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From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Adrian McMenamin <adrian@newgolddream.dyndns.info>
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Subject: Re: [PATCH] sh: maple: add support for Maple controller as a joystick
Date: Sat, 20 Dec 2008 14:17:25 -0800
Cc: Matt Fleming <matt@console-pimps.org>, LKML <linux-kernel@vger.kernel.org>,
       linux-sh <linux-sh@vger.kernel.org>, Andrew Morton <akpm@osdl.org>,
       Paul Mundt <lethal@linux-sh.org>
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On Dec 20, 2008, at 12:47 PM, Adrian McMenamin <adrian@newgolddream.dyndns.info 
 > wrote:

> On Sat, 2008-12-20 at 12:19 -0800, Dmitry Torokhov wrote:
>> On Saturday 20 December 2008 08:07:27 Adrian McMenamin wrote:
>>> On Sat, 2008-12-20 at 15:16 +0000, Matt Fleming wrote:
>>>> On Fri, Dec 19, 2008 at 11:15:42PM +0000, Adrian McMenamin wrote:
>>>>> Add support for the SEGA Dreamcast Maple controller as a joystick
>>>>
>>>> [snip]
>>>>
>>>>> +    struct dc_pad *pad = maple_get_drvdata(mapledev);
>>>>> +    struct input_dev *dev = pad->dev;
>>>>> +    unsigned char *res = mq->recvbuf;
>>>>> +
>>>>> +    buttons = ~cpu_to_le16(*(unsigned short *)(res + 8));
>>>>> +
>>>>
>>>> I may be wrong but shouldn't this use the I/O accessor functions?
>>>
>>> It's not iomemory, so I don't think so. The maple bus writes to a  
>>> buffer
>>> in the physical RAM.
>>>
>>
>> Hmm, but why we are going _from_ cpu order to le16??? I'd expect we
>> wanted to do conversion the other way around. I pulled it out of  
>> 'next'
>> for now.
>>
> The data is transmitted here by the bus in 32 bit little endian blocks
> but makes sense in le16 bit lumps. Off is 1 and on is 0 - hence the ~
>
> The first 16 bits are the button data the next 16 bits are the (start
> of) the axis data. Both cpu and bus data (in this case - it can vary)
> are little endian.

Right, so I think we need le16_to_cpu here, you want the data in  
native CPU order to manipulate it.

-- 
Dmitry

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