Re: [linux-usb-devel] Hotplug Driver Interface to User Level Programs: A Driver Example

[Vojtech Pavlik <vojtech@suse.cz>]

On Sun, Sep 12, 2004 at 04:33:13AM +0400, Alexander N. Kozhushkin wrote:

>     Unfortunately, now I cannot present  a full list of device drivers
> which   use   the   first   and  second   approaches.   However,   the
> "drivers/input/mousedev.c"  file by  Vojtech Pavlik  is an  example of
> such a device driver. It  offers the first approach for the mouse-like
> devices with  the minor numbers from 0  to 30, and the  second one for
> the so-called "MOUSEDEV_MIX" device with minor number 31.

This is intentional, as the only reason for mousedev to exist are old
application that cannot use the event interface (which has the third
approach semantics).

There are applications (IIRC old versions of qtembedded and others) that
break when read on a PS/2 device returns an error. Hence we can't.

>     Here is a  patch for kernel 2.6.7 which makes  that driver use the
> third approach for the devices  with minor numbers 0--30. Most likely,
> the patch  can be used  for kernels 2.6.8  and 2.6.9 too.  The archive
> "tests.tgz" contains some  very simple programs to test  the patch and
> the  original mouse  driver. The  description  of the  changes to  the
> "mousedev.c" file can be found in the "patch.txt" file.
> 
>     The  'gpm' program, properly  rewritten by  me to  support hotplug
> mice,  and the  above  tests work  reliably  on my  computer with  the
> modified mouse driver. The X-server  works reliably too if it uses the
> data supplied  by the  modified version of  the 'gmp', instead  of the
> real mouse data.

Well, I think it'd be better if X and GPM were modified to use the event
interface, which would remove several more limitations (like the number
of buttons on a mouse).

> 
> Sincerely yours,
> 		Alexander.
> 
> 
> -- 
> *********************************************************
>                 Alexander N. Kozhushkin.
>             ADDR: alex@grave.users.botik.ru
> *********************************************************

> I have made the following changes to the "drivers/input/mousedev.c" file: 
> 
> 1. 'mousedev_write()', 'mousedev_read()':
>    The changes protect an application using the 'read()' function from
>    waiting indefinitely  for a disconnected mouse.  If the application
>    performs a read or write  operation on the disconnected device, the
>    call  fails with  the 'ENODEV'  error  code. This  behavior of  the
>    'read()'   and    'write()'   functions   is    proposed   in   the
>    "drivers/usb/usb-skeleton.c" file.
> 
> 2. 'mousedev_poll()':
>    The changes  protect an  application using the  'select()' function
>    from waiting  indefinitely for a  disconnected mouse. If  the mouse
>    was  disconnected, each  file descriptor  of the  respective device
>    file  is  assumed  to  be  always ready  for  reading.  The  device
>    disconnection is assumed to be a permanent exceptional condition on
>    the  file  descriptor. The  'select()'  function  treats that  file
>    descriptor as a  valid and ready one. So if  the file descriptor is
>    contained in a  descriptor set supplied to a  'select()' call as an
>    argument, then the 'select()' function adds that file descriptor to
>    the respective  output descriptor  set, i.e., sets  the appropriate
>    bit of  the respective 'fd_set'  input variable.  In this  case the
>    function  returns successfully.   The mouse  disconnection  and the
>    unusable  file descriptor can  be easily  detected by  checking the
>    results of  the successive  'read()' calls, which  make use  of the
>    results of the 'select()' call, for the 'ENODEV' error.
> 
> 3. 'mousedev_disconnect()':
>    a) Sending a 'SIGIO' signal.
>       This   'SIGIO'  signal   protects  an   application   using  the
>       'sigsuspend()' function from  waiting indefinitely for a 'SIGIO'
>       signal.  If a file  descriptor corresponds  to a  device special
>       file representing a mouse, and  the 'O_ASYNC' status flag is set
>       on that file descriptor, a  'SIGIO' signal is generated when the
>       mouse becomes disconnected.
>    b) The  pointers  in  the  'mousedev_table' are  used  only by  the
>       'mousedev_open()'  and  'mousedev_connect()'  functions, so  the
>       'mousedev_disconnect()' function  can safely remove  the pointer
>       corresponding    to   the    disconnected    mouse   from    the
>       'mousedev_table'.
> 
> 4. 'mousedev_free()':
>    The  line "mousedev_table[mousedev->minor] =  NULL;" is  moved into
>    the 'mousedev_disconnect()' function.



-- 
Vojtech Pavlik
SuSE Labs, SuSE CR


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