Allow unaligned input/output to SPI::transferBytes

Fixes esp8266#4967

Support any alignment of input and output pointers and transfer lengths
in SPI::transferBytes.  Use 32-bit transfers and FIFO as much as
possible.

Author: earlephilhower

libraries/SPI/SPI.cpp

@@ -509,9 +509,6 @@ void SPIClass::writePattern(const uint8_t * data, uint8_t size, uint32_t repeat)
 }
 
 /**
- * Note:
- *  in and out need to be aligned to 32Bit
- *  or you get an Fatal exception (9)
  * @param out uint8_t *
  * @param in  uint8_t *
  * @param size uint32_t
@@ -531,48 +528,90 @@ void SPIClass::transferBytes(const uint8_t * out, uint8_t * in, uint32_t size) {
 }
 
 /**
- * Note:
- *  in and out need to be aligned to 32Bit
- *  or you get an Fatal exception (9)
  * @param out uint8_t *
  * @param in  uint8_t *
  * @param size uint8_t (max 64)
  */
 void SPIClass::transferBytes_(const uint8_t * out, uint8_t * in, uint8_t size) {
-    while(SPI1CMD & SPIBUSY) {}
-    // Set in/out Bits to transfer
+    while(SPI1CMD & SPIBUSY) {} // Make sure we're IDLE on the SPI
 
-    setDataBits(size * 8);
+    int bytesToTransfer = size; // How much to read/write from the FIFO in bytes
 
-    volatile uint32_t * fifoPtr = &SPI1W0;
-    uint8_t dataSize = ((size + 3) / 4);
+    if (out && !in) {
+        int bytesLeft = size;
 
-    if(out) {
-        uint32_t * dataPtr = (uint32_t*) out;
-        while(dataSize--) {
-            *fifoPtr = *dataPtr;
-            dataPtr++;
-            fifoPtr++;
+        // Only transmitting, get out 32b aligned
+        while (bytesLeft && ((uint32_t)out & 3)) {
+            SPI.transfer(*(out++));
+            bytesLeft--;
         }
-    } else {
-        // no out data only read fill with dummy data!
-        while(dataSize--) {
-            *fifoPtr = 0xFFFFFFFF;
-            fifoPtr++;
+
+        if (!bytesLeft) return;
+        bytesToTransfer = bytesLeft;
+
+        // Now do 32b writes until we have 0 or fewer bytes left
+        // Note we can read and store past the end of string because it will not be sent (setDataBits)
+        uint32_t *dataPtr = (uint32_t*)out;
+        volatile uint32_t *fifoPtr = &SPI1W0;
+        while (bytesLeft > 0) {
+            *(fifoPtr++) = *(dataPtr++);
+            bytesLeft -= 4;
         }
+        // Remainder will be sent out of loaded FIFO below
+    } else if (in && !out) {
+        int bytesLeft = size;
+
+        // Only receiving, get in 32b aligned
+        while (bytesLeft && ((uint32_t)in & 3)) {
+            *(in++) = SPI.transfer(0xff);
+            bytesLeft--;
+        }
+
+        if (!bytesLeft) return;
+        bytesToTransfer = bytesLeft;
+
+        // Now send 32b writes of all 0xff, actual read will be done below
+        // Note we can write add'l 0xffs and they won't be sent thanks to setDataBits below
+        volatile uint32_t *fifoPtr = &SPI1W0;
+        while (bytesLeft > 0) {
+            *(fifoPtr++) = 0xffffffff;
+            bytesLeft -= 4;
+        }
+    } else if (in && out && (((uint32_t)in & 3) || ((uint32_t)out & 3))) {
+        // Bidirectional and we have one or both pointers misaligned
+        while (size) {
+            *(in++) = SPI.transfer(*(out++));
+            size--;
+        }
+        // All the data was transferred byte-wise, we're done
+        return;
     }
 
+    // At this point we've got aligned (or null) in and out and the FIFO is filled
+    // with whatever data we're going to transmit as aligned
+
+    // Set in/out bits to transfer
+    while (SPI1CMD & SPIBUSY) {} // Paranoia, make sure SPI is idle
+    setDataBits(bytesToTransfer * 8);
+
+    // Start the transfer and wait for done
     SPI1CMD |= SPIBUSY;
-    while(SPI1CMD & SPIBUSY) {}
+    while (SPI1CMD & SPIBUSY) { /* noop, waiting */ }
+
+    if (in) {
+        // Bulk read out by 4 until we have 0..3 left
+        uint32_t *dataPtr = (uint32_t*)in;
+        volatile uint32_t *fifoPtr = &SPI1W0;
+        while (bytesToTransfer >= 4) {
+            *(dataPtr++) = *(fifoPtr++);
+            bytesToTransfer -= 4;
+            in += 4; // Keep track of the in ptr for any stragglers
+        }
 
-    if(in) {
-        uint32_t * dataPtr = (uint32_t*) in;
-        fifoPtr = &SPI1W0;
-        dataSize = ((size + 3) / 4);
-        while(dataSize--) {
-            *dataPtr = *fifoPtr;
-            dataPtr++;
-            fifoPtr++;
+        // Bytewise read out the remainder
+        volatile uint8_t *fifoPtrB = (uint8_t*)fifoPtr;
+        while (bytesToTransfer--) {
+            *(in++) = *(fifoPtrB++);
         }
     }
 }