Add example which uses the in-built 'DMA sniff' capability to check a CRC32

Author: andygpz11

README.md

@@ -44,6 +44,7 @@ App|Description
 [hello_dma](dma/hello_dma)| Use the DMA to copy data in memory.
 [control_blocks](dma/control_blocks)| Build a control block list, to program a longer sequence of DMA transfers to the UART.
 [channel_irq](dma/channel_irq)| Use an IRQ handler to reconfigure a DMA channel, in order to continuously drive data through a PIO state machine.
+[sniff_crc](dma/sniff_crc)| Use the DMA engine's 'sniff' capability to calculate a CRC32 on a data buffer.
 
 ### Flash
 

dma/CMakeLists.txt

@@ -2,4 +2,5 @@ if (NOT PICO_NO_HARDWARE)
     add_subdirectory(channel_irq)
     add_subdirectory(control_blocks)
     add_subdirectory(hello_dma)
+    add_subdirectory(sniff_crc)
 endif ()

dma/sniff_crc/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_executable(sniff_crc
+        sniff_crc.c
+        )
+
+target_link_libraries(sniff_crc pico_stdlib hardware_dma)
+
+# create map/bin/hex file etc.
+pico_add_extra_outputs(sniff_crc)
+
+# add url via pico_set_program_url
+example_auto_set_url(sniff_crc)

dma/sniff_crc/sniff_crc.c

@@ -0,0 +1,100 @@
+/**
+ * Copyright (c) 2023 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+// Use the DMA engine's 'sniff' capability to calculate a CRC32 on data in a buffer.
+// Note:  This does NOT do an actual data copy, it 'transfers' all the data to a single
+// dummy destination byte so as to be able to crawl over the input data using a 'DMA'.
+// If a data copy *with* a CRC32 sniff is required, the start address of the suitably sized
+// destination buffer must be supplied and the 'write_increment' set to true (see below).
+
+#include <stdio.h>
+#include <string.h>
+#include "pico/stdlib.h"
+#include "hardware/dma.h"
+
+#define CRC32_INIT                  ((uint32_t)-1l)
+
+#define DATA_TO_CHECK_LEN           9
+#define CRC32_LEN                   4
+#define TOTAL_LEN                   (DATA_TO_CHECK_LEN + CRC32_LEN)
+
+// commonly used crc test data and also space for the crc value
+static uint8_t src[TOTAL_LEN] = { 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x00, 0x00, 0x00, 0x00 };
+static uint8_t dummy_dst[1];
+
+// This uses a standard polynomial with the alternate 'reversed' shift direction.
+// It is possible to use a non-reversed algorithm here but the DMA sniff set-up
+// below would need to be modified to remain consistent and allow the check to pass.
+static uint32_t soft_crc32_block(uint32_t crc, uint8_t *bytp, uint32_t length) {
+    while(length--) {
+        uint32_t byte32 = (uint32_t)*bytp++;
+
+        for (uint8_t bit = 8; bit; bit--, byte32 >>= 1) {
+            crc = (crc >> 1) ^ (((crc ^ byte32) & 1ul) ? 0xEDB88320ul : 0ul);
+        }
+    }
+    return crc;
+}
+
+int main() {
+    uint32_t crc_res;
+
+    stdio_init_all();
+
+    // calculate and append the crc
+    crc_res = soft_crc32_block(CRC32_INIT, src, DATA_TO_CHECK_LEN);
+    *((uint32_t *)&src[DATA_TO_CHECK_LEN]) = crc_res;
+
+    printf("Buffer to DMA: ");
+    for (int i = 0; i < TOTAL_LEN; i++) {
+        printf("0x%02x ", src[i]);
+    }
+    printf("\n");
+
+    // UNcomment the next line to deliberately corrupt the buffer
+    //src[0]++;  // modify any byte, in any way, to break the CRC32 check
+
+    // Get a free channel, panic() if there are none
+    int chan = dma_claim_unused_channel(true);
+
+    // 8 bit transfers. The read address increments after each transfer but
+    // the write address remains unchanged pointing to the dummy destination.
+    // No DREQ is selected, so the DMA transfers as fast as it can.
+    dma_channel_config c = dma_channel_get_default_config(chan);
+    channel_config_set_transfer_data_size(&c, DMA_SIZE_8);
+    channel_config_set_read_increment(&c, true);
+    channel_config_set_write_increment(&c, false);
+
+    // (bit-reverse) CRC32 specific sniff set-up
+    channel_config_set_sniff_enable(&c, true);
+    dma_sniffer_set_data_accumulator(CRC32_INIT);
+    dma_sniffer_set_output_reverse_enabled(true);
+    dma_sniffer_enable(chan, DMA_SNIFF_CTRL_CALC_VALUE_CRC32R, true);
+
+    dma_channel_configure(
+        chan,          // Channel to be configured
+        &c,            // The configuration we just created
+        dummy_dst,     // The (unchanging) write address
+        src,           // The initial read address
+        TOTAL_LEN,     // Total number of transfers inc. appended crc; each is 1 byte
+        true           // Start immediately.
+    );
+
+    // We could choose to go and do something else whilst the DMA is doing its
+    // thing. In this case the processor has nothing else to do, so we just
+    // wait for the DMA to finish.
+    dma_channel_wait_for_finish_blocking(chan);
+
+    uint32_t sniffed_crc = dma_sniffer_get_data_accumulator();
+    printf("Completed DMA sniff of %d byte buffer, DMA sniff accumulator value: 0x%lx\n", TOTAL_LEN, sniffed_crc);
+
+    if (0ul == sniffed_crc) {
+        printf("CRC32 check is good\n");
+    }
+    else {
+        printf("ERROR - CRC32 check FAILED!\n");
+    }
+}