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

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Message-ID: <CADYu3083da+hW_uLg3WWqgotvOZD1f3V3TdabgX6gDTY93GB7w@mail.gmail.com>
Date:	Wed, 6 Apr 2016 20:26:39 +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

On Sat, Apr 2, 2016 at 10:31 PM, Aniroop Mathur
<aniroop.mathur@...il.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@...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?
>


Hello Mr. Torokhov,

Could you please update about this?
It would be appreciating if you could help out to conclude it quickly.  Thanks!


> 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