From ad6734eb6d1ce489ad2a7d9edb84125b226434ac Mon Sep 17 00:00:00 2001
From: Jacob Pease <jacobpease@protonmail.com>
Date: Sat, 24 Aug 2024 21:36:29 -0500
Subject: [PATCH] Improved the speed of the bootloader by 60s. CRC16 is now
 calculated with a table and a byte is now sent for every byte read, keeping
 the FIFO full.

---
 fpga/zsbl/boot.c | 87 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 74 insertions(+), 13 deletions(-)

diff --git a/fpga/zsbl/boot.c b/fpga/zsbl/boot.c
index e98eda2c1..4af852361 100644
--- a/fpga/zsbl/boot.c
+++ b/fpga/zsbl/boot.c
@@ -52,6 +52,42 @@ when 8 bytes are transferred
 
  */
 
+// crc16 table to reduce byte processing time
+static const uint16_t crctable[256] = {
+  0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7, 
+  0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef, 
+  0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6, 
+  0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de, 
+  0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485, 
+  0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d, 
+  0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4, 
+  0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 
+  0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823, 
+  0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b, 
+  0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12, 
+  0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a, 
+  0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41, 
+  0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49, 
+  0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70, 
+  0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78, 
+  0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f, 
+  0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067, 
+  0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e, 
+  0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256, 
+  0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d, 
+  0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405, 
+  0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c, 
+  0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634, 
+  0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab, 
+  0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3, 
+  0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a, 
+  0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92, 
+  0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9, 
+  0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1, 
+  0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8, 
+  0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0 
+};
+
 int disk_read(BYTE * buf, LBA_t sector, UINT count) {
   uint64_t r;
   UINT i, j;
@@ -86,6 +122,7 @@ int disk_read(BYTE * buf, LBA_t sector, UINT count) {
   for (i = 0; i < count; i++) {
     uint16_t crc, crc_exp;
     uint64_t n = 0;
+    uint64_t readCount = 0;
 
     // Wait for data token
     while((r = spi_dummy()) != SD_DATA_TOKEN);
@@ -98,21 +135,45 @@ int disk_read(BYTE * buf, LBA_t sector, UINT count) {
     /*   crc = crc16(crc, x); */
     /* } while (--n > 0); */
 
-    n = 512/8;
-    do {
-      // Send 8 dummy bytes (fifo should be empty)
-      for (j = 0; j < 8; j++) {
+    /* n = 512/8; */
+    /* do { */
+    /*   // Send 8 dummy bytes (fifo should be empty) */
+    /*   for (j = 0; j < 8; j++) { */
+    /*     spi_sendbyte(0xff); */
+    /*   } */
+
+    /*   // Reset counter. Process bytes AS THEY COME IN. */
+    /*   for (j = 0; j < 8; j++) { */
+    /*     while (!(read_reg(SPI_IP) & 2)) {} */
+    /*     uint8_t x = spi_readbyte(); */
+    /*     *p++ = x; */
+    /*     // crc = crc16(crc, x); */
+    /*     crc = ((crc << 8) ^ crctable[x ^ (crc >> 8)]) & 0xffff; */
+    /*   } */
+    /* } while(--n > 0); */
+
+    n = 512;
+    // Initially fill the transmit fifo
+    for (j = 0; j < 8; j++) {
+      spi_sendbyte(0xff);
+    }
+
+    
+    while (n > 0) {
+      // Wait for bytes to be received
+      while (!(read_reg(SPI_IP) & 2)) {}
+      // Read byte
+      uint8_t x = spi_readbyte();
+      // Send another dummy byte
+      if (n > 8) {
         spi_sendbyte(0xff);
       }
-
-      // Reset counter. Process bytes AS THEY COME IN.
-      for (j = 0; j < 8; j++) {
-        while (!(read_reg(SPI_IP) & 2)) {}
-        uint8_t x = spi_readbyte();
-        *p++ = x;
-        crc = crc16(crc, x);
-      }
-    } while(--n > 0);
+      // Place received byte into memory
+      *p++ = x;
+      // Update CRC16 with fast table based method
+      crc = ((crc << 8) ^ crctable[x ^ (crc >> 8)]) & 0xffff;
+      n = n - 1;
+    }
     
     // Read CRC16 and check
     crc_exp = ((uint16_t)spi_dummy() << 8);