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    Subject[PATCH/RFC] Resolve 2 year old issue with different demands on EVIOCGRAB

    I'll let the comments in the patch below to most of the talking.
    This came up because I wanted to use EVIOCGRAB in some code on an
    Openmoko Freerunner, and found that EVIOCGRAG does different things on
    that kernel to elsewhere. I looked into why, and found that there was
    a good reason but that the issues hadn't been completely resolved. I
    hope to help resolve the issues so that EVIOCGRAB can behave the same
    everywhere, and still meet everybody's needs.

    I would have Cc:ed to Magnus Vigerlof who wrote the original patch on
    which this is based, but his Email address doesn't appear in lkml.org.

    NeilBrown



    Support multiple styles of input device GRABbing with EVIOCGRAB

    From: NeilBrown <neilb@suse.de>

    This is a 2 year old issue (almost to the day!) which does not
    appear to have been properly resolved yet.
    See http://lkml.org/lkml/2006/8/12/64

    An input device (e.g. touch pad, key board) can be attached to
    multiple handlers (/dev/mouseX, console, etc) one of which can be
    evdev which provides the /dev/input/event* files.
    evdev can support multiple clients on a single device, as multiple
    processes can open /dev/input/eventX. They each get to see all the
    events.

    EVIOCGRAB is an ioctl on an evdev device which causes all events to go
    to that handler, and that client, exclusively.

    It is being used for two distinct purposes.

    1/ Stop events going to other handlers, so that e.g. absolute touchpad
    events don't get mapped to relative mouse events. Keyboard events
    processed by X server don't get processed by console and control-C
    kills the X server.
    2/ Stop events going to any other client, so they can temporarily be
    used for a different purpose. e.g. GRAB all events when putting a
    device partially to sleep (e.g. screen blank) so the events can
    cause a wakeup without being processed by any application.

    So there are three levels of grab that are required:

    0/ no grab : all events go to all handlers and all clients
    1/ evdev grab: events only go to evdev, but can then go to any
    client of evdev
    2/ total grab: events only go to the specific evdev client that
    requested the grab.


    0 is the default
    mainline allows you to select 2 but not 1.
    There is a patch floating around (in the above mail thread, and
    in the Openmoko kernel for example) that allows you to select 1, but not 2.

    It seems that the "obvious" thing to do is to interpret the argument
    to EVIOCGRAB to select between these options.
    We cannot use 0, 1, 2 as above - despite the fact that it makes sense
    - because that would be a regression in mainline: working programs could break.
    Maybe it's best to use 0, 2, 1 and document it clearly. The chance of
    anyone using 2 already has got to be very close to zero.

    With this patch in place, clients that are interested in purpose '1'
    above would need to be changed to pass '2' to EVIOCGRAB, then (I
    think) everyone would be happy.

    Note that this patch causes any value other than 0, 1, 2 to return
    EINVAL. I believe this is appropriate defensive practice in general
    to allow for future enhancements. However it does mean that any
    current code which uses a value other than 0 or 1 would break. Is
    there any such code?

    Note also that it is not possible to switch between '1' and '2'. A
    client must always release the grab first. Such a switch could be
    implemented, but there doesn't seem much point.

    Signed-off-by: NeilBrown <neilb@suse.de>

    ---

    drivers/input/evdev.c | 53 ++++++++++++++++++++++++++++++++++++++++---------
    include/linux/input.h | 10 ++++++++-
    2 files changed, 52 insertions(+), 11 deletions(-)


    diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
    index 2d65411..ff4169f 100644
    --- a/drivers/input/evdev.c
    +++ b/drivers/input/evdev.c
    @@ -29,6 +29,7 @@ struct evdev {
    struct input_handle handle;
    wait_queue_head_t wait;
    struct evdev_client *grab;
    + int grabcnt;
    struct list_head client_list;
    spinlock_t client_lock; /* protects client_list */
    struct mutex mutex;
    @@ -39,6 +40,7 @@ struct evdev_client {
    struct input_event buffer[EVDEV_BUFFER_SIZE];
    int head;
    int tail;
    + int grab;
    spinlock_t buffer_lock; /* protects access to buffer, head and tail */
    struct fasync_struct *fasync;
    struct evdev *evdev;
    @@ -129,17 +131,38 @@ static void evdev_free(struct device *dev)
    }

