am7xxx: improve the comment about setting a new configuration

[libam7xxx.git] / src / am7xxx.c

diff --git a/src/am7xxx.c b/src/am7xxx.c
index 1c3f704..b4e3e54 100644 (file)
--- a/src/am7xxx.c
+++ b/src/am7xxx.c
@@ -489,8 +489,8 @@ static int send_data_async(am7xxx_device *dev, uint8_t *buffer, unsigned int len
 
        /* Make a copy of the buffer so the caller can safely reuse it just
         * after libusb_submit_transfer() has returned. This technique
-        * requires more allocations than a proper double-buffering approach
-        * but it takes a lot less code. */
+        * requires more dynamic allocations compared to a proper
+        * double-buffering approach but it takes a lot less code. */
        transfer_buffer = malloc(len);
        if (transfer_buffer == NULL) {
                error(dev->ctx, "cannot allocate transfer buffer (%s)\n",
@@ -734,16 +734,34 @@ static int open_device(am7xxx_context *ctx,
         */
 
        current_configuration = -1;
-       libusb_get_configuration((*dev)->usb_device, &current_configuration);
+       ret = libusb_get_configuration((*dev)->usb_device,
+                                      &current_configuration);
+       if (ret < 0) {
+               debug(ctx, "libusb_get_configuration failed: %s\n",
+                     libusb_error_name(ret));
+               goto out_libusb_close;
+       }
+
        if (current_configuration != (*dev)->desc->configuration) {
                /*
-                * In principle kernel drivers bound to each interface should
-                * be detached before setting the configuration, but in
-                * practice this is not necessary for most devices.
+                * In principle, before setting a new configuration, kernel
+                * drivers should be detached from _all_ interfaces; for
+                * example calling something like the following "invented"
+                * function _before_ setting the new configuration:
+                *
+                *   libusb_detach_all_kernel_drivers((*dev)->usb_device);
+                *
+                * However, in practice, this is not necessary for most
+                * devices as they have only one configuration.
                 *
-                * For example something like the following function would be
-                * called:
-                *      libusb_detach_all_kernel_drivers((*dev)->usb_device);
+                * When a device only has one configuration:
+                *
+                *   - if there was a kernel driver bound to the device, it
+                *     had already set the configuration and the call below
+                *     will be skipped;
+                *
+                *   - if no kernel driver was bound to the device, the call
+                *     below will suceed.
                 */
                ret = libusb_set_configuration((*dev)->usb_device,
                                               (*dev)->desc->configuration);
@@ -772,16 +790,26 @@ static int open_device(am7xxx_context *ctx,
         * http://libusb.sourceforge.net/api-1.0/caveats.html
         */
        current_configuration = -1;
-       libusb_get_configuration((*dev)->usb_device, &current_configuration);
+       ret = libusb_get_configuration((*dev)->usb_device,
+                                      &current_configuration);
+       if (ret < 0) {
+               debug(ctx, "libusb_get_configuration after claim failed: %s\n",
+                     libusb_error_name(ret));
+               goto out_libusb_release_interface;
+       }
+
        if (current_configuration != (*dev)->desc->configuration) {
                debug(ctx, "libusb configuration changed (expected: %hhu, current: %hhu\n",
                      (*dev)->desc->configuration, current_configuration);
                ret = -EINVAL;
-               goto out_libusb_close;
+               goto out_libusb_release_interface;
        }
 out:
        return ret;
 
+out_libusb_release_interface:
+       libusb_release_interface((*dev)->usb_device,
+                                (*dev)->desc->interface_number);
 out_libusb_close:
        libusb_close((*dev)->usb_device);
        (*dev)->usb_device = NULL;