Re: Questions about implementing hid driver or input driver

On Thu, 6 Sep 2012, Dmitry Torokhov wrote:

quoted

Hi list,
I have a tablet that shows it is a HID device.
But the report descriptor looks broken.

Below shows the device information:

lsusb -v:

Bus 002 Device 004: ID 04b4:3045 Cypress Semiconductor Corp.
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0
  bDeviceProtocol         0
  bMaxPacketSize0         8
  idVendor           0x04b4 Cypress Semiconductor Corp.
  idProduct          0x3045
  bcdDevice            1.00
  iManufacturer           1 Ping-IT
  iProduct                2 RS232_USB
  iSerial                 0
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           34
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          3 Bus power
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              4 HID
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.10
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      26
         Report Descriptors:
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              10
Device Status:     0x0000
  (Bus Powered)

And below is the report descriptor ( by usbhid-dump / hidrd-convert )

0x05, 0x0D, /*  Usage Page (Digitizer),         */
0x09, 0x08, /*  Usage (3D Digitizer),           */
0xA1, 0x01, /*  Collection (Application),       */
0x09, 0x01, /*      Usage (Digitizer),          */
0xA1, 0x00, /*      Collection (Physical),      */
0x05, 0x01, /*          Usage Page (Desktop),   */
0x09, 0x3B, /*          Usage (Byte Count),     */
0x15, 0x00, /*          Logical Minimum (0),    */
0x25, 0x7F, /*          Logical Maximum (127),  */
0x75, 0x08, /*          Report Size (8),        */
0x95, 0x07, /*          Report Count (7),       */
0x81, 0x02, /*          Input (Variable),       */
0xC0,       /*      End Collection,             */
0xC0        /*  End Collection                  */

The report descriptor looks broken to me:
 1. The report descriptor does not have Usage for TOOL_PEN/STYLUS/STYLUS2/PRESSURE,
    so evdev doest not recognized it as a tablet.
    Actually, the report descriptor does not define any detail of the data.
    It only shows a 7 bytes report content.
 2. It does not have Report ID field. 
    (Does it make sense to have a report descriptor without Report ID field?)

So in this case, should I implement a HID (special device) driver or
a input driver (in drivers/input/tablet folder) for this device?

I think because the driver would hardly use any of HID infrastructure it
would make sense to do it as an input driver.

Agreed.

It however also slightly depends on the actual data it is sending. It 
might be a proper HID protocol in the end, just the report descriptor 
might be completely bogus.
We already have a couple devices supported in linux by hid-waltop driver. 
The only thing it does is that it replaces completely wrong report 
desriptor with a proper one, and it's enough to make the device work 
properly.

But that really depends on the protocol that the device is using in the 
reports.

-- 
Jiri Kosina
SUSE Labs