On Thu, Jul 09, 2009 at 01:35:51PM -0700, Steve Calfee wrote:
> > The device STALLS EP0 when asked for string descriptor 0, so it tells
> > it's not willing to hand out any string at all. Haven't checked with the
> > USB specs whether this is correct behaviour, but this is what it looks
> > like.
> >
> > When an endpoint communication stalls, the return value is -EPIPE, and
> > in case of a malformed string descriptor we seem to be getting -ENODATA.
> >
> > Handling those two case seperately should make both sides happy.
> >
> > Could you test the patch below? It's only compile tested for now.
> >
> > Thanks,
> > Daniel
> >
> >
> Your patch is wrong and maybe so is the current code, I haven't looked.
Yes, as Alan pointed out, there was a braino.
> 1) String descriptors are optional, a device does not have to have any.
>
> b) When a device cannot handle a control command, it will return a STALL pid.
>
> iii) When windows xp enumerates a gadget and it gets a stall on the
> string desc "0" fetch, it stops fetching device strings. Since most
> gadgets are only tested under windows, if you then request a string
> after it has told you there are no strings, some gadgets will hang.
Ok, I see.
> I believe that the proper response after getting a STALL from a fetch
> of string desc 0, the system should flag the device as not having
> strings and fail all future requests with a STALL (EPIPE) - without
> going out on the bus. I am working with a quite old kernel and cannot
> provide a patch to the latest kernel to try this.
What the kernel did before my patch is
- upon each string read, check whether we received the LANGID already
- if not, try to get it
- bail out if that fails
- if we got one, remember the LANGID and save a flag for that, so the
LANGID will not be retrieved a second time
The problem here is that the code was trying to unnecessarily read
LANGID over and over again, spitting an error every time.
Below is a new patch that changed the bahviour to this:
- upon the first string read, go get the LANGID descriptor
- if that fails with -EPIPE (caused by a STALL), mark the device as
unable to respond to string descriptor request
- if that fails with -ENODATA or if the LANGID descriptor is bogus, fall
back to the 0x0409 default
- on each subsequent string request, return -EPIPE immediately if the
device is not able to provide strings
Let's see if that works.
I factored out this logic to a new function to make it more readable.
Thanks,
Daniel
diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 2bed83c..9720e69 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -806,6 +806,48 @@ static int usb_string_sub(struct usb_device *dev, unsigned int langid,
return rc;
}
+static int usb_get_langid(struct usb_device *dev, unsigned char *tbuf)
+{
+ int err;
+
+ if (dev->have_langid)
+ return 0;
+
+ if (dev->string_langid < 0)
+ return -EPIPE;
+
+ err = usb_string_sub(dev, 0, 0, tbuf);
+
+ /* If the string was reported but is malformed, default to english
+ * (0x0409) */
+ if (err == -ENODATA || (err > 0 && err < 4)) {
+ dev->string_langid = 0x0409;
+ dev->have_langid = 1;
+ dev_err(&dev->dev,
+ "string descriptor 0 malformed (err = %d), "
+ "defaulting to 0x%04x\n",
+ err, dev->string_langid);
+ return 0;
+ }
+
+ /* In case of all other errors, we assume the device is not able to
+ * deal with strings at all. Set string_langid to -1 in order to
+ * prevent any string to be retrieved from the device */
+ if (err < 0) {
+ dev_err(&dev->dev, "string descriptor 0 read error: %d\n",
+ err);
+ dev->string_langid = -1;
+ return -EPIPE;
+ }
+
+ /* always use the first langid listed */
+ dev->string_langid = tbuf[2] | (tbuf[3] << 8);
+ dev->have_langid = 1;
+ dev_dbg(&dev->dev, "default language 0x%04x\n",
+ dev->string_langid);
+ return 0;
+}
+
/**
* usb_string - returns UTF-8 version of a string descriptor
* @dev: the device whose string descriptor is being retrieved
@@ -837,24 +879,9 @@ int usb_string(struct usb_device *dev, int index, char *buf, size_t size)
if (!tbuf)
return -ENOMEM;
- /* get langid for strings if it's not yet known */
- if (!dev->have_langid) {
- err = usb_string_sub(dev, 0, 0, tbuf);
- if (err < 0) {
- dev_err(&dev->dev,
- "string descriptor 0 read error: %d\n",
- err);
- } else if (err < 4) {
- dev_err(&dev->dev, "string descriptor 0 too short\n");
- } else {
- dev->string_langid = tbuf[2] | (tbuf[3] << 8);
- /* always use the first langid listed */
- dev_dbg(&dev->dev, "default language 0x%04x\n",
- dev->string_langid);
- }
-
- dev->have_langid = 1;
- }
+ err = usb_get_langid(dev, tbuf);
+ if (err < 0)
+ goto errout;
err = usb_string_sub(dev, dev->string_langid, index, tbuf);
if (err < 0)
--
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