[Qemu-devel] Absolute USB-HID device musings (was Re: VNC Terminal Server)

["Leonardo E. Reiter" <lreiter@win4lin.com>]

This is by no means a complete patch (do not apply it as it will break 
usb-hid.c), but it adjusts the report descriptor in usb-hid.c to provide 
position in 16-bits, and in absolute coordinates:

Index: usb-hid.c
===================================================================
RCS file: /cvsroot/qemu/qemu/hw/usb-hid.c,v
retrieving revision 1.1
diff -a -u -r1.1 usb-hid.c
--- usb-hid.c   5 Nov 2005 16:57:08 -0000       1.1
+++ usb-hid.c   8 Apr 2006 20:56:02 -0000
@@ -117,7 +117,7 @@
      0x15, 0x00, 0x25, 0x01, 0x95, 0x03, 0x75, 0x01,
      0x81, 0x02, 0x95, 0x01, 0x75, 0x05, 0x81, 0x01,
      0x05, 0x01, 0x09, 0x30, 0x09, 0x31, 0x15, 0x81,
-    0x25, 0x7F, 0x75, 0x08, 0x95, 0x02, 0x81, 0x06,
+    0x25, 0x7F, 0x75, 0x16, 0x95, 0x02, 0x81, 0x02,
      0xC0, 0xC0,
  };

According to: 
http://72.14.203.104/search?q=cache:wVYUTwc33f8J:www.usb.org/developers/devclass_docs/HID1_11.pdf+usb+hid+specification+absolute+relative&hl=en&gl=us&ct=clnk&cd=1

I'm still trying to figure out how the logical min/max apply if we are 
to report absolute (unsigned) positions in 16-bits.  Obviously 8-bits is 
not enough for absolute coordinates.  You could theoretically use only 
12-bits per coordinate but that would make life difficult I think, and 
probably unnecessarily frugal in a software emulation.

It's not clear to me [yet] how the scroll wheel comes into play, and 
whether or not it (the dz coordinate) can be kept relative for ease of 
implementation.  Also the code would need to be changed to report 
coordinates in 16-bits rather than 8, and of course made to report 
absolute coordinates (like from sdl.c, etc.)  Still it looks fairly easy 
to implement - the USB spec is pretty simple.

So to reiterate, my patch does virtually nothing - in fact it will break 
usb-hid.c so please don't use it.  I was just illustrating how to get it 
to report the device as providing 16-bit absolute coordinates instead of 
8-bit relative ones.  If anyone wants to chime in with more info, I'd be 
glad to make this a discussion.  *If* using the USB HID device only, not 
any real USB devices, can be done without slowing down QEMU, then I 
think this is a great way to get a tablet emulated without having to 
deal with drivers on either side.  Plus, in the long run, it probably 
means other neat stuff like being able to get away from ISA bus 
emulation, and also it's portable to other targets (for example, OS-X on 
PPC would talk to the USB HID device the same way theoretically), so 
it's likely the most portable and cleanest option.

Regards,

Leo Reiter

Brad Campbell wrote:
> Apparently USB HID supports absolute input devices natively. Given we 
> have a HID mouse driver of sorts in qemu I wonder if that is another 
> avenue perhaps ?
> 
> 

-- 
Leonardo E. Reiter
Vice President of Product Development, CTO

Win4Lin, Inc.
Virtual Computing from Desktop to Data Center
Main: +1 512 339 7979
Fax: +1 512 532 6501
http://www.win4lin.com