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

From: Aniroop Mathur
Date: Thu Mar 10 2016 - 13:57:22 EST

Hi Henrik,

On Thu, Mar 10, 2016 at 7:15 PM, Henrik Rydberg <rydberg@xxxxxxxxxxx> 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.

Aniroop Mathur

> Henrik