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    SubjectRe: [RFCv7 PATCH 2/4] poll: add poll_requested_events() and poll_does_not_wait() functions
    On Thu,  2 Feb 2012 11:26:55 +0100
    Hans Verkuil <> wrote:

    > From: Hans Verkuil <>
    > In some cases the poll() implementation in a driver has to do different
    > things depending on the events the caller wants to poll for. An example is
    > when a driver needs to start a DMA engine if the caller polls for POLLIN,
    > but doesn't want to do that if POLLIN is not requested but instead only
    > POLLOUT or POLLPRI is requested. This is something that can happen in the
    > video4linux subsystem.
    > Unfortunately, the current epoll/poll/select implementation doesn't provide
    > that information reliably. The poll_table_struct does have it: it has a key
    > field with the event mask. But once a poll() call matches one or more bits
    > of that mask any following poll() calls are passed a NULL poll_table_struct
    > pointer.
    > The solution is to set the qproc field to NULL in poll_table_struct once
    > poll() matches the events, not the poll_table_struct pointer itself. That
    > way drivers can obtain the mask through a new poll_requested_events inline.
    > The poll_table_struct can still be NULL since some kernel code calls it
    > internally (netfs_state_poll() in ./drivers/staging/pohmelfs/netfs.h). In
    > that case poll_requested_events() returns ~0 (i.e. all events).
    > Very rarely drivers might want to know whether poll_wait will actually wait.
    > If another earlier file descriptor in the set already matched the events the
    > caller wanted to wait for, then the kernel will return from the select() call
    > without waiting.
    > A new helper function poll_does_not_wait() is added that drivers can use to
    > detect this situation.
    > Drivers should no longer access any of the poll_table internals, but use the
    > poll_requested_events() and poll_does_not_wait() access functions instead.

    A way to communicate and enforce this is to rename the relevant fields. Prepend
    a "_" to them and add a stern comment.

    > Since the behavior of the qproc field changes with this patch (since this
    > function pointer can now be NULL when that wasn't possible in the past) I
    > have renamed that field from qproc to pq_proc. Any out-of-tree driver that
    > uses it will now fail to compile.
    > Some notes regarding the correctness of this patch: the driver's poll()
    > function is called with a 'struct poll_table_struct *wait' argument. This
    > pointer may or may not be NULL, drivers can never rely on it being one or
    > the other as that depends on whether or not an earlier file descriptor in
    > the select()'s fdset matched the requested events.
    > ...

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      Last update: 2012-02-02 23:51    [W:0.031 / U:11.784 seconds]
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