    /*
    + * Grabs an underlying input device for use by evdev only.
    + */
    +static int evdev_shared_grab(struct evdev *evdev, struct evdev_client *client)
    +{
    + int error;
    +
    + if (client->grab)
    + return -EBUSY;
    +
    + if (!evdev->grabcnt) {
    + error = input_grab_device(&evdev->handle);
    + if (error)
    + return error;
    + }
    + evdev->grabcnt++;
    + client->grab = 1;
    +
    + return 0;
    +}
    +
    +/*
    * Grabs an event device (along with underlying input device).
    * This function is called with evdev->mutex taken.
    */
    -static int evdev_grab(struct evdev *evdev, struct evdev_client *client)
    +static int evdev_exclusive_grab(struct evdev *evdev, struct evdev_client *client)
    {
    int error;

    if (evdev->grab)
    return -EBUSY;

    - error = input_grab_device(&evdev->handle);
    + error = evdev_shared_grab(evdev, client);
    if (error)
    return error;

    @@ -151,12 +174,17 @@ static int evdev_grab(struct evdev *evdev, struct evdev_client *client)

    static int evdev_ungrab(struct evdev *evdev, struct evdev_client *client)
    {
    - if (evdev->grab != client)
    + if (!client->grab)
    return -EINVAL;

    - rcu_assign_pointer(evdev->grab, NULL);
    - synchronize_rcu();
    - input_release_device(&evdev->handle);
    + if (evdev->grab == client) {
    + rcu_assign_pointer(evdev->grab, NULL);
    + synchronize_rcu();
    + }
    +
    + if (!--evdev->grabcnt && evdev->exist)
    + input_release_device(&evdev->handle);
    + client->grab = 0;

    return 0;
    }
    @@ -231,7 +259,7 @@ static int evdev_release(struct inode *inode, struct file *file)
    struct evdev *evdev = client->evdev;

    mutex_lock(&evdev->mutex);
    - if (evdev->grab == client)
    + if (client->grab)
    evdev_ungrab(evdev, client);
    mutex_unlock(&evdev->mutex);

    @@ -721,10 +749,15 @@ static long evdev_do_ioctl(struct file *file, unsigned int cmd,
    return 0;

    case EVIOCGRAB:
    - if (p)
    - return evdev_grab(evdev, client);
    - else
    + switch ((unsigned long)p) {
    + case EVG_NONE:
    return evdev_ungrab(evdev, client);
    + case EVG_EXCLUSIVE:
    + return evdev_exclusive_grab(evdev, client);
    + case EVG_SHARED:
    + return evdev_shared_grab(evdev, client);
    + default: return -EINVAL;
    + }

    default:

    diff --git a/include/linux/input.h b/include/linux/input.h
    index a5802c9..d1e6a8a 100644
    --- a/include/linux/input.h
    +++ b/include/linux/input.h
    @@ -79,7 +79,7 @@ struct input_absinfo {
    #define EVIOCRMFF _IOW('E', 0x81, int) /* Erase a force effect */
    #define EVIOCGEFFECTS _IOR('E', 0x84, int) /* Report number of effects playable at the same time */

    -#define EVIOCGRAB _IOW('E', 0x90, int) /* Grab/Release device */
    +#define EVIOCGRAB _IOW('E', 0x90, int) /* Grab/Release device. See GRAB types below */

    /*
    * Event types
    @@ -108,6 +108,14 @@ struct input_absinfo {
    #define SYN_CONFIG 1

    /*
    + * GRAB types
    + */
    +#define EVG_NONE 0 /* release any active GRAB */
    +#define EVG_EXCLUSIVE 1 /* No other handler or evdev gets events */
    +#define EVG_SHARED 2 /* No other handler gets events, but other
    + * evdev clients still do.
    + */
    +/*
    * Keys and buttons
    *
    * Most of the keys/buttons are modeled after USB HUT 1.12

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