linux-kernel - Re: [PATCH v2 00/10] Input - wacom: conversion to HID driver, series 2

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Date:	Fri, 25 Jul 2014 20:26:16 +0100
From:	Przemo Firszt <przemo@...szt.eu>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Ping Cheng <pinglinux@...il.com>,
	Jason Gerecke <killertofu@...il.com>,
	linux-kernel@...r.kernel.org, linux-input@...r.kernel.org
Subject: Re: [PATCH v2 00/10] Input - wacom: conversion to HID driver,
 series 2

Dnia 2014-07-25, pią o godzinie 10:54 -0400, Benjamin Tissoires pisze:
> [..]
Hi Benjamin,
>  
> > If I understand you correctly we cannot have the same entry point in
> > sysfs for OLEDs unless we can tell userspace somehow if the tablet is
> > conected over USB or over bluetooth. The hardware of Intuos4 Wireless
> > over bluetooth allows only 1-bit images. The same hardware over USB
> > allows 4-bit images. Formatting of the images is also completely
> 
> Holy crap! I missed that. I did not noticed the 1-bit vs 4-bits
> difference :(
> 
> > different and it's not "plain". Check [1] for usb and exisitng
> > hid-wacom.c/wacom_scramble function for bluetooth.
> 
> Maybe I overlooked it, but I thought that in case of USB, the scrambling
> is done in user space, and in case of BT, the same scrambling made in the
> kernel. They looks very similar so I thought the user-space scramble for
> USB would have fit. However, the 4-bits/1-bits kills that assumption.

You're correct - USB requires scrambling in the user space, but
scrambling for USB is different than for BT. Example of USB scramble is
here: https://github.com/PrzemoF/i4oled/blob/master/src/i4oled.c#L401

> > 
> > If we want to make it consistent single entry on kernel level we
> > probably have to implement image conversion in the kernel. The user land
> > would always use 4-bit, plain formatted images and kernel driver would
> > convert them to 4-bit, wacom usb format or 1-bit wacom bluetooth format
> > depending on how the tablet is connected.
> > 
> > The downside of this approach is that the user land wouldn't have 100%
> > control over 1-bit images for bluetooth as kernel would have to create
> > them from 4-bit images.
> 
> The USB interface is *very* simple:
> - if incoming data != 1024 -> reject
> - forward everything to the device without in kernel treatment*
> 
> How about just changing the expected size to 256 in case of a BT tablet,
> and let the user-space scramble in both cases and forward proper raw
> data?
> 
> This way, user space will still have control over 1-bit vs 4-bits, and
> the changes required in the kernel will be small enough.
Sounds good to me. Bit more coding in gnome, but it's a small thing.

> My concern is that I'd like to have this series in 3.17 and I will not
> have access to the hardware until next week :/
> Having all of the user-spaces breakages in the same kernel release will
> I think simplify the user space work.
I agree.

P.S. Could you send me that set of 23 patches using git-send-email that
I should apply on top of linux-next? I got it from lkml, but there is
still something messed (fails on patch 09/23). Of course, if it's not a
problem for you :-)

-- 
Kind regards,
Przemo Firszt


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