linux-kernel - Re: [PATCH 3/5] usb: gadget: gmidi: remove bus powered requirement on bmAttributes

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Message-id: <56DEA61F.1060100@samsung.com>
Date:	Tue, 08 Mar 2016 11:14:55 +0100
From:	Krzysztof Opasiak <k.opasiak@...sung.com>
To:	Felipe Balbi <balbi@...nel.org>,
	Felipe Ferreri Tonello <eu@...ipetonello.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Michal Nazarewicz <mina86@...a86.com>,
	Clemens Ladisch <clemens@...isch.de>
Subject: Re: [PATCH 3/5] usb: gadget: gmidi: remove bus powered requirement on
 bmAttributes



On 03/08/2016 08:43 AM, Felipe Balbi wrote:
(...)

>>>> This is necessary because this driver is actually wrong in which is
>>>> asking for the host to power itself. This is not specified on USB-MIDI
>>>> specification, neither makes any sense since this configuration is
>>>> device specific.
>>>>
>>>> What is your suggestion to make it configurable? Maybe at compile-time?
>>>> I really don't know what is the best solution if this is not something
>>>> you like it.
>>>
>>> well, you could use our configfs-based gadget interface. You don't
>>> really need to use gmidi.ko at all. In fact, we wanna do away with any
>>> static modules and rely only on configfs. If configfs doesn't let you
>>> change what you want/need, then we can talk about adding support for
>>> those.
>>>
>>> bMaxPower and bmAttributes sound like good things to have configurable
>>> over configfs but beware of what the USB specification says about them,
>>> we cannot let users violate the spec by passing bogus values on these
>>> fields.
>>
>> I agree that we should move to configfs, but the truth is that these
>> legacy devices are still useful. They just do one thing, mostly, but
> 
> yes, they are useful as they are. They don't need to be changed to be
> useful. Plus, you can have a gadget built with configfs that does only
> one thing. And you can do that with a simple shell script.
> 
>> its easy and simple to setup and use. So I think before we have some
> 
> so is configfs.
> 
>> sort of preset library of configfs-based gadget drivers, we still need
>> these modules.
> 
> there is already a library called libusbg.

As libusbg itself is a little bit dead there is a fork called
libusbgx[1] and it is still active;)

It already has support for f_midi so it is ready to use.

Footnotes:
1 - https://github.com/libusbgx/libusbgx

Cheers,
-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics