Enable specifying the alternate setting for the DFU interface per-target

This works by "padding" the DFU alternate setting with dummy no-op descriptors
that occupy all previous alternate setting values. AFAICT while these no-op
interfaces are not documented in the USB defined class codes, they don't
identify as anything in Linux and cause no driver to be loaded nor any warnings
in the system logs, and they should be fine for other operating systems as
well. When USB_ALT > 0, the DFU interface appears in the specific alternate
setting, which dfu-util does automatically identify and use, alternatively it
can be manually targeted with the -a flag. To have USB_ALT be a per-target
(optional) property the target.h implementor must load it (since it has the
macro value to keep everything compile-time), which is why I've had to split
out the descriptor structs into their own file to be preprocessed as part of
target_stm32f103.c or target_stm32l1.c. The USB initialization code can then
call target_usb_descriptor() to get the pointer to the configuration
descriptor defined by the target.

Author: twelho

src/stm32f103/target_stm32f103.c

@@ -29,6 +29,7 @@
 #include "target.h"
 #include "config.h"
 #include "backup.h"
+#include "usb_descriptor.h"
 
 #ifndef USES_GPIOA
 #if (HAVE_USB_PULLUP_CONTROL == 0)
@@ -157,6 +158,10 @@ const usbd_driver* target_usb_init(void) {
     return &st_usbfs_v1_usb_driver;
 }
 
+const struct usb_config_descriptor* target_usb_descriptor(void) {
+    return &usb_config;
+}
+
 bool target_get_force_bootloader(void) {
     bool force = false;
     /* Check the RTC backup register */

src/stm32l1/target_stm32l1.c

@@ -26,6 +26,7 @@
 #include "target.h"
 #include "config.h"
 #include "backup.h"
+#include "usb_descriptor.h"
 
 //#define CMD_FAST_BOOT 0xfa57b007
 static const uint32_t CMD_BOOT = 0x544F4F42UL;
@@ -75,6 +76,11 @@ const usbd_driver* target_usb_init(void)
 	return &st_usbfs_v1_usb_driver;
 }
 
+const struct usb_config_descriptor* target_usb_descriptor(void)
+{
+    return &usb_config;
+}
+
 /* This implementation will always start in bootloader, unless the app
  * has asked it to skipp straight forwards
  * You may wish to fill in button handling...

src/target.h

@@ -26,6 +26,7 @@
 extern void target_clock_setup(void);
 extern void target_gpio_setup(void);
 extern const usbd_driver* target_usb_init(void);
+extern const struct usb_config_descriptor* target_usb_descriptor(void);
 extern bool target_get_force_bootloader(void);
 extern void target_get_serial_number(char* dest, size_t max_chars);
 extern size_t target_get_max_firmware_size(void);

src/usb_conf.c

@@ -45,44 +45,6 @@ static const struct usb_device_descriptor dev = {
     .bNumConfigurations = 1,
 };
 
-static const struct usb_interface_descriptor dfu_iface = {
-    .bLength = USB_DT_INTERFACE_SIZE,
-    .bDescriptorType = USB_DT_INTERFACE,
-    .bInterfaceNumber = INTF_DFU,
-    .bAlternateSetting = 0,
-    .bNumEndpoints = 0,
-    .bInterfaceClass = 0xFE,
-    .bInterfaceSubClass = 1,
-    .bInterfaceProtocol = 2,
-    .iInterface = 4,
-
-    .endpoint = NULL,
-
-    .extra = &dfu_function,
-    .extralen = sizeof(dfu_function),
-};
-
-static const struct usb_interface interfaces[] = {
-    /* DFU interface */
-    {
-        .num_altsetting = 1,
-        .altsetting = &dfu_iface,
-    }
-};
-
-static const struct usb_config_descriptor config = {
-    .bLength = USB_DT_CONFIGURATION_SIZE,
-    .bDescriptorType = USB_DT_CONFIGURATION,
-    .wTotalLength = 0,
-    .bNumInterfaces = sizeof(interfaces)/sizeof(struct usb_interface),
-    .bConfigurationValue = 1,
-    .iConfiguration = 0,
-    .bmAttributes = 0xC0,
-    .bMaxPower = 0x32,
-
-    .interface = interfaces,
-};
-
 static const struct usb_device_capability_descriptor* capabilities[] = {
     (const struct usb_device_capability_descriptor*)&webusb_platform,
 };
@@ -119,7 +81,7 @@ usbd_device* usb_setup(void) {
     int num_strings = sizeof(usb_strings)/sizeof(const char*);
 
