Re: [PATCH v2] usbcore: Add quirk for 255-bytes initial config read

[Michal Pecio <michal.pecio@gmail.com>]

I really think it could (and should) be a simple patch.

This is what I wrote a few weeks ago. It's an unconditional change
for all devices, but it would be easy to turn it into a quirk.


--- a/drivers/usb/core/config.c
+++ b/drivers/usb/core/config.c
@@ -938,15 +938,14 @@ int usb_get_configuration(struct usb_device *dev)
 	if (!dev->rawdescriptors)
 		return -ENOMEM;
 
-	desc = kmalloc(USB_DT_CONFIG_SIZE, GFP_KERNEL);
+	desc = kmalloc(255, GFP_KERNEL);
 	if (!desc)
 		return -ENOMEM;
 
 	for (cfgno = 0; cfgno < ncfg; cfgno++) {
-		/* We grab just the first descriptor so we know how long
-		 * the whole configuration is */
+		/* Try 255 bytes first because that's what Windows does */
 		result = usb_get_descriptor(dev, USB_DT_CONFIG, cfgno,
-		    desc, USB_DT_CONFIG_SIZE);
+		    desc, 255);
 		if (result < 0) {
 			dev_err(ddev, "unable to read config index %d "
 			    "descriptor/%s: %d\n", cfgno, "start", result);
@@ -975,8 +974,12 @@ int usb_get_configuration(struct usb_device *dev)
 		if (dev->quirks & USB_QUIRK_DELAY_INIT)
 			msleep(200);
 
-		result = usb_get_descriptor(dev, USB_DT_CONFIG, cfgno,
-		    bigbuffer, length);
+		/* Don't bother if we already have it all */
+		if (length <= result)
+			memcpy(bigbuffer, desc, length);
+		else
+			result = usb_get_descriptor(dev, USB_DT_CONFIG, cfgno,
+					bigbuffer, length);
 		if (result < 0) {
 			dev_err(ddev, "unable to read config index %d "
 			    "descriptor/%s\n", cfgno, "all");