Add documentation of --freq and --opt

doc/tutorial.md

@@ -19,6 +19,17 @@ The size may be followed by an optional "k" or "m" to multiply the given value b
 When flashing a file, a checksum is calculated for the binary file, both in md5 and the sum algorithm.
 The latter is also used by the official ST-Link utility tool from STMicroelectronics as described in the document: [`UM0892 - User manual - STM32 ST-LINK utility software description`](https://www.st.com/resource/en/user_manual/cd00262073-stm32-stlink-utility-software-description-stmicroelectronics.pdf).
 
+#### --freq=n[k][m]
+
+(since v1.6.1)
+
+You can specify the frequency of SWD/JTAG interface, to override the default 1800KHz configuration. This option accpets decimal only (5K or 1.8M). `Hz` is left out in this option. Valid frequencies are `5K, 15K, 25K, 50K, 100K, 125K, 240K, 480K, 950K, 1200K(1.2M), 1800K(1.8M), 4000K(4M)`
+
+#### --opt
+
+(since v1.6.1, optional; enabled by default from v1.0.0 to v1.6.0)
+
+You can enable the optimization to skip flashing empty (0x00 or 0xff) bytes at the end of binary file. May cause some garbage data left after a flash.
 
 ## Solutions to common problems
 ### a) STLINK/v1 driver: Issue with Kernel Extension (kext) (macOS 10.11 and later only)

include/stlink.h

@@ -151,7 +151,7 @@ typedef struct flash_loader {
 
         // transport layer verboseness: 0 for no debug info, 10 for lots
         int verbose;
-        int opt;
+        int opt;            // set by main() in tools/flash.c, empty tail bytes drop optimization
         uint32_t core_id;	// set by stlink_core_id(), result from STLINK_DEBUGREADCOREID
         uint32_t chip_id;	// set by stlink_load_device_params(), used to identify flash and sram
         int core_stat;		// set by stlink_status(), values STLINK_CORE_xxxxx

include/stlink/tools/flash.h

@@ -25,8 +25,8 @@ struct flash_opts
     enum flash_area area;
     uint32_t val;
     size_t flash_size;	/* --flash=n[k][m] */
-    int opt;
-    int freq;
+    int opt;            /* enable empty tail data drop optimization */
+    int freq;           /* --freq=n[k][m] frequency of JTAG/SWD */
 };
 
 #define FLASH_OPTS_INITIALIZER {0, { 0 }, NULL, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

src/usb.c

@@ -1042,7 +1042,7 @@ stlink_t *stlink_open_usb(enum ugly_loglevel verbose, bool reset, char serial[ST
     }
 
 
-    sl->opt = freq;
+    sl->freq = freq;
     // set the speed before entering the mode
     // as the chip discovery phase should be done at this speed too
     // Set the stlink clock speed (default is 1800kHz)