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Date: Wed, 19 Aug 2009 11:41:28 -0600
Subject: Re: Purpose of parameter in sysfs binary read
From: "Rick L. Vinyard, Jr." <rvinyard@cs.nmsu.edu>
To: "Alan Cox" <alan@lxorguk.ukuu.org.uk>
Cc: "Greg KH" <greg@kroah.com>, "Linux USB" <linux-usb@vger.kernel.org>,
       "LKML" <linux-kernel@vger.kernel.org>
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Alan Cox wrote:
> On Tue, 18 Aug 2009 09:41:56 -0600
> "Rick L. Vinyard, Jr." <rvinyard@cs.nmsu.edu> wrote:
>
>> Greg KH wrote:
>> > On Tue, Aug 18, 2009 at 08:55:29AM -0600, Rick L. Vinyard, Jr. wrote:
>> > What are you using the binary sysfs attribute for?  It should ONLY be
>> > used as a pass-through to and from hardware, with no interpretation by
>> > the kernel at all.
>>
>> It's for the Logitech G13's game panel image (a simple binary bitmap of
>> the pixels).
>>
>> It doesn't do any interpretation... just a few checks on the magic
>> number
>> and size.
>
> If its a bitmap image why not register the device as a framebuffer ?
>

Would that preclude it also being registered as a HID? The primary purpose
of the device is to function as a keypad input.

But, in addition to sending keystroke output you can send a usb control
message to change the backlight color and an interrupt message to load a
bitmap as an image.

The bitmap itself is simple, but requires a specific format. Byte 0 must
be 0x03 followed by 31 bytes of null. The low bit of byte 32 is x,y (0,0)
second high bit is (0,1)... with low bit of byte 33 being (1,0)...