Re: [PATCH 4/6 v2] HID: magicmouse: remove axis data filtering

[Henrik Rydberg <rydberg@euromail.se>]

On 08/31/2010 11:27 PM, Chase Douglas wrote:

> On Tue, 2010-08-31 at 23:18 +0200, Henrik Rydberg wrote:
>> On 08/31/2010 11:16 PM, Chase Douglas wrote:
>>
>>> On Tue, 2010-08-31 at 23:06 +0200, Henrik Rydberg wrote:
>>>> On 08/31/2010 10:58 PM, Chase Douglas wrote:
>>>>
>>>>> On Tue, 2010-08-31 at 22:34 +0200, Henrik Rydberg wrote:
>>>>>> On 08/31/2010 08:41 PM, Chase Douglas wrote:
>>>>>>
>>>>>>> The Magic Mouse device is very precise. No driver filtering of input
>>>>>>> data needs to be performed.
>>>>>>>
>>>>>>> Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
>>>>>>> Acked-by: Michael Poole <mdpoole@troilus.org>
>>>>>>> ---
>>>>>>
>>>>>>
>>>>>> I am still not sure this is a good idea. Bandwidth from MT devices is a big
>>>>>> deal. A statement roughly how much data comes out of mtdev (which does the
>>>>>> filtering for type A devices) before and after this change would be reassuring.
>>>>>
>>>>> As it is right now, hid-magicmouse doesn't support MT slots. I think all
>>>>> the fuzz code ends up comparing in the MT case is between one touch and
>>>>> another touch, not between one touch's current location and its previous
>>>>> location. If I'm correct, then it means a fuzz > 0 is incorrect for
>>>>> non-slotted MT devices.
>>>>>
>>>>> In fact, the code in drivers/input/input.c around line 194 looks like it
>>>>> discards defuzzing in this case, so one could say this patch is making
>>>>> things more correct :).
>>>>
>>>>
>>>> For type A devices, the filtering is performed in userspace, in mtdev, in the
>>>> same manner as it would have been performed in the kernel in the MT slot case.
>>>> Therefore, knowing the amount of messages coming out of mtdev is a direct
>>>> measurement of the effect of filtering.
>>>
>>> Yes, but we're only interested in the kernel driver when reviewing this
>>> patch. Leaving the fuzz in as it is has no effect right now on ABS_MT_*
>>> axes. On the other axes, such as the touch orientation, it's probably
>>> more harmful than good.
>>
>>
>> "We" are interested in knowing if this patch makes any sense, given that
>> filtering is in actuality performed in userspace, and thus modifying this code
>> changes the stream rate into userspace applications, thank you.
> 
> Because in-kernel defuzzing is turned off for ABS_MT_* axes on
> non-slotted MT devices, there will be no change in the number of events
> sent to the userspace due to this patch.
> 
> Maybe I'm missing something more fundamental. In which case, I'll need
> more details to get it through my dense skull :).


All events passes through to mtdev, yes, but mtdev filters a considerable amount
of events from passing through to X drivers like evdev. Thus, the fuzz values
reported in the kernel driver impacts the performance in userspace, even if
filtering is not done in the kernel.

Henrik