On Thu, 2 Feb 2012 11:26:55 +0100 Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > From: Hans Verkuil <hans.verkuil@xxxxxxxxx> > > 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. > > ... > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
