Merge branch 'feature/spi_little_endien' into 'master'

feature(spi): provide macro to write multi-byte data straightly

See merge request idf/esp-idf!2634

components/driver/include/driver/spi_common.h

@@ -31,6 +31,34 @@ extern "C"
 //Maximum amount of bytes that can be put in one DMA descriptor
 #define SPI_MAX_DMA_LEN (4096-4)
 
+/**
+ * Transform unsigned integer of length <= 32 bits to the format which can be
+ * sent by the SPI driver directly.
+ *
+ * E.g. to send 9 bits of data, you can:
+ *
+ *      uint16_t data = SPI_SWAP_DATA_TX(0x145, 9);
+ *
+ * Then points tx_buffer to ``&data``.
+ *
+ * @param data Data to be sent, can be uint8_t, uint16_t or uint32_t. @param
+ *  len Length of data to be sent, since the SPI peripheral sends from the MSB,
+ *  this helps to shift the data to the MSB.
+ */
+#define SPI_SWAP_DATA_TX(data, len) __builtin_bswap32((uint32_t)data<<(32-len))
+
+/**
+ * Transform received data of length <= 32 bits to the format of an unsigned integer.
+ *
+ * E.g. to transform the data of 15 bits placed in a 4-byte array to integer:
+ *
+ *      uint16_t data = SPI_SWAP_DATA_RX(*(uint32_t*)t->rx_data, 15);
+ *
+ * @param data Data to be rearranged, can be uint8_t, uint16_t or uint32_t.
+ * @param len Length of data received, since the SPI peripheral writes from
+ *      the MSB, this helps to shift the data to the LSB.
+ */
+#define SPI_SWAP_DATA_RX(data, len) (__builtin_bswap32(data)>>(32-len))
 
 /**
  * @brief Enum with the three SPI peripherals that are software-accessible in it

components/driver/spi_master.c

@@ -727,9 +727,12 @@ static void SPI_MASTER_ISR_ATTR spi_intr(void *arg)
         host->hw->user.usr_addr=addrlen?1:0;
         host->hw->user.usr_command=cmdlen?1:0;
 
-        // output command will be sent from bit 7 to 0 of command_value, and then bit 15 to 8 of the same register field.
-        uint16_t command = trans->cmd << (16-cmdlen);    //shift to MSB
-        host->hw->user2.usr_command_value = (command>>8)|(command<<8);  //swap the first and second byte
+        /* Output command will be sent from bit 7 to 0 of command_value, and
+         * then bit 15 to 8 of the same register field. Shift and swap to send
+         * more straightly.
+         */
+        host->hw->user2.usr_command_value = SPI_SWAP_DATA_TX(trans->cmd, cmdlen);
+
         // shift the address to MSB of addr (and maybe slv_wr_status) register.
         // output address will be sent from MSB to LSB of addr register, then comes the MSB to LSB of slv_wr_status register.
         if (addrlen>32) {

docs/en/api-reference/peripherals/spi_master.rst

@@ -167,6 +167,10 @@ memcpy of temporary buffers.
 .. note::  Half duplex transactions with both read and write phases are not supported when using DMA. See
   :ref:`spi_known_issues` for details and workarounds.
 
+Tips
+""""
+
+1. Transactions with small amount of data:
     Sometimes, the amount of data is very small making it less than optimal allocating a separate buffer
     for it. If the data to be transferred is 32 bits or less, it can be stored in the transaction struct
     itself. For transmitted data, use the ``tx_data`` member for this and set the ``SPI_USE_TXDATA`` flag
@@ -174,6 +178,25 @@ on the transmission. For received data, use ``rx_data`` and set ``SPI_USE_RXDATA
     not touch the ``tx_buffer`` or ``rx_buffer`` members, because they use the same memory locations
     as ``tx_data`` and ``rx_data``.
 
+2. Transactions with integers other than uint8_t
+    The SPI peripheral reads and writes the memory byte-by-byte. By default,
+    the SPI works at MSB first mode, each bytes are sent or received from the
+    MSB to the LSB. However, if you want to send data with length which is
+    not multiples of 8 bits, unused bits are sent.
+
+    E.g. you write ``uint8_t data = 0x15`` (00010101B), and set length to
+    only 5 bits, the sent data is ``00010B`` rather than expected ``10101B``.
+
+    Moreover, ESP32 is a little-endian chip whose lowest byte is stored at
+    the very beginning address for uint16_t and uint32_t variables. Hence if
+    a uint16_t is stored in the memory, it's bit 7 is first sent, then bit 6
+    to 0, then comes its bit 15 to bit 8.
+
+    To send data other than uint8_t arrays, macros ``SPI_SWAP_DATA_TX`` is
+    provided to shift your data to the MSB and swap the MSB to the lowest
+    address; while ``SPI_SWAP_DATA_RX`` can be used to swap received data
+    from the MSB to it's correct place.
+
 Speed and Timing Considerations
 -------------------------------