- loaddevdesc() should request 18 bytes as per the spec.  this
avoids the problem of getting just a 17 byte descriptor.  

- loaddevstr() should request 256 bytes, as per standard.
respect the length embedded in the string.

- loaddevstr() should set a language id.  this is important for
some devices.

Reference: /n/sources/patch/maybe/usb-short-desc
Date: Wed Mar  5 15:00:36 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/lib/dev.c	Wed Mar  5 15:00:26 2014
+++ /sys/src/cmd/usb/lib/dev.c	Wed Mar  5 15:00:24 2014
@@ -188,6 +188,8 @@
 	char *s;
 	char *e;
 
+	if(n > b[0])
+		n = b[0];
 	if(n <= 2 || (n & 1) != 0)
 		return strdup("none");
 	n = (n - 2)/2;
@@ -204,42 +206,37 @@
 char*
 loaddevstr(Dev *d, int sid)
 {
-	uchar buf[128];
-	int type;
-	int nr;
+	uchar buf[256];
+	int type, langid, nr;
 
 	if(sid == 0)
 		return estrdup("none");
 	type = Rd2h|Rstd|Rdev;
-	nr=usbcmd(d, type, Rgetdesc, Dstr<<8|sid, 0, buf, sizeof(buf));
+	nr = usbcmd(d, type, Rgetdesc, Dstr<<8|sid, 0, buf, sizeof(buf));
+	if(nr < 4)
+		langid = 0x0409;		/* english */
+	else
+		langid = buf[3]<<8 | buf[2];
+	nr = usbcmd(d, type, Rgetdesc, Dstr<<8|sid, langid, buf, sizeof(buf));
+
 	return mkstr(buf, nr);
 }
 
 int
 loaddevdesc(Dev *d)
 {
-	uchar buf[Ddevlen+255];
+	uchar buf[Ddevlen];
 	int nr;
 	int type;
 	Ep *ep0;
 
 	type = Rd2h|Rstd|Rdev;
-	nr = sizeof(buf);
 	memset(buf, 0, Ddevlen);
-	if((nr=usbcmd(d, type, Rgetdesc, Ddev<<8|0, 0, buf, nr)) < 0)
+	if((nr = usbcmd(d, type, Rgetdesc, Ddev<<8|0, 0, buf, Ddevlen)) < 0)
 		return -1;
-	/*
-	 * Several hubs are returning descriptors of 17 bytes, not 18.
-	 * We accept them and leave number of configurations as zero.
-	 * (a get configuration descriptor also fails for them!)
-	 */
 	if(nr < Ddevlen){
-		print("%s: %s: warning: device with short descriptor\n",
-			argv0, d->dir);
-		if(nr < Ddevlen-1){
-			werrstr("short device descriptor (%d bytes)", nr);
-			return -1;
-		}
+		werrstr("short device descriptor (%d bytes)", nr);
+		return -1;
 	}
 	d->usb = emallocz(sizeof(Usbdev), 1);
 	ep0 = mkep(d->usb, 0);
@@ -254,6 +251,8 @@
 			d->usb->serial = loaddevstr(d, d->usb->ssid);
 		}
 	}
+	else
+		print("usbd: desc error: %r");
 	return nr;
 }