linux-kernel - Re: [PATCH] Input: Do not add SYN_REPORT in between a single packet data

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Date:	Sat, 2 Apr 2016 22:31:39 +0530
From:	Aniroop Mathur <aniroop.mathur@...il.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Henrik Rydberg <rydberg@...math.org>,
	Aniroop Mathur <a.mathur@...sung.com>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Input: Do not add SYN_REPORT in between a single packet data

Hello Mr. Torokhov,

First of all, Thank you for your reply.

On Sat, Apr 2, 2016 at 3:21 AM, Dmitry Torokhov
<dmitry.torokhov@...il.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@...math.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?

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