linux-kernel - Re: [PATCH 1/1] Bluetooth: hidp: Add support for hidraw HIDIOCGFEATURE and HIDIOCSFEATURE

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Date:	Fri, 09 Jul 2010 09:02:32 -0400
From:	Alan Ott <alan@...nal11.us>
To:	Marcel Holtmann <marcel@...tmann.org>
Cc:	David S Miller <davem@...emloft.net>,
	Jiri Kosina <jkosina@...e.cz>,
	Michael Poole <mdpoole@...ilus.org>,
	Bastien Nocera <hadess@...ess.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] Bluetooth: hidp: Add support for hidraw   HIDIOCGFEATURE   and HIDIOCSFEATURE

On 07/09/2010 04:01 AM, Marcel Holtmann wrote:
> Hi Alan,
>
>    
>>> I looked at this and I am bit worried that this should not be done in
>>> this detail in the HIDP driver. Essentially HIDP is a pure transport
>>> driver. It should not handle all these details. Can we make this a bit
>>> easier for the transport drivers to support such features?
>>>        
>> I put these changes (most notably the addition of hidp_get_raw_report())
>> in hidp because that's where the parallel function
>> hidp_output_raw_report() was already located. I figured the input should
>> go with the output. That said, if there's a better place for both of
>> them (input and output) to go, let me know where you think it should be,
>> and I'll get them moved into the proper spot.
>>
>> I'm not sure what you mean about HIDP being a pure transport driver.
>>      
> what is usb-hid.ko doing here? I would expect a bunch of code
> duplication with minor difference between USB and Bluetooth.
>
> Regards
>
> Marcel
>    

Hi Marcel,

usbhid doesn't have a lot of code for hidraw. Two functions are involved:
     usbhid_output_raw_report()
         - calls usb_control_msg() with Get_Report
     usbhid_get_raw_report()
         - calls usb_control_msg() with Set_Report
             OR
         - calls usb_interrupt_msg() on the Ouput pipe.

This is of course easier than bluetooth because usb_control_msg() is 
synchronous, even when requesting reports, mostly because of the nature 
of USB, where the request and response are part of the same transfer.

For Bluetooth, it's a bit more complicated since the kernel treats it 
more like a networking interface (and indeed it is). My understanding is 
that to make a synchronous transfer in bluetooth, one must:
     - send the request packet
     - block (wait_event_*())
     - when the response is received in the input handler, wake_up_*().

There's not really any code duplication, mostly because initiating 
synchronous USB transfers (input and output) is easy (because of the 
usb_*_msg() functions), while making synchronous Bluetooth transfers 
must be done manually. If there's a nice, convenient, synchronous 
function in Bluetooth similar to usb_control_msg() that I've missed, 
then let me know, as it would simplify this whole thing.

See the Set/Get Feature patch, including USB support, here:
     http://lkml.org/lkml/2010/6/9/222

Alan.



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