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

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Date:	Mon, 07 Mar 2016 09:34:19 +0200
From:	Felipe Balbi <balbi@...nel.org>
To:	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


Hi,

Felipe Ferreri Tonello <eu@...ipetonello.com> writes:
> [ text/plain ]
> Hi Balbi, 
>
> On March 4, 2016 7:16:42 AM GMT+00:00, Felipe Balbi <balbi@...nel.org> wrote:
>>
>>Hi,
>>
>>"Felipe F. Tonello" <eu@...ipetonello.com> writes:
>>> [ text/plain ]
>>> This gadget uses a bmAttributes and MaxPower that requires the USB
>>bus to be
>>> powered from the host, which is not correct because this
>>configuration is device
>>> specific, not a USB-MIDI requirement.
>>>
>>> This patch adds two modules parameters, bmAttributes and MaxPower,
>>that allows
>>> the user to set those flags. The default values are what the gadget
>>used to use
>>> for backward compatibility.
>>>
>>> Signed-off-by: Felipe F. Tonello <eu@...ipetonello.com>
>>> ---
>>>  drivers/usb/gadget/legacy/gmidi.c | 14 ++++++++++++--
>>>  1 file changed, 12 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/usb/gadget/legacy/gmidi.c
>>b/drivers/usb/gadget/legacy/gmidi.c
>>> index fc2ac150f5ff..0553435cc360 100644
>>> --- a/drivers/usb/gadget/legacy/gmidi.c
>>> +++ b/drivers/usb/gadget/legacy/gmidi.c
>>> @@ -63,6 +63,14 @@ static unsigned int out_ports = 1;
>>>  module_param(out_ports, uint, S_IRUGO);
>>>  MODULE_PARM_DESC(out_ports, "Number of MIDI output ports");
>>>  
>>> +static unsigned int bmAttributes = USB_CONFIG_ATT_ONE;
>>> +module_param(bmAttributes, uint, S_IRUGO);
>>> +MODULE_PARM_DESC(bmAttributes, "Configuration Descriptor's
>>bmAttributes parameter");
>>> +
>>> +static unsigned int MaxPower = CONFIG_USB_GADGET_VBUS_DRAW;
>>> +module_param(MaxPower, uint, S_IRUGO);
>>> +MODULE_PARM_DESC(MaxPower, "Used to calculate Configuration
>>Descriptor's bMaxPower parameter");
>>
>>you didn't run checkpatch, did you ? Also, are you sure you will need
>>to
>>change this by simply reloading the module ? I'm not convinced.
>
> Yes I always run checkpatch :)

do you really ? Why do you have a 98-character line, then ?

> What do you mean by reloading the module?

modprobe g_midi MaxPower=4
modprobe -r g_midi
modprobe g_midi MaxPower=10

I'm not convinced this is a valid use-case. Specially since you can just
provide several configurations and let the host choose the one it suits
it best.

>>> @@ -119,8 +127,8 @@ static struct usb_configuration midi_config = {
>>>  	.label		= "MIDI Gadget",
>>>  	.bConfigurationValue = 1,
>>>  	/* .iConfiguration = DYNAMIC */
>>> -	.bmAttributes	= USB_CONFIG_ATT_ONE,
>>
>>nack, nackety nack nack nack :-)
>>
>>USB_CONFIG_ATT_ONE *must* always be set. With your module parameter you
>>give users the oportunity to violate USB spec.
>
> You are right. It needs to check the value before it assigns to
> bmAttributes.

why check ? You can just unconditionally or USB_CONFIG_ATT_ONE. In any
case, I'm not convinced this is necessary at all.

-- 
balbi

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