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    SubjectRe: [PATCH] Input: Do not add SYN_REPORT in between a single packet data
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    On Wed, Apr 6, 2016 at 11:08 PM, Dmitry Torokhov
    <dmitry.torokhov@gmail.com> wrote:
    > On Wed, Apr 06, 2016 at 08:26:39PM +0530, Aniroop Mathur wrote:
    >> On Sat, Apr 2, 2016 at 10:31 PM, Aniroop Mathur
    >> <aniroop.mathur@gmail.com> wrote:
    >> > Hello Mr. Torokhov,
    >> >
    >> > First of all, Thank you for your reply.
    >> >
    >> > On Sat, Apr 2, 2016 at 3:21 AM, Dmitry Torokhov
    >> > <dmitry.torokhov@gmail.com> wrote:
    >> >> On Fri, Mar 11, 2016 at 12:26:57AM +0530, Aniroop Mathur wrote:
    >> >>> Hi Henrik,
    >> >>>
    >> >>> On Thu, Mar 10, 2016 at 7:15 PM, Henrik Rydberg <rydberg@bitmath.org> wrote:
    >> >>> > Hi Dmitry,
    >> >>> >
    >> >>> >>> diff --git a/drivers/input/input.c b/drivers/input/input.c
    >> >>> >>> index 8806059..262ef77 100644
    >> >>> >>> --- a/drivers/input/input.c
    >> >>> >>> +++ b/drivers/input/input.c
    >> >>> >>> @@ -401,8 +401,7 @@ static void input_handle_event(struct input_dev *dev,
    >> >>> >>> if (dev->num_vals >= 2)
    >> >>> >>> input_pass_values(dev, dev->vals, dev->num_vals);
    >> >>> >>> dev->num_vals = 0;
    >> >>> >>> - } else if (dev->num_vals >= dev->max_vals - 2) {
    >> >>> >>> - dev->vals[dev->num_vals++] = input_value_sync;
    >> >>> >>> + } else if (dev->num_vals >= dev->max_vals - 1) {
    >> >>> >>> input_pass_values(dev, dev->vals, dev->num_vals);
    >> >>> >>> dev->num_vals = 0;
    >> >>> >>> }
    >> >>> >>
    >> >>> >> This makes sense to me. Henrik?
    >> >>> >
    >> >>> > I went through the commits that made these changes, and I cannot see any strong
    >> >>> > reason to keep it. However, this code path only triggers if no SYN events are
    >> >>> > seen, as in a driver that fails to emit them and consequently fills up the
    >> >>> > buffer. In other words, this change would only affect a device that is already,
    >> >>> > to some degree, broken.
    >> >>> >
    >> >>> > So, the question to Aniroop is: do you see this problem in practise, and in that
    >> >>> > case, for what driver?
    >> >>> >
    >> >>>
    >> >>> Nope. So far I have not dealt with any such driver.
    >> >>> I made this change because it is breaking protocol of SYN_REPORT event code.
    >> >>>
    >> >>> Further from the code, I could deduce that max_vals is just an estimation of
    >> >>> packet_size and it does not guarantee that packet_size is same as max_vals.
    >> >>> So real packet_size can be more than max_vals value and hence we could not
    >> >>> insert SYN_REPORT until packet ends really.
    >> >>> Further, if we consider that there exists a driver or will exist in future
    >> >>> which sets capability of x event code according to which max_value comes out to
    >> >>> y and the real packet size is z i.e. driver wants to send same event codes
    >> >>> again in the same packet, so input event reader would be expecting SYN_REPORT
    >> >>> after z events but due to current code SYN_REPORT will get inserted
    >> >>> automatically after y events, which is a wrong behaviour.
    >> >>
    >> >> Well, I think I agree with Aniroop that even if driver is to a degree
    >> >> broken we should not be inserting random SYN_REPORT events into the
    >> >> stream. I wonder if we should not add WARN_ONCE() there to highlight
    >> >> potential problems with the way we estimate the number of events.
    >> >>
    >> >> However I think there is an issue with the patch. If we happen to pass
    >> >> values just before the final SYN_REPORT sent by the driver then we reset
    >> >> dev->num_vals to 0 and will essentially suppress the final SYN_REPORT
    >> >> event, which is not good either.
    >> >>
    >> >
    >> > Yes, right!
    >> >
    >> > I think it can be fixed by sending the rest of events but not the last event
    >> > in case number of events becomes greater than max_vals. The last event will be
    >> > saved to be sent in next set of events. This way immediate SYN_REPORT will not
    >> > be suppressed and duplicate SYN_REPORT event will not be sent as well.
    >> >
    >> > Change:
    >> > @@ -401,8 +401,7 @@ static void input_handle_event(struct input_dev *dev,
    >> > if (dev->num_vals >= 2)
    >> > input_pass_values(dev, dev->vals, dev->num_vals);
    >> > dev->num_vals = 0;
    >> > - } else if (dev->num_vals >= dev->max_vals - 2) {
    >> > - dev->vals[dev->num_vals++] = input_value_sync;
    >> > - input_pass_values(dev, dev->vals, dev->num_vals);
    >> > - dev->num_vals = 0;
    >> > + } else if (dev->num_vals == dev->max_vals) {
    >> > + input_pass_values(dev, dev->vals, dev->num_vals - 1);
    >> > + dev->num_vals = 0;
    >> > + dev->vals[dev->num_vals++] = dev->vals[dev->max_vals - 1];
    >> > }
    >> >
    >> > So, does the above patch looks good now?
    >> >
    >
    > No, consider what will happen if you need to switch slot when your queue
    > is at dev->max_vals - 1. With your patch you will end up with out of
    > bounds write.
    >

