non-scattered buffers.
+ *
+ * In other words, returns the length of the smaller of the current RX or current TX buffer.
+ * Except if either RX or TX buf length is 0, returns the length of the other.
+ * And if both are 0 then will return 0 and should indicate transfer completion.
  */
 static inline size_t spi_context_max_continuous_chunk(struct spi_context *ctx)
 {
@@ -432,11 +521,13 @@ static inline size_t spi_context_max_continuous_chunk(struct spi_context *ctx)
 	return MIN(ctx->tx_len, ctx->rx_len);
 }
 
+/* Returns the length of the longer of the current RX or current TX buffer. */
 static inline size_t spi_context_longest_current_buf(struct spi_context *ctx)
 {
 	return ctx->tx_len > ctx->rx_len ? ctx->tx_len : ctx->rx_len;
 }
 
+/* Helper function, not intended to be used by drivers directly */
 static size_t spi_context_count_tx_buf_lens(struct spi_context *ctx, size_t start_index)
 {
 	size_t n;
@@ -449,6 +540,7 @@ static size_t spi_context_count_tx_buf_lens(struct spi_context *ctx, size_t star
 	return total_len;
 }
 
+/* Helper function, not intended to be used by drivers directly */
 static size_t spi_context_count_rx_buf_lens(struct spi_context *ctx, size_t start_index)
 {
 	size_t n;
@@ -462,21 +554,33 @@ static size_t spi_context_count_rx_buf_lens(struct spi_context *ctx, size_t star
 }
 
 
+/* Returns the length of the sum of the remaining TX buffers in the buf set, including
+ * the current buffer in the total.
+ */
 static inline size_t spi_context_total_tx_len(struct spi_context *ctx)
 {
 	return spi_context_count_tx_buf_lens(ctx, 0);
 }
 
+/* Returns the length of the sum of the remaining RX buffers in the buf set, including
+ * the current buffer in the total.
+ */
 static inline size_t spi_context_total_rx_len(struct spi_context *ctx)
 {
 	return spi_context_count_rx_buf_lens(ctx, 0);
 }
 
+/* Similar to spi_context_total_tx_len, except does not count words that have been finished
+ * in the current buffer, ie only including what is remaining in the current buffer in the sum.
+ */
 static inline size_t spi_context_tx_len_left(struct spi_context *ctx)
 {
 	return ctx->tx_len + spi_context_count_tx_buf_lens(ctx, 1);
 }
 
+/* Similar to spi_context_total_rx_len, except does not count words that have been finished
+ * in the current buffer, ie only including what is remaining in the current buffer in the sum.
+ */
 static inline size_t spi_context_rx_len_left(struct spi_context *ctx)
 {
 	return ctx->rx_len + spi_context_count_rx_buf_lens(ctx, 1);