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Date: Mon, 4 Apr 2011 15:46:11 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Jeffrey Brown <jeffbrown@...roid.com>
Cc: rydberg@...omail.se, djkurtz@...gle.com,
linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] input: evdev: Make device readable only when it
contains a complete packet.
On Mon, Apr 04, 2011 at 03:16:13PM -0700, Jeffrey Brown wrote:
> Hi Dmitry,
> I don't think the new patch is completely correct.
>
> On Mon, Apr 4, 2011 at 2:36 PM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> > I think we should target SYN_REPORT directly. SYN_CONFIG is unused and
> > SYN_DROPPED is not interesting till next SYN_REPORT anyway. Given the
> > changes to the previous patch I have the following:
>
> Explicitly checking for SYN_REPORT makes sense. I wasn't sure to do
> with SYN_CONFIG before so I tried to keep the condition somewhat
> conservative.
>
> Per previous comments on an older iteration of this patch, it probably
> makes sense to calculate this flag once in evdev_event and pass it to
> evdev_pass_event.
>
> bool full_sync = (type == EV_SYN && code == SYN_REPORT);
I am not sure what is cheaper - 2 conditionals or stack manipulation
needed to push another argument if we happed to be register-starved.
>
> > @@ -41,6 +41,7 @@ struct evdev {
> > struct evdev_client {
> > unsigned int head;
> > unsigned int tail;
> > + unsigned int last_syn; /* position of the last EV_SYN/SYN_REPORT */
>
> This comment for last_syn is not quite right. We need last_syn to
> refer to the position just beyond the last sync. Otherwise the device
> will not become readable until another event is written there. The
> invariants for last_syn should be similar to those for head.
Hm, yes, comment is incorrect. Given this fact I do not like the name
anymore either (nor do I like 'end'). Need to think about something
better.
>
> Whereas tail != head means buffer non-empty, tail != last_syn should
> mean buffer is readable.
>
> It looks like we almost maintain those invariants here, except for SYN_DROPPED.
>
> > spinlock_t buffer_lock; /* protects access to buffer, head and tail */
> > struct fasync_struct *fasync;
> > struct evdev *evdev;
> > @@ -72,12 +73,16 @@ static void evdev_pass_event(struct evdev_client *client,
> > client->buffer[client->tail].type = EV_SYN;
> > client->buffer[client->tail].code = SYN_DROPPED;
> > client->buffer[client->tail].value = 0;
> > +
>
> Should use client->head here so that the SYN_DROPPED is readable.
It is readable, but we do not want to signal on it.
>
> > + client->last_syn = client->tail;
> > }
> >
> > spin_unlock(&client->buffer_lock);
>
> Can use full_sync or something equivalent instead of repeating the
> condition on EV_SYN / SYN_REPORT here.
>
> > - if (event->type == EV_SYN)
> > + if (event->type == EV_SYN && event->code == SYN_REPORT) {
>
> I don't think it's safe to modify last_syn outside of the spin lock.
> This should be done above.
This is the only writer, plus we are running under event_lock with
interrupts off, so it is safe.
>
> > + client->last_syn = client->head;
> > kill_fasync(&client->fasync, SIGIO, POLL_IN);
> > + }
> > }
>
> MISSING: We need to also modify evdev_event to only call
> wake_up_interruptible when enqueuing a sync. It does not make sense
> to wake up waiters unless the device is about to become readable
> again.
Right, I'll add it.
>
> This also means we should wake after having written SYN_DROPPED. We
> might need to make evdev_pass_event return (or take by reference) a
> boolean that indicates whether at least one client has become
> readable.
Why? Why would we not want to wait till the next SYNC to deliver
DROPPED?
>
> Pseudo-code:
>
> if (full_sync || evdev_became_readable_for_a_client_due_to_syn_dropped)
> wake_up_interruptible(&evdev->wait);
>
> > /*
> > @@ -387,12 +392,12 @@ static ssize_t evdev_read(struct file *file, char __user *buffer,
> > if (count < input_event_size())
> > return -EINVAL;
> > - if (client->head == client->tail && evdev->exist &&
> > + if (client->last_syn == client->tail && evdev->exist &&
> > (file->f_flags & O_NONBLOCK))
> > return -EAGAIN;
> >
> > retval = wait_event_interruptible(evdev->wait,
> > - client->head != client->tail || !evdev->exist);
> > + client->last_syn != client->tail || !evdev->exist);
> > if (retval)
> > return retval;
> >
> > @@ -421,7 +426,7 @@ static unsigned int evdev_poll(struct file *file, poll_table *wait)
> > poll_wait(file, &evdev->wait, wait);
> >
> > mask = evdev->exist ? POLLOUT | POLLWRNORM : POLLHUP | POLLERR;
> > - if (client->head != client->tail)
> > + if (client->last_syn != client->tail)
> > mask |= POLLIN | POLLRDNORM;
> >
> > return mask;
>
> It looks to me like this patch isn't based on top of your previous
> patch for SYN_DROPPED. Specifically, the SYN_DROPPED should be
> inserted before the newly enqueued event but I don't see that above.
Yes it does - please check the chunk for evdevPass_event again.
--
Dmitry
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