    Sorry, I know very less about MT protocol, so could not catch this.
    I have worked only on normal input device drivers.
    input_abs_set_val(dev, ABS_MT_SLOT, mt->slot); --> I guess I missed this? :(

    I have modified condition to handle it, so does it look fine now?

    diff --git a/drivers/input/input.c b/drivers/input/input.c
    index 8806059..799941c 100644
    --- a/drivers/input/input.c
    +++ b/drivers/input/input.c
    @@ -401,12 +401,11 @@ static void input_handle_event(struct input_dev *dev,
    if (dev->num_vals >= 2)
    input_pass_values(dev, dev->vals, dev->num_vals);
    dev->num_vals = 0;
    - } else if (dev->num_vals >= dev->max_vals - 2) {
    - dev->vals[dev->num_vals++] = input_value_sync;
    - input_pass_values(dev, dev->vals, dev->num_vals);
    + } else if (dev->num_vals >= dev->max_vals - 1) {
    + input_pass_values(dev, dev->vals, dev->num_vals - 1);
    dev->num_vals = 0;
    + dev->vals[dev->num_vals++] = dev->vals[dev->num_vals - 1];
    }
    -
    }

    >>
    >>
    >> Hello Mr. Torokhov,
    >>
    >> Could you please update about this?
    >> It would be appreciating if you could help out to conclude it quickly. Thanks!
    >
    > I am not sure what the urgency is. It is more of a theoretical problem
    > ans so far the proposed solutions were actually introducing more
    > problems than they were solving.
    >
    > I am sorry, bit this particular topic is not a priority for me.
    >

    There is no hurry at all. :-) As you know request is made a long time ago,
    so I am only very curious to complete it.
    >>
    >>
    >> > And may be about WARN_ONCE, do you mean to add something like below in above
    >> > code?
    >> > WARN_ONCE(1, "Packet did not complete yet but generally expected to be
    >> > completed before generation of %d events.\n", dev->max_vals);
    >> >
    >> >
    >> > Thanks,
    >> > Aniroop Mathur
    >> >
    >> >> Thanks.
    >> >>
    >> >> --
    >> >> Dmitry
    >
    > Thanks.
    >
    > --
    > Dmitry

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