usb-linux: Get the alt. setting from sysfs rather then asking the dev

At least one device I have lies when receiving a USB_REQ_GET_INTERFACE,
always returning 0 even if the alternate setting is different. This is
likely caused because in practice this control message is never used as
the operating system's usb stack knows which alternate setting it has
told the device to get into, and thus this ctrl message does not get
tested by device manufacturers.

When usb_fs_type == USB_FS_SYS, the active alt. setting can be read directly
from sysfs, which allows using this device through qemu's usb redirection.
More in general it seems a good idea to not send needless control msg's to
devices, esp. as the code in question is called every time a set_interface
is done. Which happens multiple times during virtual machine startup, and
when device drivers are activating the usb device.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/usb-linux.c b/usb-linux.c
index 353e1b1232..f4601e6e63 100644
--- a/usb-linux.c
+++ b/usb-linux.c
@@ -830,6 +830,24 @@ static uint8_t usb_linux_get_alt_setting(USBHostDevice *s,
     struct usb_ctrltransfer ct;
     int ret;
 
+    if (usb_fs_type == USB_FS_SYS) {
+        char device_name[64], line[1024];
+        int alt_setting;
+
+        sprintf(device_name, "%d-%d:%d.%d", s->bus_num, s->devpath,
+                (int)configuration, (int)interface);
+
+        if (!usb_host_read_file(line, sizeof(line), "bAlternateSetting",
+                                device_name)) {
+            goto usbdevfs;
+        }
+        if (sscanf(line, "%d", &alt_setting) != 1) {
+            goto usbdevfs;
+        }
+        return alt_setting;
+    }
+
+usbdevfs:
     ct.bRequestType = USB_DIR_IN | USB_RECIP_INTERFACE;
     ct.bRequest = USB_REQ_GET_INTERFACE;
     ct.wValue = 0;