[U-Boot] [PATCH v3 3/3] usb: Early failure when the first descriptor read fails or is invalid

[Paul Kocialkowski]

Le mardi 07 avril 2015 à 23:41 -0600, Stephen Warren a écrit :
> On 04/07/2015 11:07 PM, Stephen Warren wrote:
> > On 04/04/2015 07:12 AM, Paul Kocialkowski wrote:
> >> This may happen when using an USB1 device on a controller that only supports
> >> USB2 (e.g. EHCI). Reading the first descriptor will fail (read 0 byte), so we
> >> can abort the process at this point instead of failing later and wasting time.
> > 
> > FYI, this patch breaks USB keyboard (or perhaps any USB 1.x device)
> > support on the RPi.

Damn, I'm really sorry about that, I wish I had had more hardware
available to test it thoroughly and that I had thought twice about it
and foresaw that issue. My sincerest apologies for that mistake and of
course, thanks for noticing and picking it up!

> >> diff --git a/common/usb.c b/common/usb.c
> > 
> >> @@ -956,7 +956,7 @@ int usb_new_device(struct usb_device *dev)
> >>  	 */
> >>  #ifndef CONFIG_USB_XHCI
> >>  	err = usb_get_descriptor(dev, USB_DT_DEVICE, 0, desc, 64);
> >> -	if (err < 0) {
> >> +	if (err < sizeof(struct usb_device_descriptor)) {
> >>  		debug("usb_new_device: usb_get_descriptor() failed\n");
> >>  		return -EIO;
> >>  	}
> > 
> > The value returned here is 8 not 18 (the sizeof the descriptor), so the
> > code bails out with an error.

That makes sense on USB low speed devices.

> > Given the description of what this patch is attempting to achieve,
> > shouldn't the replacement check be:
> > 
> > 	if (err <= 0) {
> > 
> > My guess for why the value 8 is returned is because the device's
> > maxpacket is 8, and the DWC2 driver only attempts to transfer 1 packet
> > because the requested transfer size is 64, and the default packet size
> > is 64, which means 1 packet. Note that DWC2 HW (perhaps unlike e.g.
> > EHCI) requires the driver to specify the number of packets transferred,
> > not a byte count. However, I haven't validated that yet. My device is a
> > USB 1.x FS keyboard.
> 
> Yes, I can avoid the error by hacking dwc2.c's assignment of max:
> 
> > int chunk_msg(struct usb_device *dev, unsigned long pipe, int *pid, int in,
> > 	      void *buffer, int len, bool ignore_ack)
> > {
> ...
> > 	int max = usb_maxpacket(dev, pipe);
> 
> and forcing that to 8 rather than 64 for the data transfer phase of the
> first transaction to each USB device (my keyboard has a built-in hub, so
> I need to override a few different transactions).
> 
> Thinking about the fix more, perhaps something like the following is
> appropriate:
> 
> if (err < offsetof(struct usb_device_descriptor, idVendor))

I would be happy with using 8 directly since that's AFAIK the smallest
packet size that should be used on USB. Using offsetof, while not
requiring any hardcoded value, seems a bit unclean to me as it abstracts
that fact.

> ... since idVendor is the first field that's not used by the code that
> processes the results of the initial descriptor read. Hopefully there
> are no devices with a control endpoint bMaxPacketSize0 less than 8
> (which is what that offsetof evaluates to).
> http://www.usbmadesimple.co.uk/ums_3.htm seems to imply that's the case,
> saying for control transfers:

That makes sense, too.

> The max packet size for the data stage is 8 bytes at low speed, 8, 16,
> 32 or 64 at full Speed and 64 for high speed.
> 
> (although one of my keyboards has a maxPacket of 4 for EP 2, but I
> thinkt hat's something different)
> 
> Also note the comment a little before the code this patch affects in
> U-Boot's common/usb.c:
> 
> > 	/* send 64-byte GET-DEVICE-DESCRIPTOR request.  Since the descriptor is
> > 	 * only 18 bytes long, this will terminate with a short packet.  But if
> > 	 * the maxpacket size is 8 or 16 the device may be waiting to transmit
> > 	 * some more, or keeps on retransmitting the 8 byte header. */

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