linux-kernel - Re: [PATCH] HID: multitouch: do not init reports for multitouch devices

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Date:	Wed, 24 Jul 2013 17:01:01 +0200
From:	Benjamin Tissoires <benjamin.tissoires@...il.com>
To:	Henrik Rydberg <rydberg@...omail.se>
Cc:	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Stephane Chatty <chatty@...c.fr>,
	linux-input <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: multitouch: do not init reports for multitouch devices

Hi Henrik,

On Fri, Jul 19, 2013 at 11:04 PM,  <rydberg@...omail.se> wrote:
> Hi Benjamin,
>
>> >> Some multitouch screens do not like to be polled for input reports.
>> >> However, the Win8 spec says that all touches should be sent during
>> >> each report, making the initialization of reports unnecessary.
>> >> The Win7 spec is less precise, but we can safely assume that when
>> >> the module is loaded (at boot), no one is touching the screen.
>> >>
>> >> Add the quirk HID_QUIRK_NO_INIT_REPORTS so that we do not have to
>> >> introduce a quirk for each problematic device.
>> >
>> > I assume you have tested thoroughly for regressions? How about odd
>> > eGalax devices, for instance? Changes affecting existing hardware
>> > makes me nervous. Is it so bad to add this quirk on a per-device
>> > basis? Or perhaps turned on by default for win8 devices only?
>>
>> Aargh, I forgot the eGalax... (I don't have it anymore on my desk). I
>> was pretty confident because Win [7-8] is not doing any quirks for the
>> multitouch devices, and I had in mind that it did not asked for the
>> reports at startup (at least, I am sure about it for HID/I2C). I'm not
>> sure win 8 devices is a sufficient denominator, because this init
>> sequence is not mentioned anywhere in the Win 8 spec. It's true that
>> we are going to see fewer Win 7 devices, but I would say it's the
>> exact same problem for win 7 and 8. Moreover, asking this for Win 8
>> devices only will forces us to detect it in core before hid-multitouch
>> is loaded because the init reports is called before the parsing.
>
> We already branch on report specifics in hid_add_device(). Adding win8
> detection there is more or less what it was built for.

right. So I will send a following series to detect win8 multitouch
device in core.

>
>> If I capture the Win 7 & Win 8 initialization events and I observe
>> that they do not retrieve the reports, will it be sufficient as a
>> guarantee to include this patch even if it is not widely tested under
>> Linux?

Actually, the current quirk is not imitating Windows behavior. Windows
does not retrieve the input reports, but only the feature reports.
This scheme is also useful for us as the feature_mapping() callback
makes use of the ->value, and if we do not retrieve the feature
report, the field is left uninitialized.

I will also add an other quirk to retrieve only the features at init.

>
> We already have the usbhid quirks to handle odd cases, and we can add
> all sorts of generic detection during device add, so there really is
> no reason to risk regressions at all, is there?

Ok, so let's go for another patch series.. :)

Cheers,
Benjamin
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