     const usbd_driver* driver = target_usb_init();
-    usbd_device* usbd_dev = usbd_init(driver, &dev, &config, &bos,
+    usbd_device* usbd_dev = usbd_init(driver, &dev, target_usb_descriptor(), &bos,
                                       usb_strings, num_strings,
                                       usbd_control_buffer, sizeof(usbd_control_buffer));
 

src/usb_descriptor.h

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2021, Dennis Marttinen
+ *
+ * Permission to use, copy, modify, and/or distribute this software
+ * for any purpose with or without fee is hereby granted, provided
+ * that the above copyright notice and this permission notice
+ * appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+ * WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+ * AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
+ * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef DAPBOOT_USB_DESCRIPTOR_H
+#define DAPBOOT_USB_DESCRIPTOR_H
+
+#include "usb_conf.h"
+#include "dfu.h"
+
+// Macro to create a dummy (no-op) USB interface descriptor with the given alternate setting
+#define ALT_DUMMY(N) {                      \
+    .bLength = USB_DT_INTERFACE_SIZE,       \
+    .bDescriptorType = USB_DT_INTERFACE,    \
+    .bInterfaceNumber = INTF_DFU,           \
+    .bAlternateSetting = (N),               \
+    .bNumEndpoints = 0,                     \
+    .bInterfaceClass = 0,                   \
+    .bInterfaceSubClass = 0,                \
+    .bInterfaceProtocol = 0,                \
+    .iInterface = 0,                        \
+    .endpoint = NULL,                       \
+    .extra = NULL,                          \
+    .extralen = 0,                          \
+},
+
+// Functionality for creating repetitive ALT_DUMMY structs with an increasing count during compile time.
+// It doesn't look very nice, but C doesn't allow loops in preprocessor macros, so this needs to be hard-coded.
+#define ALT0
+#define ALT1 ALT_DUMMY(0)
+#define ALT2 ALT1 ALT_DUMMY(1)
+#define ALT3 ALT2 ALT_DUMMY(2)
+#define ALT4 ALT3 ALT_DUMMY(3)
+#define ALT5 ALT4 ALT_DUMMY(4)
+#define ALTC(n) ALT##n
+#define ALTN(n) ALTC(n)
+
+// The DFU interface doesn't have any special alternate setting by default
+#ifndef USB_ALT
+#define USB_ALT 0
+#endif
+
+static const struct usb_interface_descriptor altsettings[] = {
+    ALTN(USB_ALT) // Prepend USB_ALT dummy USB interface descriptors to "pad" the real one
+    {
+        .bLength = USB_DT_INTERFACE_SIZE,
+        .bDescriptorType = USB_DT_INTERFACE,
+        .bInterfaceNumber = INTF_DFU,
+        .bAlternateSetting = USB_ALT,
+        .bNumEndpoints = 0,
+        .bInterfaceClass = 0xFE,
+        .bInterfaceSubClass = 1,
+        .bInterfaceProtocol = 2,
+        .iInterface = 0,
+
+        .endpoint = NULL,
+
+        .extra = &dfu_function,
+        .extralen = sizeof(dfu_function),
+    }
+};
+
+// Tracking this is mandatory if exposing multiple altsettings
+static uint8_t cur_altsetting = 0;
+
+static const struct usb_interface interfaces[] = {
+    /* DFU interface */
+    {
+        .cur_altsetting = &cur_altsetting,
+        .num_altsetting = USB_ALT + 1,
+        .altsetting = (const struct usb_interface_descriptor*)&altsettings,
+    },
+};
+
+static const struct usb_config_descriptor usb_config = {
+    .bLength = USB_DT_CONFIGURATION_SIZE,
+    .bDescriptorType = USB_DT_CONFIGURATION,
+    .wTotalLength = 0,
+    .bNumInterfaces = sizeof(interfaces)/sizeof(struct usb_interface),
+    .bConfigurationValue = 1,
+    .iConfiguration = 0,
+    .bmAttributes = 0xC0,
+    .bMaxPower = 0x32,
+
+    .interface = interfaces,
+};
+
+#endif //DAPBOOT_USB_DESCRIPTOR_H